HPE 606A Owner's manual

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Model]
606A
Output
Attenuator
4-22
00170.

OP
ERATING AND SERVICING MANUAL
MODEL 606A
HIGH FREQUENCY
SIGNAL GENERATOR
SERIALS PREFIXED: 009-
FREQUENCY
ining, P
30 “e, ~
PERCEN?, MODULATION
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MODULATION
AMPLITUDE
'
VERNIER
MODULATION,
SELERTOR
ATTENUATOR
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1-10
MODULATION
GRYSTAL
CALIBRATOR
IMPUT-OUTPUT
PHONES
ort
SiGNAL
GENERATOR
606A
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MODEL ~
WeWeTT ) PACKARD
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Printed 3/60

Model 606A
SPECIFICATIONS
FREQUENCY RANGE: 50 ke to 65 mc in 6 bands.
50 - 170 kc 1.7
165 - 560kc 5.8 - 19.2 mc
530 - 1800 kc 9.0
FREQUENCY ACCURACY: Within +1%.
FREQUENCY CALIBRATOR: Crystal oscillator provides check points at 100 kc (useful
to 6 mc) and 1 mc intervals, accurate within 0.01% from
0°to 50°C,
RF OUTPUT LEVEL: Continuously adjustable from 0.1 pv to 3 volts into a 50 ohm
resistive load. Calibration is in volts and dbm (0 dbm is
1 milliwatt).
OUTPUT ACCURACY: Within +1 db into 50 ohm resistive load.
FREQUENCY RESPONSE: Within +1 db into 50 ohm resistive load over entire fre-
quency range at any output level setting.
OUTPUT IMPEDANCE: 50 ohms, swr less than 1.1 on 0.3 volt range; on 1 volt
and 3 volt ranges, less than 1.1 to 20 mc and less than
1.2 to 65 mc. BNC output connector mates with UG-88A/
B/C/D.
SPURIOUS
HARMONIC OUTPUT: Less than 3%.
LEAKAGE: Negligible; permits receiver sensitivity measurements
down to at least 0.1 microvolt.
AMPLITUDE MODULATION: Continuously adjustable from 0 to 100%. Indicated bya
panel meter. Modulation level is constant within +1/2 db
regardless of carrier frequency and output level changes.
INTERNAL MODULATION: 0 to 100% sinusoidal modulation at 400 cps 45% or
1000 cps +5%.
ii

Model 606A
SPECIFICATIONS (CONT’D.)
MODULATION BANDWIDTH:
EXTERNAL MODULATION:
ENVELOPE DISTORTION:
MODULATION
METER ACCURACY:
INCIDENTAL FM:
SPURIOUS FM:
SPURIOUS AM:
FREQUENCY DRIFT:
POWER:
SIZE:
WEIGHT:
ACCESSORIES AVAILABLE:
DC to 20 kc maximum, depends on carrier frequency, fo;
and percent modulation as shown in the following table:
30% Mod. 70% Mod. Square Wave Mod.
Max.Mod.Frequency “906 a. 0.021, 0.0031, (Skemax)
0 to 100% sinusoidal modulation de to 20 kc. 4.5 volts peak
produces 100% modulation at modulating frequencies from
de to 20 kc, Input impedance is approximately 600 ohms.
May also be modulated by square waves and other complex
signals,
At output levels of 1 voltor less; less than 1% at 30% modu-
lation using internal 400/1000 cps source, less than 3% from
0 to 70% modulation.
Within +5% from 0 to 90%.
At Il volt or less output and 30% amplitude modulation:
0.0025% or 100 cps, whichever is greater.
Less than 0,0001% or +20 cps whichever is greater.
Hum and noise sidebands are 70 db below carrier down to
thermal level of 50 ohm output system.
Less than 0.005% or 5 cps whichever is greater, for a
10 minute period after warm-up.
115/230 volts 410%, 50 to 1000 cps, 135 watts.
Cabinet Mount: 20-1/2 in. wide, 12-1/2 in. high,
14-3/4 in. deep.
19 in. wide, 10-1/2 in. high, 13-1/2 in.
deep behind panel.
Rack Mount:
Cabinet Mount: Net 46 lbs., shipping 66 lbs.
Rack Mount: Net 43 lbs., shipping 65 lbs.
606A-34A Output Termination. Three positions: 50 ohms,
for use into high impedance; 5ohms (10:1 voltage division);
IRE Standard Dummy Antenna (driven from 10:1 divider).
lii

SECTION I
SECTION II
SECTION III
SECTION IV
SECTION V
CONTENTS
GENERAL INFORMATION
a Am bh Wh
General Description 2... 6 cette eee ewe eww
Output Termination... 1... wee ee ee ee es
Initial. Inspection: 2-6 e404) a as aes. Se nee ee es ee ees
Installation Instructions... .......-6-2.2050505008-
Conversion for 230 Volt Operation..............
Accessories Available .. 2... 2. eee eee ee eee
OPERATING INSTRUCTIONS
NNNNNN ND NOMS WN EE
Output Termination . 2... 2. ee et ee we
Setting the Cursor . 2... we ee te ee ee
Qutput Attenuator a... ie ee we ee RR ee ed eae
Use of the 3-Volt Range .........202. 0500000
External Modulation ... 2... 1... ee ee ee ee ee
External Synchronization Signal ...............
Bt Fuse). 3 ep 8s) 6 oe Oe ee ee Sivlst ad ooh ancy e GO Sew
THEORY OF OPERATION
3-1 Over-All Operation 2... ... eee ee ee ee
3-2 Circuit: Description’ 4.28.06 8 3B ee ee ec we Sea we oe
MAINTENANCE
4-1 Introduction... 4.38) 226 8 ed eS eG) ee LA vw a ay a
4-2 Removing the Instrument from the Cabinet .........
4-3 Connections for 230 Volt Operation .............
4-4 Tube Replacement .... 2... 2... ee eee eee es
4-5 Front Panel Operation Check ...........2.-200-00-
4-6 Trouble Shooting ........ 0.2.0.0. eee eee eee
4-7 Turn-On Procedure Following Repair ...........
4-8 Test Equipment Required ........-..5456+220004
4-9 Meter Zero Adjustment .... 1... 0.2.0 ee ee eevee
4 ~- 10 General Test and Alignment Instructions ..........
4-11 Power Supplies . 2... ee ee tt tt et ee es
4-12 Audio Oscillator ... 1... 2. 2 ee ee ee ee ete
4-13 Crystal Calibrator .. 1... 1. ee tt ee
4 - 14 Tune Oscillator and Amplifier .............4...
4-15 Set Maximum Oscillator Current ............046.
4 - 16 Carrier Zero: Set. 260s sec: eace ce eed be ee
4-17 Maximum Carrier Set and Modulation ZeroSet ......
4-18 Set Percent Modulation Meter ............+8..
4-19 Calibrate RF Output Meter ..........-.+0.+000.
4 - 20 Adjust Feedback Relay 2... 1.1 ee ee ee ee ee ee
TABLE OF REPLACEABLE PARTS
5-2
Table of Replaceable Parts... 1.1... . 1. ee ee ees
iv
Model
606A
PAGE
I-1
I-1
I-1
I-1
1-2
1-2
f-1
Y-1
t-1
I-1
I-11
I-2
I - 2
I - 1
Il - 1
IV -1
IV -1
IV -1
IV -1
IV -3
IV -4
IV -11
IV - 11
IV - 12
IV - 12
IV - 12
IV - 13
W -13
IV - 13
IV - 14
IV - 14
IV - 15
W - 15
Iv - 16
Iv - 16
Vy -2

Model 606A
1-1 GENERAL DESCRIPTION
The Hewlett-Packard Model 606A is a general-
purpose signal generator with a frequency range of
50 kc to 65 mc in six ranges. The instrument has
a direct reading frequency dial calibrated to an
accuracy of 1%.
Output is held constant within +1 db andis continu-
ously adjustable from 0.1 pv to 3 volts into a 50
ohm resistive load. In addition, a termination with
a 10:1 voltage divider which lowers the minimum
output to 0.01u.v (from a5 ohm source) or a dummy
antenna which simulates the standard IREstandard
dummy antenna for precision measurements on
receivers is available. An internal crystal cali-
brator provides check points at 100 kc and 1 mc
intervals with an error of less than 0.01%. A front
panel meter accurately indicates the percent ampli-
tude modulation for frequencies within the modu-
lation bandwidth of the signal generator.
The instrument has a highly refined modulation
system which allows modulation up to 90% with low
distortion and incidental fm. This feature makes
possible precision distortion checks of receivers
from antennas to output. The instrument has built-
in 400 and 1000 cps modulation and can accept ex-
ternal modulation from 20 cps to 20 kc or more,
depending on rf frequency in use.
1-2 OUTPUT TERMINATION
This signal generator is designed to be used with
an ® 606A-34A Output Termination. THE OUTPUT
LEVEL IS CALIBRATED ONLY WHEN USED WITH
THIS TERMINATION OR A 50 OHM RESISTIVE
LOAD. Further details concerning the use of this
output termination will be found in Section Il,
Sect.I Page 1
SECTION |
GENERAL INFORMATION
1-3. INITIAL INSPECTION
Your instrument was individually tested and in-
spected before leaving the factory and met all
specifications as printed at the front of this manual.
Upon receipt, you should inspect and operate the in-
strument to verify proper operation, Ifany damage
is found, refer to the ‘‘Claim for Damage in Ship-
ment’’ page of this manual.
The only preliminary check necessary before turn-
on is to check the meter movement mechanical
zero. The meter pointer should rest directly over
zero when the instrument is turned off. Bestac-
curacy is obtained if you observe this with one eye
only. To avoid parallax, position your head so the
meter scale, pointer and your eye are all in line,
A mechanically stable adjustment can only be made
when the direction of pointer travel is opposite to
the direction of rotation of the adjust screw, The
adjusting screws may be turned in either direction,
but a good method is to turn the screw clockwise
until the pointer swings up scale and then starts
to swing down scale towards zero (i.e.: counter-
clockwise), Continue turning the screw clockvise
until the pointer (now moving to the left)is directly
over the zero mark. If you overshoot, continue
turning the screw clockwise until the pointer is
again traveling down scale towards zero,
1-4 INSTALLATION INSTRUCTIONS
This instrument should not be operated inan ambient
temperature greater than +50°C. Donot install the
rack-mount model ina rack near other equipment
discharging hot air around the Model 606A.
For personnel protection the power cord shouli be
plugged intoa 3-prong (NEMA approved) receptec Je.
If a 2-prong adapter is used, ground the instrunent
by connecting the external wire to ground. T7he
receptacle box may be used if it is grounied.

Sect.I Page 2
1-5 CONVERSION FOR 230 VOLT
OPERATION
The Model 606A may be quickly and easily converted
for operation from a nominal 230 volt 50-1000 cps
source. Refer to paragraph 4-3 CONNECTIONS
FOR 230 VOLT OPERATION for instructions.
Model 606A
1-6 ACCESSORIES AVAILABLE
Output Termination -- ® 606A-34A
Fused Attenuator Protector -- 608A-95A. Prevents
attenuator burnout when working with tranceiver
type of equipment (requires two BNC to type ‘‘N”’
adaptors UG-201A/U and UG-349A/U),

Model 606A
2-1 OUTPUT TERMINATION
This instrument is calibrated ONLY when used
with a 50 ohm resistive load, such as the @ Model
606A-34A Output Termination, into a high-im-
pedance (500 ohms or greater) input. If the signal
is fed into a lower impedance the output will no
longer be within calibration specifications and the
output of the Crystal Calibrator will be reduced,
particularly on the higher harmonics.
Any shielded rf cable of 50 ohms nominal impedance
with BNC male connectors is suitable for use with
the Model 606A. Single braid shield types are
suitable for use from maximum output to approxi-
mately -80 dbm (30 microvolts). Double braid or
solid shield types are recommended for use over
the entire attenuation range. A good general pur-
pose cable consists of three feet of RG-55U (double
braid shield) with UG-88C/U BNC connectors on
each end. See also Figure 2-3 EXTERNAL OUT-
PUT TERMINATION for information concerning
output cable termination.
The output jack on the Output Termination has been
provided as a BNC connector for maximum shield-
ing. Clip-lead connection may be provided easily
by inserting a UG-290/U connector with soldered-
on clip leads into the output jack ofthe Output Ter-
mination. Keep the length of the clip leads as
short as possible.
2-2 SETTING THE CURSOR
Set the cursor (movable index) with the CALIBRATE
knob so that it is aligned with the line under the
engraving reading FREQUENCY before setting the
frequency. The FREQUENCY dial is calibrated only
after this operation is performed.
Sect. II Page 1
SECTION Il
OPERATING INSTRUCTIONS
2-3 OUTPUT ATTENUATOR
The output attenuator contains resistors which can
be burned out by careless usage. If the output is
shorted out in the 3 VOLT range or if voltage is
fed into the attenuator accidently, these resistors
may be burned out or heated up so that they are no
longer calibrated. This may occur while measuring
the sensitivity of the receiver in a mobile trans-
mitter- receiver installation when the transmit
button is pushed accidently. An attenuator fuse is
available as an accessory when it is desired to use
this generator under conditions where the attenu-
ator may be burned out (see paragraph 1-6), The
resistors in the attenuator are NOT field re-
placeable. Do NOT open the attenuator to check
these resistors as placement of the resistors is
critical, The attenuator may be removed from the
instrument and returned separately to the factory
for repair.
2-4 USE OF THE 3-VOLT RANGE
The unusually high output range of 3 volts is useful
for driving rf bridges or other equipment requiring
a calibrated high-level high-frequency voltage.
This useful range is obtained at the expense of
operating the power amplifier stage near the over-
load point. You will obtain best life from these
tubes by not leaving the generator on the 3 VOLT
range any longer than necessary to make your
measurement. Never leave it on this range during
warm-up or stand-by.
2-5 EXTERNAL MODULATION
Take care when using external modulation vith
direct coupling. The dc level of the signal y ill
affect the average rf level. If only modulatioi is
desired or if only the ac component of the molu-
lating signal has significance, switch the MODU-
LATION SELECTOR to EXT. AC,

Sect.I] Page 2
2-6 EXTERNAL SYNCHRONIZATION SIGNAL
When the Model 606A is modulated internally a sig-
nal is available at the MODULATION Input-Output
jack for synchronization purposes. This signal is fed
from the same oscillator which modulates the car-
rier. It is of approximately 1 volt amplitude froma
high impedance source. Since the signal comes from
a high impedance source use it only as a voltage
source and do not attempt to draw current.
2-7 B+ FUSE
The regulated B+ voltage is fused on the front
panel. If you accidentally apply excessive modu-
lation to the instrument the tuned circuits may
flash over from excessive peak rf voltage. This
will blow the B+ fuse. The instrument will have
no output and the output level meter will be pinned
to left of zero. If this happens, the fuse needs to
be replaced to restore operation.
2-8 MODULATION DATA
= RATED MAL — ~
AD ee re ol
10%
oS —— Sos
\XG
100%
MAK, MOOG. FREQUENCY
SOKC 100KC IMC IOMC 65MC
CARRIER FREQUENCY
ENVELOPE DISTORTION (%)
ENVELOPE DISTORTION (%)
ENVELOPE DISTORTION (%)
Model 606A
RATED MAX. DISTORTION ———\- -
axe.
25.
tz
2 3 17 2
MODULATING
loKe 20 © 40
FREQUENCY

Model
606A
Sect,
I]
Page
3
OPERATING CONTROLS
Set
Range
Set
Read
Read
RF
Cursor
Percent
Output
Modulation
Set
Frequency <<]
MODULATION AMPLITUDE.
MODULATION SELECTOR
r, 7 INT.
Adjust
eet Output
eae
SIGNAL GENERATOR MODULATION CRYSTAL CALIBRATOR RF OUT
INPUT-OUTPUT PHONES. OFF son
wer, dB vad tts be
es ® note ( o @ =< @—+—+}— Connect
RF Output
Select Type Plug In Switch for
of Here for Calibrator
Modulation External Operation
Modulation
Adjust Plug Head-
Internal Phones In
Modulation Here

Sect.I] Page 4 Model 606A
GENERAL OPERATION
FREQUENCY
RANGE
[a
owe eeuc
c
ce
7) “
Ke = 70K
VERNIER
MOOULATION AMPLITUDE VERNIER
MODULATION SELECTOR ATTENUATOR
EXT, w INT. ———— 8
SIGNAL GENERATOR
mont
e 6068
HEWLETT PACKARD
PALO ALTO CALIFORNIA
B+ FUSE NO R.F. OUTPUT AND
OUTPUT METER PINNED TO LEFT
IF BLOWN.
Rotate
RANGE
switch
to
select
desired
band.
.
Set
amount
of
modulation
as
read
on
PER-
CENT
MODULATION
meter
with
MODUL
A-
Rotate
FREQUENCY
control
to
desired
fre-
TION
AMPLITUDE
control.
quency.
Read
frequency
on
scale
indicated
.
Rotate
ATTENUATOR
to
select
output
volt-
by
pointer.
age
level
desired.
. Rotate ATTENUATOR VERNIER to desired
Rotate MODULATION selector to INT(either output level. Read range of output meter on
400
cps
or
1000
cps
modulation
is
available).
ATTENUATOR
switch
and
value
on
meter.
Figure 2-2

Model 606A Sect. I] Page 5
EXTERNAL OUTPUT TERMINATION
FREQUENCY
PERCENT MODULATION
CALIBRATE
MODULATION AMPLITUDE VERNIER
MODULATION SELECTOR ATTENUATOR
EXT, INT. ———
SIGNAL GENERATOR MODULATION CRYSTAL CALIBRATOR
INPUT QUTPUT PHONES. OFF
i ‘one @
WODEL
HEWLETT
acd ALTO caLiFor
TO UNIT
UNDER TEST
OUTPUT
TERMINATION
606A — 344A
The attenuator on the Model 606A Signal Gener~
ator is calibrated only when used with a load of
50 ohms, For high impedance loads or receiver
inputs the output termination ( @ Stock No. 606A-
34A) is recommended. This output termination
is designed for use at the end ofa 50 ohm shielded
cable, and to operate into a high impedance
(500 ohms or greater) load.
The probe has three positions as the rear shell
of the probe is rotated clockwise as follows:
a. Dummy Antenna - output impedance varies
as per IRE Standard Dummy Antenna®* at an
output level 20 db below the input level (10:1
voltage division ratio).
Zero db attenuation - 25 ohms output im-
pedance (1:1 voltage ratio).
Twenty db attenuation - 5 ohms output im-
pedance (10:1 voltage division).
NOTE - Maximum permissible input power to
probe is 18 milliwatts (3 volts across 50 ohms),
* See ‘‘Standards on Radio Receivers’’, In-
stitute of Radio Engineers, 1938; ani
Terman, ‘‘Radio Engineers Handbook’’, any
edition - section entitled ‘‘Measurements o
Radio Receivers’’.
Figure 2-3

Sect. II Page 6
Model 606A
CALIBRATION
FREQUENCY
SIGNAL GENERATOR
MODULATION AMPLITUDE VERNIER
MODULATION SELECTOR ATTENUATOR
ExT, INT, —e
MODEL 6064
HEWLETT PACKARD
PALO ALTO CAL FORNIA,
CAUTION
Remove headphones before
switching CALIBRATOR
. Switch XTAL CALIBRATOR to 1 mc.
. Plug a set of high impedance headphones into
PHONES jack.
. Zero-beat nearest even megacycle division to
desired frequency.
. Set the cursor exactly on the megacycle
mark,
. Switch XTAL CALIBRATOR switch to 100 kc
(lower frequencies only).
. Turn VERNIER dial toward desired frequency
while counting 100 kc beats. This procedure
Figure 2-4
HEADPHONES
will set the frequency within 100 kc. If the
frequency is desired with greater accuracy
follow the remaining steps:
. Determine number of scale divisions on
VERNIER dial between zero-beat of the two
100 kc beats on either side of the desired
frequency.
. Set VERNIER dial to proportional number
of divisions from nearest 100 kc beat to the
desired frequency.
. Turn XTAL CALIBRATOR to OFF, Iflefton
it will modulate output signal.

Model 606A
Sect. II Page 7
EXTERNAL MODULATION
FREQUENCY
MODULATION AMPLITUDE VERNIER
MODULATION SELECTOR ATTENUATOR
1. oon
Ex. Cw IW o
—_"—_—
ie os °
7, SY)
tl D
0
| eum oe
it?
—"20 03
CRYSTAL CALIBRATOR RE OUT
PHONES OFF 50.0,
1. Perform procedure for CW operation (see
Figure 2-2).
2. Set MODULATION SELECTOR to EXT (AC
or DC coupling).
3. Connect modulating signal (3 volts or more
adjustable) to MOD, terminal.
4, Turn MODULATION AMPLITUDE fully
clockwise.
5. Increase signal from external generator until
a reading of 100% is obtained on the PERCENT
MODULATION Meter.
6. Reduce the percent modulation to the de-
sired level with the MODULATION AMPLI-
TUDE control.
The limits of modulation depend upon the maxi-
mum tolerable envelope distortion. For three
percent envelope distortion the limits in terms
of the carrier frequency, f,, are:
Square—
30% Mod. _70%Mod. Wave Mad.
ae ax: = . 06 fi .02 te .003 f
absolute
fmod. max. = 20 kc 20 kc 3 ke
Applying these formulas, typical bandwidths are:
Carrier Freq. Modulation Frequency
Frequency 30% Mod. 70%Mod. Square Wive
50 kc 3 kc lke 150 cps
200 ke 12kc 4 ke 600 cps
500 ke §=20 kc 10 kc 1500 cps
Imc &above 20 kc 20 kc 3 ke
NOTE - On the 3 VOLT output range and le-
tween 19 - 65 mc in frequency, modulation te-
yond 30% is not recommended,
Figure 2-5

FREQUENCY
MODULATION
SELECTOR
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MODULATION
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Figure 3-1. Block Diagram
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V909 [epoy]

Model 606A
3-1 OVER-ALL OPERATION
Refer to the block diagram Figure 3-1. The level
of the rf oscillator is stabilized by a feedback
circuit from the RF Oscillator to the Oscillator
Level Control tube. In a similar way the rf output
and modulation levels are held constant by a feed-
back
loop
from
the
rf
output
through
the
Differ-
ential
Amplifier
to
the
Modulator.
The
rest
of
the
block
diagram
represents
standard
circuitry
for
signal generators.
3-2 CIRCUIT DESCRIPTION
A.
RE
OSCILLATOR
AND
LEVEL
CONTROL
The RF Oscillator is a tuned-plate push-pull
oscillator.
The
pentode
section
of
V1
acts
asa
variable cathode resistor for V2 to control the
oscillator
level.
The
control
grid
of
this
pentode
receives a rectified portion of the oscillator signal.
This voltage decreases the current through V1
when the oscillator level rises and vice versa.
Since this current is also the cathode current of
the RF Oscillator, the level of oscillation will be
held constant. The triode section of Vlis a cathode
follower
which
provides
bias
voltage
for the
grids
of the oscillator and amplifier.
B. RF AMPLIFIER
The RF Oscillator signal is fed to the control
grids of the push-pull RF Amplifier stage, V3 and
V4. The screen grids are connected directly to
+300
volts.
The
amplifier
cathode
current
is
con-
trolled by: V6, which acts as a variable cathode
resistor, See thenext paragraph for an explanation,
C, MODULATOR
Triode V6 is inserted in the cathode circuit of
the RF Amplifier to obtain cathode modulation. The
internal resistance of the triode changes according
Sect. III] Page 1
SECTION Ill
THEORY OF OPERATION
to the applied modulation signal at the control grid.
Thus cathode current of the RF Amplifier is varied
which, in turn, amplitude modulates the rf level.
D. RF FEEDBACK AND LEVEL
CONTROL CIRCUIT
The modulated rf output signal is sampled at the
secondary winding of the rf output transformerand
rectified by crystal diodes CR2 and CR3. An rc
filter passes the de current developed. The time
constant of the rc filter is determined by the po-
sition of the frequency RANGE selector, This time
constant is selected to bypass the rf component but
not the modulation. The demodulated rf signal is
then fed to the control grid, pin 7, of the Differ-
ential Amplifier.
This demodulated signal is dc-coupled and thushas
a dc component equal to the cw level of the output
plus an ac component corresponding to the moclu-
lation, The demodulated rf signal also passes
through a second rc filter, C37, C38 and L6, with
a long time constant which removes the modulation.
At the output of this filter a dc current is obtained
which is proportional to the cw rf level. This cur-
rent is then fed to the rf output meter.
A two-position attenuator, which controls the a-
mount of feedback, is inserted between the crystal
detectors and the differential amplifier. In the
1 VOLT and lower output positions the feedback is
obtained through the divider R25 and R36. Only in
the 3 VOLT position is R36 (in feedback loop)
shunted by R26. This raises the current through the
RF Amplifier by 10 db, This switching is done au to-
matically by relay K1 whenever the ATTENUATOR
selector is switched to the 3 VOLT position.
E, RF ATTENUATOR
The rf output signal is tapped off the seconlary
winding of the output rf transformer and fed tc the
input of the output attenuator. This attenw tor

Sect. II Page 2
provides a maximum attenuation of 120 db in 10 db
ranges. The output level may be varied between
ranges by changing the input to the attenuator with
the ATTENUATOR VERNIER control. Input is
monitored by the rf level meter. The 3 VOLT rf
output is obtained by reducing the dc component of
the demodulated rf feedback.
F. CRYSTAL CALIBRATOR
A small signal, coupled from the attenuator,
is fed to the control grid (pin 2) of a mixer ampli-
fier (triode section of V10). The output of the
crystal oscillator (pentode section of V10) is also
fed to this grid. The mixed signal from V10 is
fed into an output triode amplifier, V9B. The beat-
frequency output of V9B is brought out to the front
panel CRYSTAL CALIBRATOR OUTPUT phone
jack,
The crystal oscillator consists of the pentode sec-
tion of V10 operating as an electron-coupled oscil-
lator, Positive feedback to the control grid is ob-
tained from the screen grid through the crystals.
Two crystals provide oscillation at either 1 mc or
100 kc, as selected by the XTAL CALIBRATOR
switch. The plate of the oscillator provides the
signal necessary to drive the mixer grid of the
triode section,
G. AUDIO OSCILLATOR
The audio oscillator is a modified Wien Bridge
oscillator with amplitude stabilization. The feed-
back signal is taken from the secondary winding
of the output transformer. Two oscillator fre-
quencies (400 and 1000 cps) can be selected by
Switching different resistors in the Wien Bridge.
The level of oscillation is set by the MODULATION
OSCILLATOR ADJUSTMENT (R51) which controls
the feedback tothe lamp. Increased feedback causes
the lamp to heat up, increasing its resistance. The
increased resistance causes more degeneration
in the cathode of V5, limiting the gain.
In the INTERNAL positions the modulating voltage
is available at the front panel MODULATION IN-
PUT-OUTPUT jack. This voltage is supplied for
synchronization purposes. It has a source im-
pedance of 82K ohms,
H. PERCENT MODULATION METER CIRCUIT
The modulating signal goes to pin 2 of V9A, a
cathode-follower, This tube feeds shunt diode CR5
which rectifies the signal. A dc voltage corres-
ponding to the modulation is fed to the PERCENT-
Model 606A
AGE MODULATION meter. Clamp diode CR4
prevents the cathode of V9 from going negative to
protect capacitor C56, whose voltage rating is
20 volts. This point would otherwise go towards
-200 volts if V9 was removed or was weak,
The modulating signal also goes to pin 2 of V8.
The signal level to V8 can be controlled by the
ATTENUATOR
VERNIER,
When
the
signal
level
is varied the output modulation is varied, as ex-
plained in the following paragraph. This circuit
is also used to reduce the voltage onthe RF Ampli-
fier during switching. As soon as the RANGE
switch is rotated out of the detent position, S7 dis-~
connects the +300 volt power supply, hence grid
voltage of V8 drops to ground potential. The B+
is not reconnected until after the turret has made
contact in its new position. This action keeps the
amplifier tubes from drawing excessive screen
current when the turret is disengaged from the
plate circuit. When the +300 vdc supply is dis-
connected R64 is substituted as a load to keep the
power supplies in regulation.
I, DIFFERENTIAL AMPLIFIER
The external modulation signal fed into the
MODULATION
jack
is
combined
with
the
de
refer-
ence level in the same manner as the internal
modulation. Either of these signals is applied to a
resistive network consisting of R57, R62 and R68
and is added to the de reference voltage. The
combined voltages appear at the ATTENUATOR
VERNIER (R63), This control varies the dc, which
controls the carrier, and the modulation signal at
the same rate. Thus the percentage of modulation
remains constant regardless of carrier level.
The dc (carrier level) and ac (modulation) signals
are fed through an rf filter to pin 2 grid of the
triode section of V8 as a reference signal to be
compared to a signal from the output which is fed
to the pentode grid (pin 7). This triode plus the
pentode section of the same tube forma differential
amplifier, The ac level of both of these signals
is proportional to the modulation. The modulating
signal is the reference voltage and the actual modlu-
lation of the output is compared to this reference.
The de level of the output (proportional to the rf)
is compared to a reference dc level that is pro-
portional to the desired rf level, as set by the
ATTENUATOR VERNIER control.
Since the cathodes of V8 are connected togetler,
the reference signal applied to the triode section
will also appear on the cathode of the pentode

Model 606A
section,
This
signal
will
be
compared
to
the
signal
from
the
output
which
is
applied
to
the
grid.
Any
deviation
from
a
fixed
voltage
difference
between
these
two
signals
results
in
an
output
signal
which
has
a
polarity
such
as
to
reduce
this
difference.
Thus
the
original
conditions
are
restored.
For
example,
if
the
rf
level
drops,
the
voltage
on
pin
7
of
V8
will
become
more
negative.
This
will
reduce
the
amount
of
current
flowing
through
this
tube
so
the
plate
will
become
more
positive.
The
grid
voltage
of
V6
is
proportional
to
the
plate
voltage
of
V8,
As
this
grid
voltage
becomes
more
positive
the
current
through
V6
will
increase.
But
this
current
is
also
the
cathode
current
for
the
RF
Amplifier,
so
the
output
will
increase
until
the
ori-
ginal
conditions
are
restored.
By
this
action
the
output
is
stabilized
and
is
constant
to
better
than
#1
db
over
the
entire
frequency
range.
The
rf
level
can
be
changed
by
varying
the
AT
TENUATOR
VER-
NIER
(R63)
which
will
change
the
reference
bias.
Sect. HI Page 3
In
a
similar
way
the
modulation
is
held
constant.
Since
the
crystal
detector
circuit
has
a
time
con-
stant
fast
enough
to
follow
the
modulation
envelope,
the
output
modulation
is
compared
to
the
modulating
frequency
and
distortion
is
minimized.
J,
BIAS
SUPPLY
One
half
of
V7
(pins
6,
7,
8
and
9)
is
a
pentode
cathode
follower
used
as
a
constant
voltage
source
to
furnish
the
plate
potential
needed
for
the
triode
section
of
the
differential
amplifier.
The
other
half
of
V7
supplies
the
screen
potential
for
the
pentode
section
of
the
differential
amplifier.
The
screen
potential
is
adjusted
for
RF
Amplifier
cut-off
when
both
grids
of
the
differential
ampli-
fier
are
at
ground
potential.
This
establishes
the
zero
point
of
the
ATTENUATOR
VERNIER
control.

Sect.IV Page 0
Model 606A
MAX MOD
CARRIER ZERO SET
MOD SET R69
osc R60
OUTPUT 4
CAL Re CARRIER
R37 ZERO.
SET
R35
MOD CAL
R67
osc
LEVEL
R3
MODULATION
CHASSIS
» RF OSCILLATOR
CHASSIS
RF AMPLIFIER
CHASSIS
C41
REMOVE THIS
TERMINAL BOARD TO
CONVERT TO 230V,
CALIBRATOR
CHASSIS
-~200V SET
R126
(ADJUST FROM
' FRONT )
Figure
4-1,
Location
Diagram
Model
606A