HP 712B Owner's manual

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OPERATING AND SERVICING MANUAL
MODEL 7121
POWER
SUPPLY
Serial 511 and Above
HEWLETT-PACKARD COMPANY
275 Page Mill Road • Palo Alto, California, U.S.A.

OPERATING AND SERVICING MANUAL
FOR
MODEL 7128
POWER SUPPLY
Serial 511 and Above
Copyright 1954 by Hewlett-Packard Company
The information contained in this booklet
is intended for the operation and main-
tenance of Hewlett-Packard equipment and
is not to be used otherwise or reproduced
without the written consent of the Hewlett-
Packard Company.
HEWLEn·PACKARD COMPANY
275 PAGE MILL ROAD, PALO ALTO, CALIFORNIA, U.S.A.
7121004.1

TABLE OF CONTENTS
FOR
MODEL 712B POWER
SUPPLY
Para.
Page Para. Page
SECTION I
SECTION IV
GENERAL DESCRIPTION MAINTENANCE
1 -1 General.
·1
-1 4-1 Cover and Bottom Plate
1 -2 Inspection.
1 -1
Removal
4-1
4-2
Replacements
···. .
4-1
4-3
Adjustments
··
4-3
SECTION II
4-4
Line Regulation
Adjustment 4-3
2-1
Controls and Terminals.
2-1
4-5
Adjustment of Fixed -300
2-2 Operation
······
2-2 Volt Bias Supply Output
Voltage
· · ····· ·
4-5
4-6
Adjus tment Of HV Supply
SECTION III Output Voltage •····
4-5
4-7
Setting Variable Resistor
3-1
General
.· · .····
3-1
R39
.····· · · ·
4-5
3-2 High Voltage Regulator
4-8
Setting the Stop on the H V
Circuit
.··.····...
3-1
Control
····· · · ·..
4-7
3-3
Bias Supply Circuit.
··
3-3
SECTION V
TABLE OF REPLACEABLE PARTS

TABLE OF SPECIFICATIONS
OUTPUT VOLT AGES
I,
DC REGULATED HIGH VOLT AGE:
DC REGULATED FIXED BIAS:
DC VARIABLE BIAS:
AC UNREGULATED:
o to +500 volts (without switching), 200 ma maximum load.
-300 volts, 50 ma maximum load.
o to -150 volts, 5 ma maximum load.
6.3 volts CT, 10 amps maximum load.
OUTPUT VOLTAGE REGULATION (for constant line voltage)
DC REGULATED HIGH VOLT AGE:
DC REGULATED FIXED BIAS:
DC VARIABLE BIAS:
LINE VOLTAGE REGULATION
RIPPLE
INTERNAL IMPEDANCE
DC REGULATED HIGH VOLTAGE:
RECOVERY TIME
METERING
CURRENT METER:
VOLTMETER:
TERMINALS
OVERLOAD PROTECTION
POWER SUPPLY
DIMENSIONS
WEIGHT
Less than 50 millivolts change no-load to full-load at any output
voltage.
Less than 50 millivolts change no-load to full-load.
Is tied to fixed bias, hence source regulation is same as for fixed
bias. Internal impedance 0 to 10, 000 ohms, depending on bias
control setting.
Regulated DC output voltages vary less than ±l00 millivolts for
line voltage variations 115 volts ±1O%.
Less than 500 microvolts.
(For frequencies above 20 cps.) Full-load: 0.1 ohm in series
with 25 microhenries maximum. No-load: 1 ohm in series with
50 microhenries maximum.
Upon application of full-load: 0.1 millisecond maximum. Upon
decrease from full-load to:
(a) 0 ma, 0.5 millisecond maximum.
(b) 25 ma, O. 1 millisecond maximum.
Maximum transient voltage, 1 volt.
o to 200 ma. (high voltage only.)
Three ranges, 0 to +500 volts, 0 to +150 volts and 0 to -150 volts.
Panel switch connects meter to DC regulated high voltage or DC
variable bias and selects range.
Either positive or negative DC regulated high voltage terminal may
be grounded. Positive terminals of both bias supplies and nega-
tive terminals of DC regulated high voltage are common.
AC line, DC regulated high voltage, DC regulated fixed bias and
filament supply are separately fused. DC regulated high voltage
drops to a safe value if bias fuse blows.
115 volts ±10%, 50 to 60 cps. Approximately 120 to 450 watts
depending on load and line voltage.
Cabinet Mount: 12-1/2" high, 20-1/2" wide, l4-l/4"_deep.
Rack Mount: Panel 10-1/2" high, 19" wide, 14-1/4" deep.
Cabinet Mount: 6!1lbs. shipping weight approx. 104 lbs.
Rack Mount: 62 lbs. shipping weight approx. 98 lbs.

SECTION I
GENERAL DESCRIPTION
CAUTION
Voltages which are dangerous to life
are present at the terminals and
within this instrument.
1-1 GENERAL
The Model 7l2B Power Supply is a very stable
source of continuously variable direct cur-
rent for plate and bias circuits. It also pro-
vide s up to 10 amper es, 6.3 volts alternat-
ing curr ent for filament cir cuits • A two-
range voltmeter and a milliammeter (high
voltage only) are provided to measure the
direct current output. This power supply is
veryusefulas apower sourcefor small trans-
mitters, oscillators, complex systems, cer-
tain types of klystrons, and general labora-
tory use.
CAUTION
The AC power should be turned on
for at least 30 seconds before switch-
ing on the high voltage. This allows
the regulator circuits to reach their
proper operating point and avoids
exces sive voltage at the high voltage
terminals.
1-2 INSPECTION
This instrument was thoroughly tested and
inspected before being shipped and is ready
for use when received.
After the instrument is unpacked, it should
be inspected for damage received in transit.
If any shipping damage is found, follow the
procedure outlined in the "Claim for Damage
in Shipment" page at the backof this instruc-
tion manual.
1 -1

SECTION II
OPERATING INSTRUCTIONS
2-1 CONTROLS AND TERMINALS
AC POWER
This toggle switch controls the power sup-
plied to the instrument from the power line.
When the switch is in the ON position the
red indicator lamp above the switch will
glow.
LINE 4 AMP
The fus eholder, located on the control panel,
contains a 4 ampere cartridge fuse for the
protection of the whole instrument. The
fuse may be replaced by turning the fuse-
holder cap, removing the old fuse, and in-
serting a new fuse. Replacement fuses for
this instrument must be of the "Slo-Blo"
type as specified in the Replaceable Parts
List.
-300V 1/16 AMP
The fus eholder, located on the control panel,
contains a 1/16 ampere cartridge fuse for
the protection of the -300 volt fixed bias
supply. The fus e may be replaced by turn-
ing the fuseholder cap, removing the old
fuse, and inserting a new fuse.
NOTE
If this fuse blows, the positive
high - voltage regulator circuit
will drop the HV voltage level to
less than 100 volts, thus protect-
ing external equipment or cir-
cuits against the pos s ibility of
damage due to loss of bias •. When
the openfuse is replaced, all cir-
cuits restore to normal.
+HV 1/2 AMP
The fus eholder, located on the control panel,
contains a .5 amper e cartridge fus e for the
protection of the high voltage de cir cuit.
The fuse may be replaced by turning the
fuseholder cap, removing the old fuse, and
inserting a new fuse.
2-1
CAUTION
If this fuse blows, immediately
turn off the AC POW ER switch,
and then replace the fus e. Con-
tinued operation of the instrument
with the +HV 1/2 AMP fuse open
will damage the screen grids of
the series regulator tubes (V3,
V4, V5, V6).
HIGH VOLTAGE
This toggle switch turns the high voltage
direct current off and on. The switch also
controls the red indicator light above it on
the control panel. When the switch is on,
the indicator lamp glows as a warning that
the high voltage is turned on.
6.3V AC, 10 AMP
The two fuseholders, located on the control
panel, each contain a15 ampere cartridge
fus e for protection of the 6.3 volt AC fila-
ment supply circuit. Each fuse may be re-
placed by turning its fus eholder cap, re-
moving the old fuse, and inserting a new
fuse. Replacement fuses must be of the
"Slo-Blo" type as specified in the Replace-
able Parts List.
6.3V AC 10 AMP CT
These binding posts are terminals for the
unregulated AC filament supply.
6.3V AC 10 AMP
These are the terminals for the 6.3 volt
unr egulated alternating current.
CT
This binding post is the terminal for the
center tap of the secondary winding from
which the unregulated AC is taken.
-300V 50 MA 5!vIA GND
These binding posts are terminals for the
bias supply output and chassis ground.
-300V 50 MA
Output terminal for the fixed -300V regu-
lated voltage supply.

5 MA
Output terminal for the variable (0-150
volt) regulated bias voltage supply.
GND
Terminal for making connection to the
chassis. Anyone of the output terminals
may be connected to the chassis by con-
nection to terminal GND.
COM +HV GND
These binding posts are terminals for the
high voltage supply, common conductor,
and chassis ground.
+HV
Output terminal for the regulated 0-500
volt voltage supply.
COM
Output terminal common to the HV, 5 MA
(variable bias), and -300V 50 MA (fixed
bias) cir cuits •
GND
Terminal for making connection to the
chassis.
-BIAS CONTROL
This control varies the output of the regu-
lated bias voltage supply from 0 to 150 volts.
HV CONTROL
This control varies the output of the regu-
lated high voltage supply from 0 to 500 volts.
METER RANGE
This switch connects the regulated high
voltage supply circuit to the voltmeter on
either the 0-150V or 0-500V range. Opera-
tion of the switch to 0-150 -BIAS connects
the variable bias supply circuit to the volt-
meter.
2-2 OPERATION
CAUTION
Avoid electric shock by turning the
high voltage switch to the off posi-
tion before making any connections
to the instrument.
2-2
The procedure for operating the Model 712B
Power Supply is as follows:
a. Connect the power cable to aIlS V power
line and turn the AC POWER switch to
ON.
NOTE
The triple -conductor power cable is
supplied with the new NEMA approv-
ed three-prong plug. The third con-
ductor provides a chassis ground.
An adapter may be obtained to per-
mit use of this plug with two-conduc-
tor receptacles.
b. With the HIGH VOLTAGE switch in the
off position, connect the output terminals
to the load.
c. If the bias supply is to be us ed, set the
METER RANGE switch to the 0-150V
-BIAS position and adjust the -BIAS CONTROL
for the desired voltage.
NOTE
The internal impedance of the vari-
able bias circuit can be as high as
10, 000 ohms, depending on the set-
ting of -BIAS CONTROL.
d. Set the METER RANGE switch at the 0-
150V or 500V position, and adjust the
HV C ONTR OL for the des ired output voltage.
e. Turn the HIGH VOLTAGE switch to ON,
check the bias voltage, and readjust -BIAS
CONTROL if necessary.
f. If the Model 712B is to be used for an
application which requires a power sup-
ply of more than 500 volts, up to 300 volts
additional may be made available by connect-
ing the load across +HV and -300V 50 MA.
Ground may be connected to either terminal,
as desired.

SECTION III
CIRCUIT DESCRIPTION
3-1 GENERAL
The Model 7l2B Power Supply furnishes two
separate regulated supplies: a positive high
voltage in the 0-500 volt range and a bias
voltage. A block diagram of the Model 7l2B
is shown in Figure 1 •
Voltage level of the high voltage supply is ad-
justed by means of a panel knob, HV CON-
TROL, which controls the setting of variable
autotransformer Tl. The autotransformer
supplies primary voltage to the high voltage
power transformer, T2. The comparis.on
voltage in the regulator circuit is adjusted by
means of variable resistor R39 which is me-
chanically coupled to the autotransformer
shaft. This arrangement maintains the regu-
lator circuit at its optimum operating point,
regardless of output voltage level.
After the high voltage has been rectified (dual
diodes VI and V2) and filtered, it is applied
to a group of series regulator tubes (pentodes
V3-V6), to the DC milliammeter, and then
to the high voltage output terminal, +HV. The
conduction of the series regulator tubes is
controlled by a cascade differential amplifier
(dual triodes V12 and V13). The differential
amplifier compares a sample of the output
voltage with the voltage developed across
voltage reference tube VII; any difference
between thes e voltages is amplified and the
conductivity of the series regulator tubes is
changed accordingly.
The fixed bias voltage (-300V) supply con-
sists of a conventional power transformer,
full-wave rectifier (dual diode V7), and capa-
citor -input filter. The voltage is regulated
by means of a series regulator tube (pentode
VB), which is controlled in turn by a cascade
differential amplifier (dual triodes V9 and
VlO). Operation of the regulator circuit is
similar to the +HV regulator described above.
A source of variable bias voltage is provided
by means of a voltage divider (variable re-
sistor R46) which is connected between the
-300V 50 MA terminal and the COM terminal.
This voltage is regulated against line voltage
variations only. Since the internal impedance
may be as high as 10, 000 ohms, depending
upon the setting of resistor R46, the voltage
will depend upon the actual load current be-
ing drawn (5 rna. max.). This voltage can
be measured by turning the METER RANGE
switch to the -150 V -BIAS position.
3-2 HIGH VOLTAGE REGULATOR CIRCUIT
The regulator circuit is so arranged that a
slight voltage change in the sampled output
in the positive directionwill result in a rela-
tively large voltage swing in the negative di-
rection on the grids of the series regulator
tubes, and vice versa.
Figure 2 shows the arrangement of dual tri-
odes V13 and V12 which, connected in cascade,
form a two stage differential amplifier. In
each tube, cathode connections are common
for the two sections. Thus any variation in
plate current in one section appears across
the cathode resistor common to both sections,
and results in a difference voltage in each of
the output circuits.
In the first stage, a fixed bias supplied by
voltage reference tube VII is applied to one
grid of dual triode V13. The bias on the other
grid of V13 is a portion of the +HV voltage,
sampledwithrespectto the -300voltbias sup-.
ply through a voltage divider. Any variation
in the level of the output voltage results in
voltage changes in the plate circuits of both
sections of V13, and corresponding changes
in the bias es on the grids of dual triode VIZ in
the next stage.
For example, if the voltage of the sampled
output swings in the positive direction, in-
creased current will flow through the right-
hand section of V13, which will raise the cath-
ode voltage and lower the right-hand plate
(pin 1) voltage. Since the bias on the grid in
the left-hand section of V13 remains fixed,
the increas ed cathode voltage will decrease
conduction in the left-hand half of the tube,
which result in an increase in voltage on the
left-hand plate (pin 6).
3-1

/
OUTPUT I--~_-o_ +HV
CURRENT
HIGH
VOLTAGE
SUPPLY
HV
CONTROL
~------------------------~
I
SERIES !
1 '.
REGULATOR I
""'--+ ......... ~
r---..&.- __ ~
VARIABLE
..- I--
TRANSFORMER
DIFFERENTIAL
r--o+- AMPLIFIER ~
POWER
INPUT
115V
5O-IOOO/V
VOLTAGE
REFERENCE
TUBE
DIFFERENTIAL
~ AMPLIFIER
/
OUTPUT
VOLTAGE
-
-
+
-300
VOLT
SUPPLY
SERIES
REGULATOR
__ ._ ...... -------+--00_ COM
~
">4o-------4II~O_ 5 MA
o -BIAS CONTROL
lGND
-
-300V
--
50MA
-
6.3V AC
_ CT
IOAMP
-
'FILAMENT
TRANSFORMER
...... --------------------0
Figure 1. Model 7l2B Block Diagram
3-2

In the second stage, the right-hand section of
dual triode VIZ acts as a cathode follower to
increase the signal on the grid of the left-
hand section. The plate circuit in the left-
hand section supplies the bias for the grids
of the series regulator tubes. The plate cir-
cuits of the two sections of VIZ are cross-
coupled. This arrangement results in great-
er amplifier gain and sensitivity.
To ensure that the series regulator tubes will
have sufficient conductivity to maintain regu-
lation under transient conditions, a separate
screen voltage supply from selenium recti-
fier SRI is provided for the regulator tubes.
The voltage divider in the voltage compari-
son circuit provides separate paths to the
signal grid of dual triode V13 for high-fre-
quency and low-frequency voltages. to make
the response of the regulated supply fast at
high frequencies and yet very stable. At a-
bout 1, 000 cycles, AC components are cou-
pled to the grid via resistor R38 and capaci-
tor C4. DC components pass to the grid
through a resistor network (R38, R39, R40,
R4l) •
3-3
3-3 BIAS SUPPLY CIRCUIT
Arrangement and operation of the regulated
bias supply circuit is similar to that of the
plate voltage supply circuit except that pri-
mary current to the bias supply transform.er,
T3, is not variable. The bias supply series
regulator tube, V8, is controlled by a differ-
ential amplifier (dual triodes VlO and V9).
The comparison voltage for the circuit is
furnished by voltage reference tube VII, and
the sample voltage is sampled with respect
to COM th~ough a voltage divider which in-
cludes variable resistor RZ4. The fixed bias
voltage is brought out to binding post -300V
50 MA on the control panel.
A variable bias voltage is available at bind-
ing post 5 MA. The voltage level of the vari-
able bias supply may be adjusted by means of
the -BIAS CONTROL knob on the front panel.
The control varies the setting of variable re-
sistor R46 in the voltage divider across the
bias supply output circuit.

3
R6 R36
R45
M2
R38
R39
R40
R34
R35
2
Vl3
R42
1-300V I
50MA
Figure 2. Partial Schematic of High Voltage Supply Regulator Circuit
3.4

CAUTION
SECTION IV
MAINTENANCE
Tubes
Voltages which are dangerous to life
are present within the instrument.
Disconnect the power cable from the
power line before removing the cover
or bottom plate.
4-1 COVER AND BOnOM PLATE REMOVAL
Rack Model
To remove the cover, unscrew the four screws
which fasten the cover to the back of the in-
strument, and slide the cover toward the rear
of the instrument. To remove the bottom
plate, unscrew the four screws which fasten
the plate to the bottom of the instrument, and
lift off the plate.
Cabinet Model
a. On the rear of the instrument, remove
the four screws which fasten the rear
cover to the chassis. Remove the rear cover.
b. Turn the instrument on its back.
c. Remove the two3/16inchAllensetscrews
on the bottom, next to the control panel.
d. Lift the cabinet off the instrument.
4-2 REPLACEMENTS
Variable Transformer Brush
Inspect the brush contact at periodic intervals.
1£ the brush contact on transformer TI begins
to show excessive wear, purchase a new brush
assembly, for a Type 20 variable transform-
er, from the" Superior Electric Co., Bristol,
Conn. The brush contact is made of special
material and ordinary carbon will not func-
tion. Fit the brush to the commutator by
sanding with fine crocus cloth. Carbon par-
ticles must be blown off the commutator after
fitting the brush.
Any tube in the Model 712B may be replaced
with a tube having corresponding RETMA
standard characteristics.
All tubes are accessible when the cover plate
or cabinet is removed. Location of tubes is
shown in Figure 3.
Tubes V3, V 4, V5, or V6 may be replaced
with a 6L6, 6L6-G, 6L6-GB or 5881.
Replacement of tube VII may make it neces-
sary to readjust the output voltage of the fix-
ed, -300 volt bias ~upply (see Adjustments,
paragraph 4-3).
Replacement of tube VIO may make it neces-
sary to readjust the bias supply line regula-
tion (see Adjustments, paragraph 4-3).
Replacement of tube Vl3 may make it neces-
sary to readjust the HV supply line regula-
tion (see Adjustments, paragraph 4-3).
1£ ripple is present in the regulated voltage
after replacement of tube V9, VIO, Vl2, or
V13, probably it is due to excessive heater-
cathode leakage in the replacement tube.
Variable Resistors
Connections to all variable resistors are
shown in Figure 5.
Resistor R39 is ganged on the shaft with vari-
able transformer T1. Its location, behind
the control panel, is shown in Figure 3. The
procedure to be followed after a replacement
of variable resistor R39 is described under
Adjustments, paragraph 4-3. The location
of resistor R46, which is adjusted by the
-BIAS CONTROL knob, also is shown in Fig-
ure 3.
Location of resistors R40, R42, R24, RIO,
and R8 on the bottom of the instrument are
shown in Figure 4.
1£ resistor R24 is replaced, readjust the level
of the fixed -300 volt bias supply (see Adjust-
ments, paragraph 4-3).
4-1

~
I
tv
Figure
3.
Model
7l2B
Top
View
Cover
Plate
Removed
"

MODEL 712 B
If resistor R42 or resistor R40 is replaced,
readjust the maximum and minimum levels
of the HV supply output voltage (see Adjust-
ments, paragraph 4-3).
If resistor R8 is replaced, readjust the bias
supply line regulation, and if resistor RIO is
replaced, readjust the HV supply line regu-
lation (see Adjustments, paragraph 4-3).
Lamps
If either the AC POWER or HIGH VOLTAGE
lamp should burn out, replace with the 6-
volt lamp specified in the Replaceable Parts
List. The lamps are of the bayonet type and
are plugged into sockets accessible from the
front of the control panel. To remove a burn-
ed out lamp:
a. Turning in a counterclockwise direction,
unscrew the lamp cap and remove it.
b. Firmly press the light bulb into the soc-
ket, and then give the bulb a twist in the
counterclockwise direction to free it. Pick
the bulb out of the socket.
4-] ADJUSTMENTS
Auxiliary Equipment Required
1 - variable transformer
1 - AC voltmeter 0 -150 ) Hewlett-Packard
volts ) Model 4l0B or
1 - DC voltmeter 1000) equal.
volt range )
1 -
High impedance vacuum)
tube voltmeter )
1 -
300 volt battery
AC
VOLT
METER
VARIABLE
TRANSFORMER
NOTE
The settings of variable resistor R39
and the HV C ONTR OL stop ar e set
at the factory, and should never be
touched in the field unless trans-
former Tl or R39 should require re-
placement. Adjustment procedures
to be performed in the event of the
replacement of either component are
given at the end of this Adjustments
paragraph. If either Tl or R39 is
replaced, its respective adjustment
procedure should be performed be-
fore making the line regulation or
output voltage adjustments.
4-4 LINE REGULATION ADJUSTMENT
To adjust the Model 7l2B line regulation, it
is necessary to have (1) a stable source of
DC voltage for comparison purposes, (2) a
variable transformer, and (3) a sensitive
voltmeter (1.0 volt full scale) with an input
resistance of 1 megohm or greater (Model
4l0B). A convenient test set-up for measur-
ing line regulation is illustrated below. Con-
nections shown at the Model 7l2B are those
used when measuring and adjusting the HV
supply line regulation.
Adjustment of Bias Supply Line Regulation
a. Make connections as shown in the test
set - up below, except that the positive
side of the battery is to be connected to the
C OM terminal on the Model 7l2B control panel
and the negative terminal of the VTVM is to
be connected to the -300V 50 MA terminal.
V.T.V.M.
0-1 V
4-3

~
I
~
Figure
4.
Mode17l2B
Bottom
View
Bottom
Plate
Removed

b. Set the variable transformer to l03volts
and allow about 15 seconds for the cir-
cuit to stabilize. Note the reading of the
VTVM.
c. Change the variable setting to 128 volts.
Mter about 5 seconds the VTVMwill in-
dicate a temporary slow drift in the -300 volt
supply voltage. When the circuit again sta-
bilizes, note the new reading of the VTVM.
d. If the new reading of the VTVM differs
by more than. 01 - .02 volt, adjust vari-
able resistor R8 (see Figure 4), and repeat
steps band c.
f. Repeat steps b - d until the -circuit sta-
bilizes to within .01 - .02 volt for line
voltage changes of 103-128 volts.
Adjustment of HV Supply Line Regulation
a. Make connections as shown on the sug-
gested test set-up below.
b. The procedure is the same as for meas-
uring and adjusting the bias supply line
regulation except that variable resistor RIO
is adjusted to stabilize the HV supply circuit.
4-5 ADJUSTMENT OF FIXED -300 VOLT
BIAS SUPPLY OUTPUT VOLTAGE
a. Check that the Model 7l2B power cable is
disconnected from the power sour ce.
Remove the cover and bottom plates. Place
the instrument on its back so that both the
front panel and the under side of the instru-
ment will be accessible.
b. Attach a DC voltmeter (Model 4l0B) be-
tween the COM and -300V 50 MA termi-
nals on the control panel.
c. Connect a variable transformer between
the power source and the Model 7l2B
power cable. Turn the AC POWER switch to
ON, and allow the instrument to warm up for
about one minute. Adjust the variable trans-
former so that 115 volts is applied to the
Model 712B.
d. Adjust variable resistor R24 until the
test voltmeter reads -300 volts.
4-5
4-6 ADJUSTMENT OF HV SUPPLY OUTPUT
VOLTAGE
a. Set up the instrument as in steps a and c
of the previous adjustment procedure.
b. With the METER RANGE switch in the
0-500V position and the HV CONTROL
in its maximum clockwise position, adjust
variable resistor R42 (see Figure 4) until the
DC voltmeter on the control panel reads +500.
c. Rotate the HV CONTROLto its maximum
counterclockwise position (minimum out-
put) and switch the METER RANGE switch to
the 0-150Vposition. Adjust variable resistor
R 40 until the DC voltmeter reads 0 volts.
d. Since the settings of R40 and R42 are
interdependent, it may be necessary to
repeat steps band _c several times.
NOTE
The indicated meter zero could be
off a volt or so due to change in me-
ter position when instrument is on
its back.
4-7 SEnlNG VARIABLE RESISTOR R39
This adjustment is to be made only if vari-
able resistor R39 or transformer Tl has been
replaced. Resistor R39 is mechanically cou-
pled to the shaft of the variable transformer
Tl. The assembly is located behind the con-
trol panel (see Figure 3) above the deck, and
is accessible when the cover plate is removed.
a. Check that the power cable is disconnect-
ed down from the power source.
b. With an Allen wrench loosen the two set-
screws which secure the ~oupling to the
transformer shaft. (Do not loosen the set-
s crews which secure the coupling to the re-
sistor shaft.)
c. Set the HV CONTROL to its maximum
counterclockwise position. Set the ME-
TER RANGE switch to the 0-150V -BIAS posi-
tion.
d. Connect an ohmmeter across the R39
lead.

R38
PIN 2
VI3
R41
R45
PIN 2
VIO
TERM.D5
T5
RI
SRi
Figure 5. Model 7l2B Detail of Variable Resistors and Selenium Rectifier
4-6

e. By means of the coupling, adjust the posi-
tion of the resistor shaft until an ohm-
meter reading of £lli ohms is obtained.
f. Hold the shaft in this position, and tight-
en the setscrews which secure the cou-
pling to the transformer shaft. Remove the
ohmmeter.
g. Check and if necessary readjust the level
of the HV output as described in Adjust-
ment of HV Supply Output Voltage, paragraph
4-6.
4-8 SEnlNG THE STOP ON THE HV
CONTROL
This adjustment is to be made only if vari-
able transformer Tl has been replaced. The
movable stop is a small "C II clamp which is
attached to the wiper ring on the back of the
variable transformer.
a. Check that the power cable is dis connect-
ed from the power source.
b. Use an Allen wrench to loosen the set-
screw which secures the stop to the rim
of the transformer wiper ring, and remove
the stop.
4-7
c. Attach a DC voltmeter (Model 4l0B, 1,000
volt range) oetween terminal COM on the
control panel an4 socket pin 3 of tube V3.
d. Plug the Model 7l2B power cable into a
variable transformer set for 115 volts
(measured) •
e. Turn off the HIGH VOLTAGE switch.
Set the METER RANGE switch at 0-500V
and the HV C ONTR OL to its maximum coun-
terclockwise position. Turn the AC POWER
switch to ON, and allow at least a 3D-second
warm-up.
f. Advance the HV CONTROL in a clock-
wise direction until the test voltmeter
reads +380 volts.
g. Turn off the AC POWER switch, dis con-
nect the Model 7l2B power cable and at-
tach the stop to the transformer wiper ring
so that the movable stop rests against the
stationary stop in such a manner that the HV
CONTROL cannot return in a counterclock-
wise direction. Note that the stop sets the
minimum voltage applied to the series regu-
lator tubes.
h. Remove the voltmeter.
i. Check the setting of variable resistor
R39, as described above.

TROUBLE SHOOTING CHART
The following information is designed to aid in trouble shooting a defective instrument:
SYMPTOMS
Instrument NOT
operating, pilot
light NOT on.
Instrument NOT
operating, pilot
light ON.
POSSIBLE CAUSE
Line fuse Fl blown
due to defective fuse
or overload in either
power supply.
Defective tube or
component causing
improper operation.
TEST PROCEDURE
Replace fuse. If this fuse blows, re-
move tubes VI, V2, and V7, and again
replace fuse Fl. If this fuse blows,
it indicate s:
1. Short circuit in wiring associated
with power transformer T2 or T3.
2. Short circuit in filament wiring.
3. Transformer T2 or T3 is defective.
If fuse does not blow when rectifying
tubes are removed, it indicates:
1. Defective filter capacitor.
2. Short circuit in a tube.
3. Short circuit in the direct current
wiring.
To localis e the circuit in trouble,
measure the DC resistance across the
output from each of the three recti-
fiers. Disconnect the instrument from
the power line before making the mea-
surements.
Normal DC Resistances Across Power
Supply Circuits:
Recti-
Connect Ohm- Approx.Normal
fier
meter Across
Resist. (Ohms)
VI, V2 Socket pin 8,
50K
VI or V2, and
term. COM
V7
Socket pin 8
2M
V7,
and term.
-300V 50 MA
SRI
Terms. of
1.5M
capacitor C9
(connect meter
neg. lead to
junction of C 9
and R13)
Set METER RANGE switch at 0-150V
-BIAS and turn -BIAS CONTROL to its
maximum clockwise position. If volt-
meter on control panel does not deflect;
or reads les s than 150 volts, trouble is
in bias supply circuit. Check bias sup-
ply DC voltages against those indicated
on schematic diagr am (should agr ee
within ±10%).
4-8
REMEDIES
1. Locate and clear
short circuit.
2. Locate and clear
short circuit.
3. Replace defective
transformer.
1. Locate and replace
defective capacitor.
2. Locate and replace
defective tube.
3. Locate and clear
short circuit.
Locate defective com-
ponent, and replace.