RCA Victor 648PTK User manual

295
Model 648PV
Walnut or Mahogany
rcaV ict0R
PROJECTION TELEVISION,
AM-FM RADIO, COMRINATION
MODEL 648PTK
Chassis Nos. KCS 24-1, KRS 20-1, KRS 21-1
KRK 1-1, RK-121A and RS-123A
PROJECTION TELEVISION
AM-FM RADIO PHONOGRAPH
COMRINATION
MODEL 848P¥ '
Chassis No. KCS 24A-I, KRS 20-1, KRS 2IA-I
KRK-iA, RK-I2IA and RS-I23B
Mfr. No. 274
Service Data
- 1947 No. 12 -
- 1948 No. T4 -
RADIO CORPORATION Of AMERICA
RCA VICTOR DIVISION
CAMDEN, N. J., U. S. A.
GENERAL DESCRIPTION
Model 648PTK is a forty-eight tube Projection Television,
AM-FM Radio, console combination. The television receiver em¬
ploys four chassis with a total of thirty-five tubes and a five-
inch projection kinescope. A Reflective Optical System pro¬
vides a 15" x 20" picture on the screen.
Features of the television unit are full thirteen channel cov¬
erage; FM sound system; improved picture brilliance; picture
A-G-C; A-F-C horizontal hold; stabilized vertical hold; two
stages of video amplification; noise saturation circuits; three-
stage sync separator and clipper; four me band width for pic¬
ture channel and reduced hazard high voltage supply.
The radio receiver employs an eight-tube tuner unit and a
four-tube audio-amplifier, power-supply unit.
The radio chassis is provided with a Phono input jack to
permit the use of an external record player.
Model 648PV is .a forty-eight tube Projection Television,
AM-FM Radio, Phonograpn, console combination. The television
receiver employs four chassis with a total of thirty-five tubes
and a five-inch projection kinescope. A Reflective Optical
System provides a 15" x 20" picture on the screen.
Features of the television unit are full thirteen channel cov¬
erage; FM sound system; improved picture brilliance; picture
A-G-C; A-F-C horizontal hold; stabilized vertical hold; two
stages of video amplification; noise saturation circuits; three-
stage sync separator and clipper; four me band width for pic¬
ture channel and reduced hazard high voltage supply.
The radio receiver employs an eight-tube tuner unit and a
four-tube audio-amplifier, power-supply unit.
An automatic record changer of the "slicer” type is em¬
ployed and features a crystal pickup with the "Silent Sap¬
phire” stylus.
ELECTRICAL AND MECHANICAL SPECIFICATIONS
TELEVISION R-F FREQUENCY RANGES
Television
Picture
Sound
Tel. Rec.
Channel
Channel
Carrier
Carrier R-F Osc.
Number Freq. Me.
Freq. Me. Freq. Me. Freq. Me.
1.
.44-50.
.45.25.
.49.75. .71
2 .
.54-60.
. 55.25 . .59.75. .81
3.
.60-66.
.61.25. .65.75.
.87
4..
......66-72.
.67.25.
.71.75 . .§3
5 .
.76-82.
.......77.25. .81.75.
.103
6.
.82-88.
.83.25.
.87.75....... .. . 1 09
7.........
....174-180.
.175.25.
.179.75....... .201
8.
....180-186.
.181.25.
.185.75..:....
.........307
9 .
....186-192.
.187.25.
.191.75.
..213
10 .
....192-198.
.193.25.
.197.75. .219
11.
....198-204.
.199.25. .203.75.
.225
12 .
....204-210.
.205.25.
.209.75. .231
13 .
....210-216.
.211.25. .215.75.
.237
TELEVISION FINE TUNING RANGE
Plus and minus approximately 800 kc on channel 1, and
plus and minus approximately 1.9 me on channel 13.
PICTURE SIZE .....................................15" x 20"
RADIO TUNING RANGE
Broadcast ..... 540-1,600 kc
Short Wave . 9.2-18 me
Frequency Modulation . 88-108 me
Intermediate Frequency—AM . 455 kc
Intermediate Frequency—FM . 10.7 me
RECEIVER ANTENNA INPUT IMPEDANCE..300 ohms balanced
Specifications continued on page 3

296
648PTE. 8 4 SPY TABLE OF CONTENTS
Page
Alignment Procedure (Television)
Detailed ......... 22
Table ......... 28
Alignment Procedure (Radio) . 44
Antenna ............. 11
Circuit Description (Television) .... 12
Circuit Description (Radio) .. 33
Drawings (Major)
Television
Adjustment Location (Alignment) . 30
Adjustment Location (Installation) . 9
R-F, I-F Chassis Views . 20
Horizontal Deflection Chassis View .. 21
Chassis Wiring Diagrams . 56
Circuit Schematic .59-63
R-F Unit Wiring Diagram .57-61
Radio
Adjustment Location . 45
Chassis Wiring Diagram .46-47
Circuit Schematic .48-49
Pag®
General Description .. 1
Interference ............ jj
Installation Instructions ....._ §
Operating Instructions .......... 4
Photographs
Cabinet . j
Test Pattern . «
Waveform . gg
Precautions
Kinescope Handling . 2
Ventilation . H
Reflections ...
Replacement Parts . 05
Service Suggestions .. 32
Specifications (Elec. & Mech.) . 1
Test Equipment . 22
Tube Complement . g
Voltage Chart (Television) . 42
Voltage Chart (Radio) . 5 q
Warning (High Voltage) . 2
HIGH VOLTAGE WARMING
OPERATION OF THIS RECEIVER OUTSIDE THE CABINET OR WITH THE COVERS REMOVED,
INVOLVES A SHOCK HAZARD FROM THE RECEIVER POWER SUPPLIES. WORK ON THE RE¬
CEIVER SHOULD NOT BE ATTEMPTED BY ANYONE WHO IS NOT THOROUGHLY FAMILIAH
WITH THE PRECAUTIONS NECESSARY WHEN WORKING ON HIGH VOLTAGE EQUIPMENT.
DO NOT OPERATE THE TELEVISION RECEIVER WITH THE HIGH VOLTAGE COMPARTMENT
SHIELD REMOVED.
KINESCOPE HANDLING PRECAUTIONS
DO NOT OPEN THE KINESCOPE SHIPPING CARTON, INSTALL, REMOVE OR HANDLE THE KINE¬
SCOPE IN ANY MANNER UNLESS SHATTERPROOF GOGGLES AND HEAVY GLOVES ARE
WORN. PEOPLE NOT SO EQUIPPED SHOULD BE KEPT AWAY WHILE HANDLING KINE¬
SCOPES. KEEP THE KINESCOPE AWAY FROM THE BODY WHILE HANDLING.
The kinescope bulb encloses a high vacuum and, due to its large surface area, is subjected to considerable air pressure.
For these reasons, kinescopes must be handled with more car© than ordinary receiving tubes.
Th® large end of the kinescope bulb—particularly that part at the rim of the viewing surface—must not be struck, scratched
or subjected to more than moderate pressure at any time. In installation, if the tube sticks or fails to slip smoothly into Its
socket, or deflecting yoke, investigate and remove th© cause of the trouble. Do not force the tube. Refer to the receiver
Installation Instructions section for detailed Instructions on kinescope installation. All RCA kinescopes are shipped in special
cartons and should be left in the cartons until ready for installation in the receiver.. Keep the carton for possible future use.

297
ELECTRICAL AMD MECHANICAL
POWER SUPPLY BATING
Television Operation . 115 volts, 60 cvcles, 530 watts
Radio Operation . 115 volts, 60 cycles, 145 watts
Phonograph Operation . 115 volts, 60 cycles, 165 watts
AUDIO POWER OUTPUT RATING
Undistorted Power Output .... iq watts
Maximum Power Output ....." ’ ’’ n watts
.CHASSIS DESIGNATIONS 648PTK
Television R-F, I-F Chassis .KCS24-1
Horizontal Deflection Chassis . KRS20-1
Television Power Supply Chassis .KRS21-1
Optical Barrel . KRK1-1
Radio Chassis .RK121A
Audio Amplifier . RS123A
Record Player .
648PV
KCS24A-1
....KRS20-1
KRS21A-1
.. KRK-IA
. RK121-A
... RS123B
. RP176
LOUDSPEAKER (92567-2)
Type .
Voice Coil Impedance .
WEIGHT
Chassis with Tubes in Cabinet
Shipping Weight .
12-inch Electrodynamic
2.2 ohms at 400 cycles
648PTK 648PV
■ • 295 lbs. 323 lbs.
360 lbs . 407 lbs.
DIMENSIONS (inches) Width Height
Cabinet (outside) . 648PTK. 36 % 471/2
Cabinet (outside) . 648PV. 48 39 V 4
Depth
22 %
251/2
TELEVISION CHASSIS DATA
PICTURE I-F FREQUENCIES
Picture Carrier Frequency . 25.75 me
Adjacent Channel Sound Trap . 27.25 me
Accompanying Sound Traps . 21.25 me
Adjacent Channel Picture Carrier Trap . 19.75 me
SOUND I-F FREQUENCIES
Sound Carrier Frequency . 21.25 me
Sound Discriminator Band Width (between peaks) . 350 kc
VIDEO RESPONSE . To 4 me
FOCUS . Electrostatic
SWEEP DEFLECTION ... Magnetic
SCANNING . Interlaced, 525 line
HORIZONTAL SCANNING FREQUENCY . 15,750 cps
VERTICAL SCANNING FREQUENCY . 60 cps
FRAME FREQUENCY (Picture Repetition Rate) . 30 cps
NON-OPERATING CONTROLS (not including r-f and i-f adjust¬
ments)
SPECIFICATIONS (Continued) 648PTK , 648PV
P.CA TUBE COMPLEMENT
KCS24-1, KCS24A-1 R-F, I-F CHASSIS
Tube Used Function
( 1 ) RCA- 6 J 6 ..... R-F Amplifier
(2) RCA- 6 J 6 . R-F Oscillator
(3) RCA- 6 J 6 ..... Converter
(4) RCA- 6 BA 6 . 1 s t Sound I-F Amplifier
(5; RCA- 6 BA 6 . 2nd Sound I-F Amplifier
( 6 ) RCA- 6 AU 6 .. 3rd Sound I-F Amplifier
(') RCA-6AL5 ... Sound Discriminator
( 8 ) RCA- 6 AT 6 ... A-G-C Amplifier
(9) RCA-6AL5 . A-G-C Diode and D-C Restorer
( 1 0 ) RCA-6AG5 . 1 st Picture I-F Amplifier
ul) RCA-6AG5 . 2nd Picture I-F Amplifier
(.12) RCA-6AG5 . 3rd Picture I-F Amplifier
(13) RCA-6AG5 . 4th Picture I-F Amplifier
(14) RCA-6AL5 . Picture 2nd .Detector and A-G-C Detector
(15) RCA- 6 AU 6 . 1 s t Video Amplifier
(16) RCA- 6 V 6 GT . 2nd Video Amplifier
(17) RCA-6SK7 . 1 st Sync Amplifier
(18) RCA-6SH7 . 2nd Sync Amplifier
(19) RCA-6J5 . 3 rd Sync Amplifier
(20) RCA-6J5 . Vertical Sweep Oscillator and Discharge
(21) RCA- 6 K 6 GT . Vertical Sweep Output
KRS20-1 HORIZONTAL DEFLECTION CHASSIS
Tube Used Function
(1) RCA- 6 H 6 . Horizontal Sync Discriminator
( 2 ) RCA- 6 K 6 GT . Horizontal Sweep Oscillator
(3) RCA-6J5 . Horizontal Discharge
(4) RCA-6AC7 . Horizontal Sweep Oscillator Control
(5) RCA- 6 BG 6 G . Horizontal Sweep Output (2 tubes)
( 6 ) RCA-5V4G . Horizontal Damper
(7) RCA-6AS7G . Horizontal Damper
( 8 ) RCA-1B3-GT/8016 . High Voltage Rectifier (3 tubes)
(9) RCA-5TP4 .... Projection Kinescope
KRS 21-1 , KRS 21 A -1 TELEVISION POWER SUPPLY CHASSIS
Tube Used Function
(1) RCA-5U4G . Rectifier (3 tubes)
Horizontal Centering .. Horizontal Deflection chassis adjustment
Vertical Centering ... R-F, I-F chassis rear adjustment
.... R-F, I-F chassis rear adjustment
Vertical Linearity . R-F, I-F chassis rear adjustment
Width .... Horizontal Deflection chassis screwdriver adjustment
Horizontal Linearity ....Horizontal Deflection chassis adjustment
Horizontal Drive . Horizontal Deflection chassis adjustment
Horizontal Oscillator Frequency
Horizontal Deflection chassis adjustment
Horizontal Oscillator Phase
Horizontal Deflection chassis adjustment
Locus (Electrical) .. Horizontal Deflection chassis rear adjustment
Focus (Mechanical) . Optical Barrel adjustment
Deflection Coil . Optical Barrel adjustment
Video Peaking Switch . R-F, I-F chassis rear switch
Horizontal Optical Centering . Optical Barrel adjustment
Lateral Optical Centering ... Optical Barrel adjustment
RK121A RADIO CHASSIS
Tube Used Function
(1) RCA- 6 BA 6 ..... R-F Amplifier
\2) PlCA- 6 BE 6 . Oscillator
(3) RCA- 6 BA 6 . Mixer
(4) RCA- 6 BA 6 . 1st I-F Amplifier
(5) RCA- 6 AU 6 . 2nd I-F and Phono Amplifier
( 6 ; RCA- 6 AU 6 . Driver
(7) RCA-6AL5 . Ratio Detector
( 8 ) RCA- 6 AT 6 . AM Detector, AVC and Audio Amplifier
RS123A, RS123B AUDIO AMPLIFIER
Tube Used Function
(1) RCA-5U4G . Recti{ier
(2) RCA-6J5 . p hase i nverte r
(3) R 0 A- 6 F 6 G . Power Output (2 tubes)
3

298
648PTI, 848PV
RECEIVER OPERATING INSTRUCTIONS
TELEVISION OPERATION
The following adjustments are necessary when turning the
receiver on lor the first time.
1. Turn the radio FUNCTION switch to Tel.
2. Turn the receiver "ON" and advance the sound VOL¬
UME control to approximately mid-position.
3. Set the STATION SELECTOR to the desired channel.
4. Turn the PICTURE control fully counter-clockwise.
5. Turn the BRIGHTNESS control clockwise, until a glow ap¬
pears on the scieen, then counter-clockwise until the glow just
disappears.
6. Turn the PICTURE control clockwise until a glow or
pattern appears on the screen.
7. Adjust the FINE TUNING control for best sound fidelity
and sound VOLUME for suitable volume.
8. Adjust the VERTICAL hold control until the pattern stops
vertical measurement.
9. Adjust the HORIZONTAL hold control until a picture is
obtained and centered.
10. Adjust the PICTURE control for suitable picture con¬
trast.
11. After the receiver has been on for some time, it may be
necessary to readjust the FINE TUNING control slightly for
improved sound fidelity.
12. In switching from one station to another, it may be
necessary to repeat steps number 7 and 10.
13. When the set is turned on again after an idle period, it
should not be necessary to repeat the adjustments if the posi¬
tions of the controls have not been changed. If any adjustment
is necessary, step number 7 is generally sufficient.
14. If the position of the controls have been changed, it
may be necessary to repeat steps number 2 through 10.
RADIO OPERATION
1. Turn the radio FUNCTION switch to the desired band
(BC, SW or FM).
2. Tune in the desired station with the TUNING control.
PUSH-BUTTON OPERATION
1. Turn the radio FUNCTION switch to the desired band
(BC, SW or FM).
2. Push the appropriate push button to receive the desired
station.
PHONOGRAPH OPERATION
1. Turn the radio FUNCTION switch to the phono position.
2. Slide the changer power switch to "ON.
HORIZONTAL
BRIGHTNESS
STATION
SELECTOR
CHANGER
CONTROL
SWITCH
CHANGER
POWER
SWITCH

299
INSTALLATION INSTRUCTIONS
648PTK, 648PV
MODEL 648PTK
Access to the front tubes in the r-f, i-f chassis may be had
through the front of the cabinet by raising the door stop as
shown in Detail A of Figure 2 and then sliding the right
television door all the''way back. When this check is com¬
pleted, close the door to its normal position and drop the door
stop back in place.
MODEL 648PV
A heat shield is placed over the RSI 2 3. audio amplifier to pre¬
vent the rectifier tube from coming in contact with the optical
barrel dust shield when the cabinet is closed. Care should be
taken to see that this shield is replaced after the cardboard
shipping shields have been removed from the RSI23 amplifier.
To get at the optical barrel adjustments, take out three screws
on each side of the front of the speaker grill and remove the
Remove the shipping material as shown in Figure 2. Make
sure that all tubes are firmly seated in their sockets.
Untie the canvas dust cover for the optical barrel and tie it off
to one side.
Caution: Handle the corrector lens with care. This lens is
made of a plastic material, is soft and can be easily scratched
by improper handling or even by rubbing with a cloth. Do not
use cleaning fluid on the lens as it may be attacked by some
of the chemicals used in such solutions. In short, the lens
should be given the care due any precision optical equipment.
Remove the corrector lens from the top of the optical barrel by
loosening the three screws holding the clamp springs as shown
in Figure 4. Caution: Do not loosen the three screws holding
the corrector lens mounting plate.
Figure 2—Removal
of Shipping Material
MODEL 648PTK
Figure 2—Removal
of Shipping Material
MODEL 648PV

648PTK, 648PV
INSTALLATION INSTRUCTIONS
Although the high voltage filter capacitors of a new receiver
are not likely to be charged, it is a good idea to form the habit
of discharging the optical barrel before making any internal
adjustments. Take a clip lead, fasten the clip end to the bar¬
rel and discharge the unit by making repeated contacts to the
kinescope holder shown in Figure 3.
Clean the back of the screen, the front of the 45° mirror and
the optical barrel spherical mirror by "sweeping" the surface
with a small camel's hair brush. Any dust on the spherical
mirror should be swept into the black center portion where it
can be picked up with a piece of scotch tape. Caution: Do
not touch the silvered portion of the mirrors. The mirrors are
surface silvered and can be damaged by contact with the
moist hand. If the screen or mirrors require cleaning, a solu¬
tion of "Dreft" and water should be employed.
Figure 3—Kinescope Holder
Place a type 202—B—1 test lamp in the kinescope holder
and adjust the ball screws to center the lamp in the holder.
Connect the lamp cord into a 110-volt power outlet and turn
the lamp on. Replace the corrector lens, taking care that the
arrow on the edge of the lens points to the rear of the cabinet.
Rotate the lamp so as to produce a picture on the screen in
the proper aspect. Cover the center hole in the corrector lens
with a piece of black cardboard in order to prevent light from
this source from lowering the resolution. Pull the dust cover
down around the barrel.
Observe the raster on the screen by use of a mirror placed in
front of the set. A chrome-plated photographic ferrotype tin
is excellent for this purpose.
Loosen the optical focus adjustment lock screws and adjust
the optical focus adjustment for the best overall definition on
the screen. The optical system should show at least 900 line
resolution over all the screen. If the system shows less
definition, it will be necessary to make the adjustments under
"Alignment of Optical Barrel."
ALIGNMENT OF OPTICAL BARREL—With the test lamp in
place as described above, turn -the optical focus adjustment
until the vertical and horizontal lines become double. When
the test lamp is properly centered, the lines are parallel. If
the lines are not parallel, the Horizontal or Lateral optical cen¬
tering controls require readjustment.
Lateral Optical Adjustment—If the vertical lines are not par¬
allel, loosen the lateral adjustment set screws and turn the
lateral adjustment until the vertical lines are parallel. Tighten
the adjustment set screws.
Horizontal Optical Adjustment—-If the horizontal lines are not
parallel, loosen the optical horizontal centering lock screws and
turn the optical horizontal centering adjustment until the lines
are parallel. Tighten the adjustment lock screws.
Corrector Lens Centering —Turn the focus adjustment until a
halo appears around the dot in the center of the test lamp.
If the halo is not symmetrical around the dot, loosen the three
corrector lens lock screws and the three corrector lens mount¬
ing clip screws and shift the lens until the halo is symmetrical.
Tighten the lens centering lock screws with the lens in this
position.
Check of Optical Barrel Tilt —Adjust the optical focus control
to and through the focus range. The picture should go through
focus all over at the same time. This does not mean that the
definition will be equal over all the picture, but it should be
the best definition obtainable. If this is not the case, the op¬
tical barrel is not in alignment with the cabinet and requires
adjustment as outlined in the following paragraph.
Optical Barrel Tilt Alignment —Turn the optical focus adjust¬
ment counterclockwise until the picture is out of focus then
clockwise until the picture begins to come in focus. If one side
comes into focus before the rest of the picture, it indicates that
that side of the optical barrel should be raised. Loosen the
lock nuts and turn the inner jack nuts, shown in Figure 4, to
raise that side of the barrel and the other jack nut down to
lower the other side of the barrel, until both sides of the pic¬
ture come into focus at the same time.
If the top of the picture comes into focus first as the optical
focus adjustment is turned clockwise, it indicates that the outer
jack nut (nearest the focus controls) should be adjusted to
lower the back of the optical barrel, until
top and bottom come into focus at the same
time.
When the barrel is properly adjusted, the
entire picture will come into best focus all
over at the same time as the locus control
is rocked through the focus point. At this
point the pattern should be in the center
of the screen. When this condition of align¬
ment is obtained, tighten the lock nuts
being careful not to disturb the adjustments.
If the optical barrel tilt adjustments are
made, it will be necessary to recheck the
adjustments under Horizontal Optical Ad¬
justments and Lateral Optical Adjustments.
Loosen all the kinescope mounting wing
screws equally and just sufficiently to per¬
mit removal of the test lamp.
Figure 4 —Optical Barrel Adjustments
6

INSTALLATION INSTRUCTIONS
S48PTK, 648PV
KINESCOPE HANDLING PRECAUTION— Do not open the kine¬
scope shipping carton, install, remove, or handle the kinescope
in any manner, unless shatterproof goggles and heavy gloves
are worn. People not so equipped should be kept away while
handling the kinescope. Keep the kinescope away from the
body while handling. The shipping carton should be kept for
use in case of future moves.
INSTALLATION OF KINESCOPE—The kinescope second anode
contact is a recessed metal well in the side of the bulb. A
small brass clip (from the carton containing the deflection yoke
and front panel control knobs) must be placed in the kine¬
scope anode contact and the tube inserted in the holder as
shown in Figure 3. The tube must be installed so that the
socket key on the base of the tube is pointed towards the
horizontal deflection chassis. Make sure that the anode clip is
horizontal so that it cannot protrude out of the holder.
Tighten the three ball screws equally to center the tube in
the support. Caution: Do not apply too much pressure to
tightening the ball screws as the tube can be cracked by so
doing.
Wipe the corrector lens clean with a piece of lens tissue and
replace on the barrel. Turn the lens mounting clips in place
and tighten the clip screws.
Turn the deflection yoke so that the slotted end of the bakelite
center tube is up and slide the yoke down over the neck of the
kinescope. Connect the kinescope socket to the base of the
tube.
Slip the yoke cables out through the cable sleeve in the op¬
tical barrel dust cover. The three-prong plug on the un¬
shielded yoke cable should be plugged into the television r-f
i-f chassis as shown in Figure 5. The two-prong plug on the
shielded yoke cable should be plugged into the horizontal de¬
flection chassis. The shield braid extension from this cable
should be grounded to the chassis by means of the screw pro¬
vided for this purpo^p.
Caution—Do not turn the television receiver on with the deflec¬
tion yoke cables disconnected. To do so may cause the de¬
struction of the kinescope screen.
Reconnect the speaker. Check all chassis interconnecting
cables to make sure that all are plugged into the proper
sockets as shown in Figure 5. It is possible to insert the re¬
ceiver antenna and ground plug backwards. The ground wire
should go to the middle connector at the radio chassis as
shown.
Open the kinescope shipping
carton and remove the tube.
Handle this tube by the neck.
Do not cover the envelope of
the tube with fingermarks as
it will produce leakaqe paths
between the high voltage rim
near the screen and the
grounded coating on the neck.
If this portion of the tube has
inadvertently been handled,
wipe it clean with a soft cloth
moistened with ''dry' 1 carbon
tetrachloride, which is obtain¬
able at most drug stores.
Wipe the kinescope screen
clean of all dust or finger
marks with a soft cloth moist¬
ened with the Drackett Co.'s
"Windex" or similar cleaning
□ gent.
MODEL 648PV
Figure 5 —Chassis
Interconnecting Cables

302
INSTALLATION INSTRUCTIONS
848PTK, 648PV
Remove the cover from the horizontal deflection chassis and
take out the strings holding the high voltage filter capacitors
in the clips during shipment. Replace the chassis cover.
The antenna and power connections should now be made.
Turn the power switch to the ''on" position, the function switch
to television, the picture control counterclockwise and the
brightness control clockwise until a glow appears on the
screen.
Adjust the electrical focus control R331 on the horizontal de¬
flection chassis until the raster lines are in sharpest focus as
seen when looking down into the barrel. If necessary, reduce
the brilliance control setting, and readjust the focus control.
Pull the dust cover down around the optical barrel.
Adjust the optical focus adjustment until the raster lines are
in focus on the screen. Turn the deflection yoke until the
raster lines are horizontal on the screen and tighten the yoke
clamp in this position.
Picture Adjustments—It will now be necessary to obtain a test
pattern picture in order to make further adjustments. See step
3 through step 10 of the receiver operating instructions on
page 4.
CHECK OF HORIZONTAL OSCILLATOR ALIGNMENT—The
sync link (see Figure 7) must be in the normal position (2 to
3). Turn the horizontal hold control to the extreme counter¬
clockwise position. The picture should remain in horizontal
sync. Momentarily remove the signal by turning the picture
control fully counterclockwise and then returning it to the op¬
erating position. Normally the picture will pull into sync.
Turn the horizontal hold control to the extreme clockwise po¬
sition. The picture should remain in sync. Momentarily remove
the signal. Again the. picture should normally pull into sync.
If the receiver passes the above checks and the picture is
normal and stable, the horizontal oscillator is properly aligned.
Sk f p "Alignment of Horizontal Oscillator" and proceed with
HEIGHT AND VERTICAL LINEARITY ADJUSTMENTS.
ALIGNMENT OF HORIZONTAL OSCILLATOR—If in the above
check the receiver failed to hold sync with the hold control
at either extreme or failed to pull into sync after momentary
removals of the signal, make the adjustments under "Slight
Retouching Adjustments." If, after making these retouching
adjustments, the receiver fails to pass the above checks or
if the horizontal oscillator is completely out of adjustment,
then make the adjustments under "Complete Realignment."
Slight Retouching Adjustments—Tune in a Television Station
and adjust the fine tuning control for best sound quality. Sync
the picture and adjust the picture control for slightly less
than normal contrast. Turn the horizontal hold control to the
extreme position in which the oscillator fails to hold or to
pull in. Momentarily remove the signal. Turn the T301 fre¬
quency adjustment on the chassis rear apron until the oscil¬
lator pulls into sync. Check hold and pull-in for the other
extreme position of the hold control.
Complete Realignment—Tune in a Television Station and adjust
the fine tuning contiol for best sound quality.
With the sync link in the normal position (2-3), turn the T301
frequency adjustment (on rear apron), until the picture is syn¬
chronized. (If the picture is not synchronized vertically, adjust
the vertical hold.) Adjust the picture control so that the pic¬
ture is somewhat below average contrast level.
I urn the T301 phase adjustment screw (under chassis, see Fig¬
ure 23) until the blanking bar, which may appear in the pic¬
ture, moves to the right and off the raster. The range of this
adjustment is such that it is possible to hit an unstable condi-
ion (ripples in the raster). The screw must be turned clock¬
wise from the unstable position. The length of stud beyond
the bushing in its correct position is usually about Vz inch.
Turn horizontal hold to extreme counterclockwise position.
Turn T301 frequency adjustment clockwise until the picture
falls out of sync. Then turn it slowly counterclockwise to the
point where the picture falls in sync again.
Readjust T301 phase adjustment so that the left side of the
picture is close to the left side of the raster, but does not begin
to fold over.
Turn horizontal hold to extreme clockwise. The right side of
the picture should be close to the right side of the raster, but
should not begin to fold over. If it does, readjust the phase.
Momentarily remove the signal. When the signal is restored,
the picture should fall in sync. If it doesn't, turn T301 fre¬
quency adjustment counterclockwise until the picture falls in
sync.
Turn horizontal hold to extreme counterclockwise position. Re¬
move the signal momentarily. When signal is restored, the
picture should fall in sync.
NOTE: If the picture does not pull in sync after momentary
removals of signal in both extreme positions of horizontal hold,
the pull-in range may be inadequate, though not necessarily.
A pull-in through % of the hold control range may still be sat¬
isfactory.
There is a difference between the pull-in range and hold-in
range of frequencies. Once in sync, the circuit will hold
about 50% to 100% more variation in frequency than it can
pull in. The range of the horizontal hold control is only ap¬
proximately equal to the pull-in range, considerable variation
may be found due to variations in the cut-off characteristic
of the horizontal oscillator control tubes, V303.
Excessive pull-in is objectionable because the higher sensi¬
tivity of the control circuits means also greater susceptibility
to noise, and to the vertical sync and equalizing pulses which
tend to cause a bend in the upper part of the raster. This
effect is more noticeable when the sync link is in the 1-2 po¬
sition.
Now that a picture has been obtained we may proceed with
the picture adjustments.
Adjust the electrical and optical focusing adjustments for
maximum definition in the vertical wedge of the test pattern.
HEIGHT AND VERTICAL LINEARITY ADJUSTMENTS- • Adjust
the height control (R149 on r-f, i-f chassis rear apron) until the
picture fills the screen vertically. Adjust vertical linearity
(R175 on rear apron), until the test pattern is symmetrical from
top to bottom. Adjustment of either control will require a re¬
adjustment of the other. Adjust vertical centering to align the
picture with the mask. In some cases it may be necessary to
shift the position of the kinescope in the holder (see Figure 3)
in order to obtain proper centering of the picture.
8

INSTALLATION INSTRUCTIONS
303
VERTICAL HEIGHT VERTICAL
LINEARITY CONTROL CENTERING
Figure 6 — R-F, I-F Rear Chassis Adjustments
WIDTH AND HORIZONTAL LINEARITY ADJUSTMENTS—Turn
the horizontal drive, R340, clockwise as far as possible without
causing crowding of the right of the picture. Adjust the hori¬
zontal linearity control, R351, until the test pattern is sym¬
metrical left to right. A slight readjustment of the horizontal
drive control may be necessary when the linearity control is
used. Adjust the width control, L302, until the picture just fills
the screen horizontally. Adjust horizontal centering to align the
picture with the mask. In some cases it may be necessary to
shift the position of the kinescope in the holder in order to
obtain proper centering of the picture.
Do not turn the horizontal drive control beyond approximately
/& of its maximum clockwise position. To do so may cause the
output stage to oscillate and result in the loss of horizontal sync.
Figure 7—Horizontal Deflection Chassis Adjustments
FOCUS—Adjust the focus control for maximum definition in
the test pattern vertical "wedge." Adjust the optical focus
adjustment for best overall focus on the screen.
Tighten all yoke and optical barrel lock screws.
Pull the dust cover down around the top of the optical barrel
and tie it securely in place. Tie the cable sleeve tight around
the leads. These precautions are very important for if dust is
permitted to enter and settle on the corrector lens, the optical
efficiency of the system will be greatly impaired.
CHECK OF R-F OSCILLATOR ADJUSTMENTS—Tune in all
available television stations to see if the receiver r'-f oscillator
is adjusted to the proper frequency on all channels. If adjust¬
ments are required, these should be made by the method
outlined in the alignment procedure on page 22 • The adjust-
848PTK, 648P¥
ments for channels 1 through 5 and 7 through 12 are available
from the front of the cabinet by removing the station selector
escutcheon as shown in Figure 8. Adjustments for channels 6
and 13 are under the chassis. See Figure 17.
Figure 8 — R-F Oscillator Adjustments
VIDEO PEAKING SWITCH—A video peaking switch is pro¬
vided (see Figure 6) to permit changing the video response.
Normally the switch should be left open. However, if the pic¬
tures from the majority of stations look better with the switch,
closed, the switch should be closed. If transients are produced
on high contrast pictures, the switch should be left open.
ANTENNA TRAP—A series resonant trap across the r-f ampli¬
fier grid circuit is provided to eliminate interference from an
FM station on the image frequency of a television station or
interference on channel 6 from a station on channel 10 or on
channel 5 from a station on channel 7. To adjust the trap in
the field, tune in the station on which the interference is
observed. Tune both cores of the trap for minimum interference
in the picture. See Figure 16 for the location of the trap. Keep
both cores approximately the same by visual inspection. Then,
turn one core V 2 turn from the original position and repeak the
second for maximum rejection. Repeat this process until the
best rejection is obtained.
RADIO OPERATION—Turn the receiver function switch to AM
and FM positions and check the radio for proper operation.
PUSH-BUTTON ADJUSTMENT—To adjust the radio push but¬
tons, set the function switch to the broadcast band position,
tune the receiver to the desired station. Adjust the push but¬
tons as instructed on page 81 .
Figure 9 — Push-Button Adjustments
Select the proper station call letter tab, moisten the back of
it and insert in the appropriate recess in the push button bezel.
Place the tab cellophane cover in the recess over the tab.
Replace the cabinet back. Make sure the screws which hold
the back in place are tight, otherwise it may rattle or buzz
when the receiver is operating at high volume.

304
648PTK , 648PV INSTALLATION INSTRUCTION TABLE
The following table is provided as a check-off list for use when installing the receivers.
Step
Nc.
Proceed as Indicated
1
Remove front of shipping carton.
■
Slide cabinet out of carton.
3
Remove cabinet back.
■
Take off two nuts inside cabinfei and remove cab¬
inet from skid.
■
Unpack yokes, knobs, anode clip, and kinescope
holder ball head screws.
6
Remove shipping materials.
■
Remove radio brackets.
8
Remove shipping tapes.
9 Install control knobs.
10 Make sure all tubes are firmly seated in their
sockets.
11 Remove optical barrel dust cover.
12
Remove corrector lens and warning label.
n
Clean screen and mirrors.
n
Insert test lamp in kinescope holder.
15
Replace corrector lens, cover center hole.
16
Misadjust optical focus.
17
Check optical, horizontal and lateral centering.
m
Adjust centering if necessary.
19
Adjust corrector lens centering if necessary.
■
Refocus.
H
If focus is uneven, adjust optical barrel tilt.
22
Repeat steps 17 through 21 if necessary to obtain
proper resolution.
23 Remove corrector lens.
24
Remove test lamp.
25
Unpack and clean kinescope.
H
Insert kinescope in kinescope holder.
27
Clean and replace corrector lens.
Step
No. Proceed as Indicated
H
Install deflection yoke, connect cables and kinescope
socket.
B
Check all chassis interconnecting cables.
Remove high voltage capacitors shipping strings.
B
Connect receiver to an a-c line and antenna.
32
Turn receiver on, function switch to Tel.
33
Tune in station per Operating Instructions, steps 3
through 10.
34 Adjust electrical and optical focus control.
35
Check horizontal oscillator for hold and pull-in with
horizontal hold control at each extreme.
36 Align horizontal oscillator (T301) if necessary.
37 Rotate yoke for horizontal pattern, tighten.
38
Adjust height and vertical linearity and vertical
centering controls.
39
Adjust width, horizontal drive, linearity and hori¬
zontal centering controls.
40
Adjust focus control R331 for max definition of ver¬
tical wedge and optical focus adjustment for best
overall focus.
41
MAKE SURE ALL OPTICAL ADJUSTMENT LOCKS
ARE TIGHT.
B
Replace optical barrel dust cover.
B
Check r-f oscillator frequency on all channels.
44 Observe picture from all available stations.
45 Set video peaking switch S101
46
Check radio for operation on BC, SW, and FM
bands.
47
Set push buttons.
48
Adjust antenna trap.
U
Insert station call letter tabs in push button es¬
cutcheon.
50
Replace cabinet back.

INSTALLATION INSTRUCTIONS
305
RECEIVER LOCATION—The owner should be advised of the
Importance of placing the receiver in the proper location in
the room.
The location should be chosen—•
—Away from bright windows and so that no bright light
will fall directly on the screen. (Some illumination in
the room is desirable, however.)
—To give easy access for operation and comfortable
viewing.
—To permit convenient connection to the antenna.
—Convenient to an electrical outlet.
—To allow adequate ventilation.
VENTILATION CAUTION—The receiver is provided with ade¬
quate ventilation holes in the bottom and back of the cabinet.
Care should be taken not to allow these holes to be covered
or ventilation to be impeded in any way.
If the receiver is to be operated with the back of the cabinet
near a wall, at least a two-inch clearance should be main¬
tained between cabinet and wall.
ANTENNAS—The finest television receiver built may be said
to be only as good as the antenna design and installation. It
is therefore important to use a correctly designed antenna,
and to use care in its installation.
RCA Television Antennas, stock #225 and #226 are designed
for reception on all thirteen television channels. These an¬
tennas use the 300-ohm RCA "Bright Picture" television trans¬
mission line. Installation personnel are cautioned not to make
any changes in the antenna or substitute other types of trans¬
mission line as such changes may result in unsatisfactory pic¬
ture reproduction.
The stock #226 antenna is bi-directional on channels one
through six (44 to 88 Me). When used on these channels, the
maximum signal is obtained when the antenna rods are broad¬
side toward the transmitting antenna.
The stock #225 antenna with reflector is uni-directional on
channels one through six. When used on these channels, the
maximum signal is obtained when the antenna rods are broad¬
side toward the transmitting antenna, with the antenna ele¬
ment beween the reflector and the transmitting antenna.
When operated on channels seven through thirteen, (174 to
216 Me), both types of antennas have side lobes. On these
channels, the maximum signal will be obtained when the
antenna is rotated approximately 35 degrees in either direc¬
tion from its broadside position toward the transmitting an¬
tenna.
In general, the stock #225 antenna should be used if re¬
flections are encountered, if the signal strength is weak, or if
the receiving location is noisy. If these conditions are not en¬
countered, the stock #226 antenna will probably be satisfac-
lory.
In some cases, the antenna should not be installed permanently
until the quality of the picture reception has been observed on
a television receiver. A temporary transmission line can be
run between receiver and the antenna, allowing sufficient
648PTK, 648PV
slack to permit moving the antenna. Then, with a telephone
system connecting an observer at the receiver and an assistant
at the antenna, the antenna can be positioned to give th®
most satisfactory results on the received signal. A shift of
direction or a few feet in antenna position may effect a tre¬
mendous difference in picture reception.
REFLECTIONS—Multiple images, sometimes known as echoes
or ghosts, are caused by the signal arriving at the antenna
by two or more routes. The second or subsequent image oc¬
curs when a signal arrives at the antenna after being re¬
flected off a building, a hill or other object. In severe cases
of reflections, even the sound may be distorted. In less sever®
cases, reflections may occur that are not noticeable as re¬
flections but that will instead cause a loss of definition in the
picture.
Depending upon the circumstances, it may be possible to elim¬
inate the reflections by rotating the antenna or by moving It
to a new location. In extreme cases, it may be impossible to
eliminate the reflection.
Under certain extremely unusual conditions, it may be possible
to rotate or position the antenna so it receives the cleanest
picture over a reflected path. If such is the case, the antenna
should be so positioned. However, such a position may give
variable results as the nature of reflecting surfaces may vary
with weather conditions. Wet surfaces have been known to
have different reflecting characteristics than dry surfaces.
INTERFERENCE—Auto ignition, street cars, electrical machin¬
ery and diathermy apparatus may cause interference which
spoils the picture. Whenever possible, the antenna location
should be removed as far as possible from highways, hos¬
pitals, doctors' offices and similar sources of interference. In
mounting the antenna, care must be taken to keep the antenna
rods at least Va wave length (at least 6 feet) away from other
antennas, metal roofs, gutters or other metal objects.
Short-wave radio transmitting and receiving equipment may
cause interference in the picture in the form of moving ripples.
In some instances it may be possible to eliminate the inter¬
ference by the use of a trap in the antenna transmission line.
However, if the interfering signal is on the same frequency as
the television station, a trap will provide no improvement.
WEAK PICTURE—When the installation is near the limit of the
area served by the transmitting station, the picture may be
speckled, having a "snow" effect, and may not hold steady on
the screen. This condition is due to lack of signal strength
from the transmitter.
LIGHTNING ARRESTOR—The lightning arrestor contained in
the antenna kit should be installed in accordance with the
instructions. The mast used to mount tho antenna should be
provided with a direct ground.
INFORMATION REFERENCES—In short, a television receiving
antenna and its installation must conform to much higher
standards than an antenna for reception of International Short
Wave and Standard Broadcast signals. For further informa¬
tion on antennas and antenna installation see the RCA Booklet
entitled "Practical Television by RCA," and also the specific
instructions accompanying the RCA Television Antenna,
11

306
848PTK, 648P¥ TELEVISION CIRCUIT DESCRIPTION
It is advisable that the reader be familiar with a recent stand¬
ard textbook of television principles in order to understand
the receiver circuits and their functions. Such knowledge is
assumed for the purpose of this publication. The discussions
which follow will not dwell on the operation of conventional
circuits used which have been used in previous receivers and
which should be well known. In general, the circuits dis¬
cussed will be only those that are new to the field.
For ease of understanding the basic operation of the television
receiver, a 14 unit block diagram of it is shown in Figure 10.
The circuit description will follow the numerical order of these
blocks in order to follow a signal through the set in a logical
manner.
R-F UNIT (block #1)—The r-f unit is a separate subchassis of
the receiver. On this subchassis are the r-f amplifier, converter,
oscillator, fine tuning control, channel switch, converter trans¬
former, r-f, converter and oscillator coils and all their tuning
adjustments. The unit provides operation on all thirteen of
the present television channels. It functions to select the
desired picture and sound carriers, amplifies and converts to
provide at the converter plate, a picture i-f carrier frequency
of 25,75 me. and a sound i-f carrier of 21.25 me.
R-F Amplifier—Referring to the schematic diagram (page 59),
T1 is a center tapped coil used for the short circuiting of low
frequency signals picked up by the antenna which would
otherwise be directly applied to the control grids of the 6J6
r-f amplifier, VI. Cl and C2 are antenna isolating capacitors.
The d-c return for the grids of VI is through R3 and R13 which
also serve to terminate the 300 ohm antenna transmission line.
C3 and C4 are neutralizing capacitors necessary to counteract
the grid to plate capacitance of the triode r-f amplifier.
In the plate circuit of the r-f amplifier are a series of induct¬
ances LI to L25 and L2 to L26 inclusive. These inductances
may be considered as a quarter wave section of a balanced
transmission line which can be tuned over a band of frequen¬
cies by moving a shorting bar along the parallel conductors.
Adjustable coils 25 and L26 provide the correct length of line
for the thirteenth channel, 210—216 me. L13 to L23 and L14
to L24 are fixed sections of line which are added to L25 and
L26 as the shorting bar is moved progressively down the line.
The physical construction of each one of these inductances is
a small non-adjustable silver strap between the switch con¬
tacts. Each strap is cut to represent a six-megacycle change
in frequency. In order to make the jump between the lowest
high frequency channel (174-180 me) and the highest low fre¬
quency channel (82-88 me), adjustable coils Lll and L12 are
inserted. To provide for the remaining five low frequency
channels, LI to L9 and L2 to L10 are progressively switched
in to add the necessary additional inductance.
Coils LI to L9 and L2 to L10 are unusual in that they are
wound in figure 8 fashion on fingers protruding from the
switch wafer. This winding form produces a relatively non-
critical coil since the coupling between turns is minimized. A
maximum amount of wire is used for the small inductance
which is required, thus permitting greater accuracy in manu¬
facturing.
Figure 10—Television Receiver Block Diagram
12

307
TELEVISION CIRCUIT DESCRIPTION 648PTK, I4SPV
Converter—The converter grid line operates in a similar man¬
ner and is so arranged on the switch to provide coupling
between it and the r-f line. CIO, C12, C13 and a link, provide
additional coupling which is arranged to produce at least a
4.5 megacycle band pass on each of the channels.
L80' and C14 form a series resonant circuit used to prevent
i-f feedback in the converter by grounding its grids for i-f
frequency. They also act as a trap to reject short-wave signals
of i-f frequency which arrive at the converter grids in a push
push manner.
A 6J6 twin triode is used as converter. Since the grids are fed
in push pull by both the signal and the oscillator, the hetero¬
dyne products (i-f signals) are in phase on the converter plates
so the two plates are connected in parallel. Unwanted sig¬
nals 'of i-f frequency that arrive at the converter grid in a
push pull manner are out of phase on the converter plates.
Since the plates are tied together, these signals tend to cancel
thus reducing the possibility of interference from this source.
R-F Oscillator—The oscillator line is similar except that trimmer
adjustments are provided for each channel and the low fre¬
quency coils are not figure 8 windings. For tuning each chan¬
nel, brass screws are used in close proximity to the high
frequency tuning straps L66 to L76, and adjustable brass cores
are provided for coils L54 to L62. It is obvious that the high
frequency adjustments should be made before each lower fre¬
quency one.
C15 is a fine tuning adjustment which provides approximately
plus or minus 800 kc. variation of oscillator frequency on
channel 1 and approximately plus or minus 1.9 me. on channel
The physical location of the oscillator line with respect to the
converter grid line is such as to provide some coupling to the
converter grids. This coupling is augmented by the link shown
on the schematic and provides a reasonably uniform oscillator
voltage at the converter grids over the entire tuning range
of the unit.
The converter transformer T2 is a combination picture i-f trans¬
former, sound trap, and sound i-f transformer. The converter
plate coil is assembled within the structure of a high Q reso¬
nant circuit tuned to the sound i-f frequency. This high Q coil
absorbs the sound i-f component from the primary. Thus on
the T2 primary (from which the picture i-f is fed), the sound
carrier is attenuated with relation to the picture channel.
SOUND I-F AMPLIFIES AND DISCRIMINATOR (block #2)—A
portion of the energy absorbed by the T2 trap circuit is fed to
the first sound i-f amplifier. Three stages of amplification are
used to provide adequate sensitivity. A conventional dis¬
criminator is used to demodulate the signal. The discriminator
band width is approximately 350 kc. between peaks.
The output from the discriminator is fed into the radio audio
syst'-m and. is controlled by the radio volume and tone con¬
trols.
PICTURE I-F AMPLIFIER AND DETECTOR (block #3)—The
picture i-f amplifier departs considerably from the conventional
coupled system. To obtain the necessary wide band char¬
acteristic with adequate gain, four stages of i-f amplification
are employed. The converter plate and each successive i-f
transformer utilize one tuned circuit each and each is tuned to
a different frequency. The effective Q of each coil is fixed
by the shunt plate load or grid resistor so that the response
product of the total number of stages produces the desired
overall responsive curve. Figure II shows the relative gains
and selectivities of each coil and the shape of the curve of the
quintuple combination.
Figure 11-—Stagger Tuned I-F Response
In order to obtain this band pass characteristic, the picture i-f
transformers are tuned as follows:
Converter transformer . 21.8 me. (T2 primary)
First pix i-f transformer . 25.3 me. (T104 primary)
Second pix i-f transformer . 22.3 me. (T105 primary)
Third pix i-f coil . 25.2 me. (L104)
Fourth pix i-f coil . 23.4 me. (L106)
In such a stagger tuned system variations of individual i-f am¬
plifier iube gain do not affect the shape of the overall i-f re¬
sponse curve if the Q's and center frequencies of the stages re¬
main unchanged. This means that the i-f amplifier tubes are
non-critical in replacement because variations in Gm do not
affect the response curve.
To align the i-f system, the transformers are peaked to the
specified frequencies with a signal generator. The overall
i-f response is then observed by use of a sweep generator and
oscilloscope. Slight deviations from design center circuit Q
are compensated for with slight shifts in tuned-circuit center
frequency until the desired response curve is obtained. If this
response cannot be obtained, the difficulty is likely to be in a
component that affects either the frequency or Q of one or
more of the i-f coils.
The response curve does shift slightly as the picture control is
varied due to the Miller effect. This effect is the change in
tube input capacitance as its gain is varied by grid bias
changes. The change of input capacitance causes a slight de¬
tuning of the preceding i-f coil and a small shift in response
curve shape. This effect is slight, however, and when the
receiver is aligned with the specified grid bias, no difficulty
from this source should be encountered.
For familiarization with the frequencies which are important
in the receiver's operation, Figure 12 shows the relative posi¬
tion of the picture and sound carriers for channels 2, 3 and 4.
If a station on channel 3 is transmitting a picture with video
frequencies up to 4 me., the picture carrier will have upper side
band frequencies up to 65.25 me. The lower side bands are
suppressed at the transmitter.
l
1
r*-CMANNEL t -
! I
| 9
t i
=4®-— CWANMSL 3-
1
J
— CWAMWBL A -
j 54 - 60 MC
i
i 60 -66 MC
i
|
S6 -72 MC
!
i
5 9.75MC
1 65.75 MC
j
vi.rsfiwe
j
■ SOUND
l SOUND
9
SOUND
\
] CARRIER
j CfeSKISR
1
CARRIE®
j
! 55.25 MC
! 61.25 MC
1
€1.25 MC
i
, PIX
? PiX
j
P IX
j C&RRIEK
*
5
1
9
i
CARRIES
i
54- SO 66 72
MC MC MC MC
Figure 12—Television Channel Frequencies
13

308
TELEVISION CIRCUIT DESCRIPTION
648PTK, 648PV
With th© receiver r-f oscillator operating at a higher frequency
than the received channel, the i-f frequency relation of picture
to sound carrier is reversed as shown in Figure 13.
Figure 13—Overall Picture I-F Response
Traps—Since it is necessary for the picture i-f to pass fre¬
quencies quite close to the sound carrier frequency, the sound
carrier would produce interference in the picture. In order
to prevent this interference, traps must be added to the picture
i-f amplifier to attenuate the sound carrier. If the receiver
should be operating on channel 3, it is possible that inter¬
ference would be experienced from the channel 2 sound carrier
and the channel 4 picture carrier. The adjacent channel traps
are provided to attenuate these unwanted frequencies.
The first three traps are absorption circuits. The first trap (T2
secondary) is tuned to the accompanying sound i-f frequency.
The second trap (T104 secondary) is tuned to the adjacent
channel sound frequency. The third trap (T105 secondary) is
tuned to the adjacent channel picture carrier frequency. The
fourth trap (T106 secondary) is in the cathode circuit of the
fourth picture i-f amplifier VI11 and is tuned to the accom¬
panying sound carrier i-f frequency. The primary of T106 in
series with Cl37 forms a series resonant circuit at the fre¬
quency to which L106 is tuned (23.4 me.). This provides a low
impedance in the cathode circuit at this frequency and per¬
mits the tube to operate with a gain. However, at the resonant
frequency of the secondary (21.25 me.), a high impedance is
reflected into the cathode circuit, and the gain of the tube
for this frequency is reduced by degeneration. The rejection
at 21.25 me. with this circuit is limited to the gain of the tube.
Picture Second Detector—The detector is a conventional half
wave rectifier connected to produce a video signal of the
proper polarity.
PICTURE A-G-C DETECTOR, AMPLIFIER AND DIODE (block
#4)—An automatic gain control circuit is employed in con¬
nection with the picture i-f system to hold the output from the
i-f's substantially constant over a wide range of signal inputs.
The a-g-c system of the picture i-f amplifier (shown in Figure
14) differs considerably from the a-v-c system used in broad¬
cast receivers. In broadcast receivers, it is customary to use
the filtered d-c drop across the diode resistor as the source
of the a-v-c voltage. This is satisfactory, because the d-c
voltage thus obtained is directly proportional to the average
carrier amplitude at the diode. If it maintains the average
carrier amplitude substantially constant, then the a-v-c oper¬
ates as it should.
In the transmission of television pictures, however, the aver¬
age carrier amplitude varies greatly with picture content, and
an a-g-c system operating on the principle of maintaining a
substantially uniform average carrier amplitude therefore is
not suitable.
The RMA Standard Television Signal calls for a transmission
system known as d-c negative transmission. Under this sys¬
tem, the carrier always reaches a uniform maximum ampli¬
tude during the periods when synchronizing pulses are being
transmitted, and a white portion of the scene is represented by
minimum or zero carrier condition. Thus, if there is no fading,
the peaks of the synchronizing pulses will always represent
some constant amplitude, and they, therefore, form a conveni¬
ent reference for operating a satisfactory picture a-g-c system.
A portion of the output from the fourth i-f amplifier is fed into
V105A, the a-g-c detector. Since the time constant of the diode
load resistor and filter (RMS and C153) is somewhat greater
than one horizontal line, the detector is essentially a peak
reading voltmeter at sync frequency (15,750 cps). The d-c
voltage that appears on the cathode of VI05A is therefore pro¬
portional to the peak strength of the received signal and sub¬
stantially independent of the picture content.
Such a system will also tend to read the peak of noise pulses.
To prevent this, R151 and the diodes of V106 are used as a
two-stage clipper or noise-limiting network. For further pro¬
tection against noise, the d-c output is fed through an integrat¬
ing network (R157 and C158) which tends to remove the effects
due to random noise.
The d-c output from the integrator is less than that required to
control the gain, and since it increases in the positive direc¬
tion with increases in signal strength, it is necessary to am-
Figure 14—Picture A-G-C Circuit
plify and "invert." To accomplish this, the output from the
integrator is d-c coupled to the VI06 a-g-c amplifier grid,
V106 is operated with approximately minus one hundred and
ten volts on the cathode and the plate at or slightly below
ground potential. The voltage available from the plate is suit¬
able for use as a control bias.
With a weak signal input, the bias on V106 (obtained CSCTOiiS
R152 and R158) is sufficient to cause the V106 plate current to
be nearly cut off. The VI06 plate is at approximately ground
potential, no bias is applied to the r-f and i-f grids and the
receiver operates at maximum gain. When a strong signal is
applied to the receiver, the d-c output from the a-g-c detector
opposes the fixed bias on V106 and causes more plate current
to flow. As a consequence, the plate goes negative with re¬
spect to ground and this negative voltage is applied to the
r-f and i-f grids reducing gain and maintaining constant output
from the i-f system.
Since the grid control characteristic of the pentode i-f ampli¬
fiers is different from that of the triode r-f amplifier, different
bias voltages are required and must be taken from different
points in the system.
Also, in order to obtain the maximum signal to noise ratio
from the receiver, it is desirable to allow the r-f amplifier to
run essentially at full gain on any signal which will not cause
14

309
TELEVISION CIRCUIT DESCRIPTION 848PII, 648PV
overloading of the first i-f stage. The circuit arrangement of
Figure 14 including the a-g-c diode (V107A) permits maximum
use of r-f gain on weak signals and prevents overloading of
the i-f amplifier on strong signals.
With an input signal of 1000 microvolts (and the picture control
set for normal contrast) the V106 plate is at approx. —2 volts.
Since the a-g-c diode plate is placed at approx, a -2.5 volt
tap on the dividers R193 and R194, the diode does not con¬
duct and the -2 volts on the V106 plate is applied to the i-f
grids. With a signal of 10,000 microvolts, the a-g-c amplifier
plate is at approx. -5 volts. Under this condition, the a-g-c
diode conducts and due to the drop in R165, prevents the i-f
bias from rising appreciably above approx. -3 volts. The r-f
bias, however, is not limited and can therefore rise above the
i-f bias.
Noise Saturation Circuit—Since the synchronizing pulse is
blacker than black" and "black" information must drive the
kinescope grid toward cut-off, the video signal polarity must
be such that the sync is negative when applied to the kine¬
scope grid. It is obvious that for the two-stage video amplifier
used, the sync pulse from the second detector must also be
negative at the first video amplifier grid. The first stage is
designed so that with a normal signal input level at its grid,
the tube will be working over most of its operating range.
Any large noise signal above sync will drive the grid to
cut-off and the noise will be limited. In effect, the signal to
noise ratio is thus improved.
D-C Restorer—Since the video amplifier is an a-c amplifier,
the d-c component of the video signal that represents the
average illumination of the original scene will not be passed.
This high value of bias on the r-f amplifier is necessary to
reduce the triode nearly to cut-off. Although triodes are not
generally considered to be remote cut-off tubes, sufficient cur¬
vature is present in the grid control characteristic to provide
approximately a ten to one reduction in gain when the bias
approaches the plate current cut-off point.
Figure 15 shows a graph of the r-f and i-f bias versus signal
input.
m
<
<0
0
-1
-2
-3
-4
-5
-6
-7
t 16 100 1000 10000 tooooo
SIGNAL INPUT ( yU. VOLTS) CV 44
Figure 15 —Bias versus Signal Input
Picture Control—A manual gain control is also provided since
it is necessary to vary the picture contrast because of varia¬
tions in room lighting, transmitting technique and to suit per¬
sonal preference in picture balance. The control varies the i-f
gain by varying the initial bias on the a-g-c amplifier which
in turn varies the r-f and i-f bias.
VIDEO AMPLIFIER AND D-C RESTORER (block #5)—The func¬
tion of this section of the receiver is to amplify the video output
of the second detector. Two amplifier stages are employed.
The gain from the first video grid to output plate is 30X and
the frequency response extends to 4 me.
The 648PTK is aligned to give a normal test pattern when re¬
ceiving a signal from a station v-employing standard RMA
vestigal side band transmission. If the station deviates from
this transmission characteristic, then a properly aligned re¬
ceiver may produce an output with an excessive amount of
low frequency video causing the picture to smear.
The 648PTK provides a back panel Video Peaking Switch S101
to modify the video response to compensate for the above
mentioned transmitter characteristic. S101 switches a 680
mmf. capacitor across the VI13 cathode resistor, R176. This
reduces the cathode degeneration for high frequencies and
thus increases the high video response. Closing the switch for
operation of the receiver on such a station will generally im¬
prove the good picture. However, if the receiver is then tuned
to a station with proper side band suppression, transients may
be produced on high contrast pictures such as test patterns.
Therefore, it must be determined at the time of installation, if
the video peaking switch S101 is to be open or closed
Unless this d-c component is restored, difficulty will be ex¬
perienced in maintaining proper scene illumination For any
given scene, this average illumination could be set properly
by the brightness control. However, a change of scene would
probably necessitate resetting this control. The d-c restorer
accomplishes this setting automatically thus assuring proper
picture illumination at all times. For a detailed explanation
of the operation of the d-c restorer, see "Practical Television
by RCA."
KINESCOPE AND REFLECTIVE OPTICAL SYSTEM (block #6)
—The picture tube employed is a 5TP4, a five inch projection
kinescope. The tube operates at approximately 27 kv and
employs magnetic deflection and electrostatic focusing. The
kinescope screen is backed by a microscopic aluminum film.
This coating is porous to the electron stream. However, it is
opaque to light and prevents radiation at the back of the
screen from reducing picture contrast by illuminating dark
areas of the picture. Instead, this light is reflected out the
front of the screen thus increasing the picture brilliance by
approximately two to one. The aluminum film also prevents
a negative charge from building up on the screen. Such a
charge tends to repel the electron beam thus reducing the ve¬
locity with which the beam strikes the screen with consequent
reduction of light output. The aluminum coating provides some
protection against screen burns produced by ions in the elec¬
tron stream. The thick screen employed in high voltage kine¬
scopes also prevents a burn on the back of the screen from
being visible on the outer surface of the screen.
The reflective optical system is employed to project the image
from the kinescope on to a large screen. The system consists
of the kinescope mounted above and facing a spherical mirror.
The spherical mirror reflects the light up through the cor¬
rector lens to a forty-five degree plane mirror which in turn re¬
flects the image on to the back of a translucent screen, as
shown in Figure 16.
The center section of the spherical mirror is painted black so
that the illumination which falls on this sector will not be re¬
flected back on to the face of the kinescope to reduce the pic¬
ture contrast by illuminating dark areas of the picture.
Since a large spherical mirror by itself will not produce an in
focus image, the corrector lens must be employed to bring the
image to focus at all points on the screen. The spherical mir¬
ror and the forty-five degree mirror are front surfaced mirrors
to prevent ghosts which would occur from reflections at the
surface of the glass of a rear surfaced mirror.
The screen is composed of two lucite sheets with a partial
diffusing layer between them. The back sheet has a fresnel
lens molded into its rear surface. The front sheet has vertical
ribs molded into its outer surface. The fresnel lens functions
to concentrate the light into a narrow viewing angle. The
vertical ribs act to increase the horizontal viewing angle above
that obtained with a flat surface. The diffusing layer is em¬
ployed to eliminate interference patterns between the fresnel
15

310
TELEVISION CIRCUIT DESCRIPTION
648PTK, 648PV
lens and the vertical ribs. The screen and lens combination
give a gain of approximately five over that which would be
obtained from a ground glass type screen. This gain is ob¬
tained at the expense of the illumination at extreme side, up¬
per or lower viewing angles. Since such extreme angles are
impractical due to foreshortening of the picture, no disad¬
vantage is achieved and the brilliance from practical viewing
angles is increased.
The leads from the deflection yoke and the kinescope socket
pass through the optical path directly above the corrector lens.
However, due to the fact that the light from any given point
on the kinescope passes through all points on the corrector
lens, as shown in Figure 16, the leads do not cast a shadow
on the picture, but instead reduce the optical efficiency of the
system by a very slight amount proportional to the percent¬
age of the corrector lens area blocked by the leads.
This reflective optical system has a resolution of approxi¬
mately 1500 lines and an efficiency equivalent to an F.8 lens.
Conventional projection optics of this speed for this size kine¬
scope and screen would be prohibitive from the standpoint of
cost and size.
The inside and outside of the flaring portion of the kinescope
bulb are given a metallic coating. The inner coating, which
is the second anode, is connected to the high voltage supply.
The outer coating is grounded by means of two small springs
on the deflection yoke support. The capacity between the two
coatings is used as a high voltage filter capacitor.
The vertical axis of the optical barrel is approximately. 7 de¬
grees off vertical and the 45 degree mirror is in reality ap¬
proximately 48 degrees from the horizontal as shown in Fig¬
ure 16. This arrangement is employed in order to permit
placing the barrel slightly forward of the mirror thus making
the optical system as compact as possible.
Figure 16—Reflective Optical System
SYNC AMPLIFIERS (block $7)—The function of this system is
to amplify the sync signal and effect the separation of sync
from the video.
First Sync Amplifier—The first sync amplifier VI14 is a 6SK7
which has a remote cut-off characteristic. The signal from the
d-c restorer is fed into this amplifier with the polarity such that
the sync is in the negative direction. Noise pulses above sync
that remain after the limiting action of the first video grid are
thus further compressed and the sync to noise ratio is again
improved.
Second Sync Amplifier—The sync at the grid of VI15, the
second sync amplifier grid is positive in polarity. The op¬
erating voltages applied to the grid, screen and plate, are
such that the negative portion of the applied signal is cut off.
Thus, the video and blanking pulses are removed and only
the sync pulses appear at the plate.
Third Sync Amplifier—The sync pulses appearing at the third
sync amplifier (VI16), grid are negative in polarity and must
be inverted before they can be injected into the sweep oscil¬
lators. The signal at the VI16 grid is sufficient to drive the
tube beyond cut-off and the signal is again clipped. This final
clipping removes all amplitude variations between sync pulses
due to noise, hum, etc., and it appears with the correct po¬
larity at the plate.
Integrating Network—The purpose of this network is to sepa¬
rate the horizontal from the vertical sync and to pass the
vertical to the vertical oscillator.
Since the horizontal sync pulse is of short duration (5 micro¬
seconds) and the vertical pulse is of much longer duration
(190 microseconds), they can be separated by an r-c filter
which is responsive to wave shape. The integrating network
which is such a filter is composed of R142, R143, R144, C148,
C149 and C150. In operation it can be considered to be a low-
pass filter which by-passes the narrow or high frequency
horizontal sync but passes the broad or low frequency vertical
sync.
VERTICAL OSCILLATOR DISCHARGE AND OUTPUT (block
#8)—The function of these circuits is to provide a sawtooth
of current of the proper frequency and phase to perform the
vertical scanning for the kinescope. To produce such a current
in the vertical deflection coil, a somewhat differently shaped
voltage wave is required.
Since the vertical trace is slow, requiring approximately 16,000
microseconds, and the vertical deflection coil inductance is
small, approximately 50 millihenries, the majority of the volt¬
age across the coil during trace is across its resistive com¬
ponent. In order to produce a linear change of current through
a resistance, a linear change of voltage is necessary. Retrace,
however, must be accomplished within the 666 microsecond
vertical blanking time and therefore requires a much faster
rate of change of current through the coil. During this time,
the effect of the inductance of the coil becomes appreciable
because of the required fast rate of change of current. It is
therefore necessary to apply a large pulse of voltage across
the coil in order to obtain rapid retrace. The composite wave¬
form required to produce a sawtooth of current in the coil is a
sawtooth of voltage with a sharp pulse as shown in Figure
17D. VI17 and VI18 supply such a voltage.
Vertical Oscillator and Discharge—A single 6J5 triode, VI17,
with its associated components form a blocking oscillator and
discharge circuit. The wave form of the voltage at the control
grid of this tube with respect to time, is a small, positive surge
followed by a large negative drop which returns to the posi¬
tive condition at a relatively slow rate as shown in Figure 17A.
During the negative part of the cycle, the grid is beyond cut¬
-off and the discharge capacitor, Cl60, charges through re¬
sistors R148 and R149. When the grid reaches a voltage that
permits plate to cathode conduction, Cl 60 discharges through
T107 secondary and VI17. The discharge current of Cl60
builds up a magnetic field in T107 that in turn induces a posi¬
tive voltage at the grid of VI17. This positive voltage on the
VI17 grid lowers the plate resistance of the tube and allows
C160 to discharge more rapidly. This process builds up very
rapidly until Cl60 is nearly discharged. The magnetic field
in T107 then collapses and drives the VI17 grid negative. The
charge placed on Cl55 due to grid conduction during the posi¬
tive pulse now holds the grid negative. As the charge on
C155 leaks off through R155 and R156, the grid slowly be¬
comes less negative and approaches the point which will al-
16

311
TELEVISION CIRCUIT DESCRIPTION
low plate to cathode conduction. Just before the conduction
point is reached, the BO cycle vertical synchronizing pulse
from the integrating network is applied to the ¥117 grid. This
pulse is sufficient to drive the tube to conduction and the
process is repeated. In this manner, the incoming sync main¬
tains control of vertical scanning.
648PTS, 648PV
HORIZONTAL SYNC DISCRIMINATOR, HORIZONTAL OSCIL¬
LATOR AND OSCILLATOR CONTROL (block #9, 10 and 11)
These circuits are a radical departure from the conventional
systems used for framing the picture in the horizontal direc¬
tion. Their features are ease of operation, stability and good
noise immunity.
On the plate of VI17, a sawtooth of voltage appears due to
the slow charging and rapid discharging of C160. A sharp
negative pulse also occurs during the discharge period. See
Figure 17B. This pulse appears because of the action of R164
and C160, an action which is known as peaking. When VI17
is conducting, the plate voltage drops nearly to cathode po¬
tential. C160 discharges during this time. However, since the
conduction time is short, C160 cannot be completely discharged
due to the time constant of R164 in series with C160. When
VI17 becomes non-conducting, the plate voltage does not
have to rise slowly from cathode potential but instead rises
immediately to an appreciable value due to the charge that
remains on C160. The plate voltage then slowly rises from
this value as C160 charges through R148 and R149. Ad¬
justment of the height control R149 varies the amplitude of
the sawtooth voltage on VI17 plate by controlling the rate
at which Cl60 can charge.
The voltage present on the VI17 plate is of the shape re¬
quired to produce a sawtooth of current in the vertical de¬
flection coil. It is now necessary to amplify it in a tube
capable of supplying a sufficient amount of power.
Vertical Output—A 6K6GT is connected as a triode for the
output stage, VI18. The vertical output transformer T108
matches the resistance of the vertical deflection coils to the
plate impedance of the 6K6GT.
Vertical Linearity Control—R175 is provided as a vertical
sweep linearity control. Since the grid-voltage, plate-current
curve of VI18 is not a straight line over its entire range, the
effect of adjustments of R175 is to produce slight variations in
the shape of the sawtooth by shifting the operating point of
the tube to different points along the curve.
Since the slope of the curve varies at these different points
and thus varies the effective gain of the tube, it is apparent
that adjustments of, linearity affect picture height and that such
adjustments must be accompanied by readjustments of the
height control R149. Adjustments of the height control affect
the shape of the sawtooth voltage on the VI17 plate so that
adjustments of height must be accompanied by readjustments
of linearity.
VERTICAL OSCILLATOR I
I
I
VERTICAL DEFLECTING COIL
CURRENT
IM THE
COIL
- \ _ N |
I\ C
\
voltage
across
the
- \ -
D
€08 L
--
MS-E25
Figure 17-—Vertical Sweep Waveforms
HORIZONTAL OSCILLATOR (block # 10)—The horizontal oscil¬
lator is an extremely stable Hartley oscillator operating at the
scanning frequency 15,750 cps. The primary of T301 (ter¬
minals A, B and C) is the oscillator coil. This coil is closely
coupled to the secondary winding (terminals D, E and F) and
thus feeds a sine wave voltage to V301.
HORIZONTAL SYNC DISCRIMINATOR (block #9)—The sync
discriminator, V301, is a 6H6 dual diode in a circuit which
produces a d-c output voltage proportional to the phase dis¬
placement between the incoming sync pulses and the sine
wave horizontal oscillator voltage.
The sine wave oscillator voltages applied to the plates of V301
are equal in amplitude and opposite in phase. The syn¬
chronizing pulses from the third sync amplifier are fed
through a small capacitor (C301) to attenuate the vertical sync
and then applied to the center tap of T301. The horizontal sync
pulses thus appear in phase and of equal amplitude on the
diode plates as shown in Figure 18. When the pulse and sine
wave from the oscillator are properly phased as in (A), both
diodes will produce equal voltage across their load resist¬
ances, R301 and R303. However, these voltages are of op¬
posing polarity and therefore the sum of the voltages across
these two load resistors will be zero If the phase of the sine
wave changes with respect to the pulse as in (B), the top
diode will produce more voltage across R301 than the bottom
diode produces across R303. Thus, the voltage across the two
will be positive. In (C) the reverse condition exists. It is ob¬
vious that the output of the discriminator can swing from
positive through zero to negative dependent upon the phase
relation of the synchronizing signal and the oscillator. This
d-c output is applied to the grid of V303.
TOP DIODE
®
©
JV
HORIZONTAL OSCILLATOR CONTROL (block #11)—V303
the oscillator control is a 6AC7 connected as a reactance tube
across the V302 oscillator coil. A change in the d-c output
of the sync discriminator produces a change in Gm of V303
which in turn changes the frequency of the oscillator. If the
phase of the oscillator shifts with respect to the synchronizing
pulse, the corresponding change in d-c from the sync discrim¬
inator causes the oscillator to be brought back into correct
phase.
C304 and C306 form a voltage divider to attenuate rapid
changes in d-c from the sync discriminator such as are pro¬
duced by the vertical sync or bursts of noise.
17

312
TELEVISION CIRCUIT DESCRIPTION
648PTK, S48PV
Sync Link—-If any phase modulation is present in the trans¬
mitted sync, a condition which unfortunately still exists in
some transmitters to date, a faster response to fluctuations in
the sync phase is needed than is provided by the ratio of
C304 to C306.
The sync discriminator will demodulate sync phase variation
quite faithfully, however, the filter resistor R305 together with
the capacity attenuator, C304 and C306 is just as effective in
removing this information as it is with respect to the noise
disturbances for which it is intended. The removal of this in¬
formation will produce a horizontal displacement of portions
of the picture.
It may be necessary in some instances to sacrifice some noise
immunity to compensate for phase modulation in the trans¬
mitted sync. By switching the link provided for this purpose,
C303 is added across C304 and the speed of response is in¬
creased by several times. Therefore, the link of J304 should
be connected between terminals 1 and 2 whenever this condi¬
tion exists.
Before making this change, however, it should first be definitely
determined that distortion of the raster is due to phase modu¬
lation of the sync. Horizontal "jitter" and distortion of the
raster can be caused by operating the picture control at too
great a gain setting considering the r-f signal input. Such a
setting produces an excessive video signal at the first video
amplifier grid. This stage is designed to limit an excessive
input in order to improve the signal to noise ratio. If the
video input is excessive, the sync is limited and thus removed.
At the same time picture information may be introduced into
the sync circuits. With extreme excesses of video level, both
horizontal and vertical sync may be lost. If the receiver oper¬
ating instructions on page 4 are followed, no difficulty should
be experienced with the picture control setting.
HORIZONTAL DISCHARGE, OUTPUT AND DAMPERS (block
#12)—The purpose of these circuits is to produce a sawtooth
of current in the deflection coils to provide horizontal scanning
for the kinescope.
Horizontal Discharge—A 615 is employed for the discharge
tube V304. The function of this stage is to produce a saw¬
tooth voltage for use in the horizontal sweep circuits.
The oscillation in V302 takes place between screen-grid and
cathode. Since the peak to peak voltage on its grid is ap¬
proximately 100 volts, a square wave of voltage is produced
on its plate. This wave is differentiated by C312 and R314,
and the pulse so obtained is applied to the grid of the dis¬
charge tube V304.
The discharge tube is normally cut off due to bias produced by
grid rectification of these incoming pulses. The pulse from
V302 overcomes this bias and drives the tube into heavy
momentary conduction. During this period the plate voltage
falls nearly to cathode potential and C318 discharges rapidly.
Then when V304 again becomes non-conducting, the plate
voltage rises slowly and approximately linearly as C318
charges through R316 and C315.
Horizontal Output and Dampers—The operation of these two
circuits is so interconnected that it will be necessary to discuss
them 'simultaneously. The function of the output tubes V305
and V306 is to supply sufficient current of the proper wave
form to the horizontal deflection coils in order to provide hori¬
zontal scanning for the kinescope. The function of the damper
tubes V310 and V3J.1 is to stop oscillation of certain compo¬
nents at certain times and thus help provide a linear trace.
Other functions of these circuits include the Utilization of
energy stored in the horizontal deflection coil to furnish retrace
and kinescope high voltage. The damper circuit also recovers
some of the energy from the yoke kickback and uses it to
help supply the plate power requirements of the output tubes.
In operation, the visible portion of the horizontal trace Is ap¬
proximately 53 microseconds in duration. Although the induct¬
ance of the horizontal deflection coil is in the order of 8 milli¬
henries, at the horizontal scanning frequency, the reactance of
the coil predominates over its resistance. This is a different
case than that encountered in the vertical deflection system
and so a different method of operation must be employed.
Horizontal blanking is approximately 10 microseconds in dura¬
tion. During this time, the kinescope beam must be returned
to the left side of the tube, the trace started and made linear.
To accomplish all this within the horizontal blanking time,
only 7 microseconds can be allowed for the return trace. In
order to obtain such rapid retrace, the horizontal deflection
coil, output transformer and associated circuits are designed
to resonate at a frequency such that one-half cycle of oscilla¬
tion at this frequency vfill occur in the 7 microseconds retrace
time limit. This represents a frequency of approximately 71 kc.
During the latter part of the horizontal trace, the output tubes
conduct very heavily and build up a strong magnetic field
in the deflection coil and output transformer. When the nega¬
tive pulse from the horizontal discharge tube is applied to the
output tube grids, their plate currents are suddenly cut off and
the magnetic field in the transformer and deflection coil begins
to collapse at a rate determined by the resonant frequency oi
the system. Actually the system is shock excited into oscilla¬
tion. Since the output tubes are cut off and since the voltage
generated by the collapsing field is negative on the damper
tube plates so that they are non-conductive, there is essen¬
tially no load on the circuit and it oscillates vigorously lor one-
half cycle. If the damper tubes were not present, the circuit
would continue to oscillate as shown in Figure 19 (C), curve 1.
This condition however is not permitted. One-half cycle of os¬
cillation is permitted because at the end of such a time the
current in the deflection coil has reached a maximum in the
opposite direction to which it was flowing at the end of the
trace period. This reversal of the direction of flow of current
is the requirement for retrace and it is accomplished in the
allotted 7 microseconds.
Now that retrace has been completed, it is necessary to start
the next trace. The energy which was placed in the deflec¬
tion coil by the output tubes in the latter part of the last trace
has not been dissipated. During the one-half cycle of oscilla¬
tion, retrace was accomplished with very little loss of energy.
The field in the coil was merely reversed in polarity. So at
this point, a strong field exists in the deflection coil.
As mentioned previously if the coil were not damped, it would
continue to oscillate at its natural frequency as shown in Fig¬
ure 19 (C), curve 1. To prevent such an oscillation the damper
tubes are brought into action. These tubes are effectively con¬
nected across the deflecting coil.
In the oscillating circuit, the current in the deflection coil lags
the voltage by approximately 90 degrees (one-quarter cycle
at oscillation frequency) and when the current has reached
its maximum negative value, the voltage across the coil being
90 degrees ahead, has begun to swing positive. When the
voltage on the damper plates becomes positive with respect
to their cathodes, they begin to conduct heavily. This places
such a load across the deflection coil that it cannot oscillate.
Instead the field begins to decay at a rate permitted by the
load which the damper tubes placed on the coil. The circuit
constants are such that this decay is linear and at a rate
suitable for the visible trace.
If no additional energy were fed into the coil the field would
fall to zero and the kinescope beam would come to rest in
the center of the tube. In such an r-1 circuit, as the current
approaches its final value, it does not do so linearly but
asymptotically as indicated in Figure 19 (C), curve 2. It is
therefore necessary to have the output tubes begin to supply
18

313
TELEVISION CIRCUIT DESCRIPTION 648PTK 643PV
power to the deflection coil before the energy in the coil is
completely dissipated. Figure 19 (C), curve 3 shows the shape
of the current supplied by the output tubes. Although the cur¬
rents supplied by the output tubes and by the decaying field
are curved at the cross over point, together they produce a
coil current that is linear.
By the time the beam has reached the right side of the kine¬
scope, the output tubes are conducting heavily and have built
up a strong field in the transformer and coil. At this point,
the output tubes are again suddenly cut off and the process is
repeated.
The 6BG6G plate voltage is supplied through the 5V4G which
is conducting over the major portion of the trace. Capacitor
C324A is charged during this period and this charge is suf¬
ficient to supply the 6BG6G plates when the 5V4G is not
conducting.
The charge is placed on this capacitor by the receiver d-c
supply and by the current from the collapse of the field in
the horizontal deflecting coil. The a-c axis of the sweep volt¬
age is 475 volts above ground since the T302 secondary is
connected to the receiver 475 volt bus. The charge placed on
this capacitor by the coil kick-back is therefore in addition
to that from the d-c supply and thus the capacitor is
charged to a voltage greater than the d-c supply. This per¬
mits operation of the output tubes at a higher voltage than is
obtainable from the receiver power supply and produces an
increase in the system efficiency by salvaging energy that
would otherwise have been wasted.
PLATE
CURRENT
Figure 19—Horizontal Sweep Waveforms
Width Control—L302 is provided to vary the output and hence
the picture width by shunting a portion of the T302 secondary
winding. Clockwise rotation of the adjustment increases the
picture width and causes the right side of the picture to
stretch slightly.
damping action over the entire .trace. V310 a dual triode is
employed to provide the extra damping action required during
the first portion of the trace. When the voltage on the damper
plate swings positive at the start of the trace, the differentiat¬
ing network (C331, R350, and R351) in the grid circuit of
V310 produces a positive pulse on the damper grid due to
the steep wave front of the sweep voltage (shown in Figure
19 (D) at point 1. This positive pulse lowers the plate re¬
sistance of the triodes and permits heavy damping current
to flow. Then due to the short time constant of the grid net¬
work, the positive pulse decays and the bias due to grid
rectification of the pulses cuts the triode damper eff, leaving
the 5V4G to provide the damping for the remainder of the
trace.
The horizontal linearity control R351 changes the time con¬
stant of the differentiation network in the 6AS7G grid circuit
and determines the portion of the trace over which the tube
conducts, thus controlling linearity on the left side of the pic¬
ture. Counterclockwise rotation of the control causes the left
side of the picture to stretch.
HIGH VOLTAGE POWER SUPPLY (block #13)—The kinescope
high voltage supply is unusual in that the power is obtained
from the energy stored in the deflection inductances during
each horizontal scan. When the 6BG6G plate currents are cut
off by the incoming signal, a positive pulse appears on the
T302 primary due to the collapsing field in the deflection coil.
This pulse of voltage is stepped up by the auto transformer
action of T302 and applied to the plate of the high voltage
rectifiers. At the same time, a negative pulse is applied to
the cathodes of the rectifiers.
Three type 8016 tubes are employed in a voltage trippler cir¬
cuit which produces approximately 27kv d-c for operation of
the kinescope. The pulses are first rectified by V307 and
charge capacitor C326 to near peak-to-peak voltage applied
between the plate and cathode. Since the cathode of V307 is
connected to the plate of V308 by resistors R342 and R343, ca¬
pacitor C327 will charge to the same voltage as C326. The
charge on C327 is thus added to the incoming pulse and V308
rectifies the sum of these voltages thus charging C328 to
double the pulse voltage. The cathode of V308 is connected
to the plate of V309 through R344 and R345 charging C329
to the same voltage as C328. The charge on C329 is added
to the incoming pulse. V309 rectifies the incoming pulse and
the d-c charge on C229 to charge C330 to three times pulse
voltage.
In practice, due to a slight loss between stages and a small
phase shift between the positive and negative pulses, the d-c
output is approximately 2.8 rather than 3 times the applied
pulse.
Horizontal Drive Control—The horizontal drive control R340
varies the amount of high peaking on the grid of the hori¬
zontal output tubes and thus affects the point on the trace at
which the tubes conduct. The negative pulse is applied to
the sawtooth by feeding back a portion of the pulse from the
secondary of the horizontal output transformer. Clockwise
rotation of the control increases picture width, crowds the
right side of the picture and stretches the left side.
Horizontal Linearity Control—In order to describe the action
of the linearity control, some additional facts about damper
circuits must be presented.
When two horizontal output tubes are employed as in the
648PTK, proper damping cannot be obtained by a single
damper tube due to the heavy damping action required dur¬
ing the first quarter of the trace. V311 a 5V4G provides
Since the frequency of the supply voltage is high (15,750 cps),
relatively little filter capacity is necessary Since the filter ca¬
pacity is small, the stored energy is small, and the high volt¬
age supply is made less dangerous.
Corona rings are employed on the rectifier tube sockets, the
high voltage capacitor lugs and on nearby sharp edges in
order to prevent carona discharge.
LOW VOLTAGE POWER SUPPLY (block #15)—The low volt¬
age power supply chassis contains two separate power sup¬
plies. One supply provides the filament and plate voltages
for the r-f, i-f chassis and the other supply provides for the
horizontal deflection chassis. This latter supply employs an
interlock cable to the horizontal deflection chassis and a fuse
in the power transformer primary to protect the supply in case
of short circuits in the horizontal deflection chassis.
19

314
648PTK 9 648PV
TELEVISION CHASSIS VIEWS
Ml and L82 are
omitted in some units.
In Model 648PV—T109
(3rd. Fix I-F) is used in¬
stead oi L104
figure 20 — R-F, I-F Chassis Top View
20