GENERAL RADIO COMPANY 1231-B User manual

OPERATING INSTRUCTIONS
TYPE 1231-B AMPLIFIER
AND NULL DETECTOR
GENERAL RADIO COMPANY

OPERATING INSTRUCTIONS
TYPE 1231-B AMPLIFIER
AND NULL DETECTOR
Form 634-H
May, 1961
GENERAL RADIO COMPANY
WEST CONCORD, MASSACHUSETTS, USA

TABLE OF CONTENTS
Section 1. INTRODUCTION •
1.1 Purpose .
1.2 Description.
1.2.1 General
1.2.2 Controls
1.2.3 Connections
1.2.4 Accessories .
Section 2. PRINCIPLES OF OPERATION.
2.1 General .
2.2
Linear Amplifier .
2.3
Semilogaritlunic Null Detector
2.4 Use with Type 874-LBA Slotted Line
2.5 Frequency Response.
1
1
1
1
1
2
3
3
3
3
3
4
4
Section 3. INSTALLATION 4
3.1
Battery . 4
3.2
A-C Power Supply. 4
3.3
Relay-Rack Model 5
Section 4. OPERATING PROCEDURE. 6
4.1
General . 6
4.2 Use as an Amplifier . 6
4.3
Use as a Null Detector 6
4.4
Operation as Standing-Wave Indicator • 6
4.4.1
Frequency Selection . 6
4.4.2 Test for Adequate R-F Input for Low VSWR
Measurement Assuming 50% Modulation. 7
4.4.3 Measuring Standing-Wave Ratio 7
Section 5. ACCESSORIES 7
5.1
Tuned Circuits. 7
5.2
Adjustable Attenuator 9
5.3 Adjustable Filter . 9
Section 6. SERVICE AND MAINTENANCE . 12
6.1 General .
6.2
Cabinet Removal
6.3 Tube Voltages and Resistance Measurements
6.4 Zero Coincidence.
6.5 Meter Sensitivity .
PARTS LIST
12
12
12
12
12
14

SPECIFICATIONS
TYPE 1231-B AMPLIFIER AND NULL DETECTOR
Input Impedance: 1 megohm
in parallel with 20J.Lj.Lf; alternate 10-megohm input available.
Maximum Gain: Greater than 83 db at 1 kc with 1-megohm load.
Meter
Scales: NORM scale, calibrated in volts with accuracy of ±5% of full scale; SENS
scale, calibrated in db with arbitrary zero. DB ratios accurate within 30% of correct value
in db, if one reading is above holf scale.
Null-Detector Sensitivity: Less than 25J.Lv input gives 1% indication on meter at 1 kc.
Amplifier
Sensitivity: Less than 8j.Lv input at 1 kc for 1% indication on SENS range of meter.
Output Impedance: Approximately 50,000 ohms.
Maximum Output Voltage: 5
volts into 20,000 ohms; 20 volts into one megohm.
Open-Circuit Noise and Hum Level: Less than 0.5 volt at full gain, battery operated; less
than one volt, a-c operated with Type 1261-A Power Supply.
Battery Life: 200 to 250 hours at 8 hours a day.
Dimensions: Height, 8 in.; width,
12-1/4 in.; depth, 10-3/4 in.; over-all.
Weight:
23-3/4 pounds, including batteries, {cabinet model).
TYPES 1231·P2 AND -P3 TUNED CIRCUITS
Tuning Accuracy: ±2% at normal voltage levels.
Attenuation: At least 25 db to second harmonic.
Dimensions: Height,
4-1/8 in.; width, 3-9/16 in.; depth, 4 in.; over-all.
Weight:
3-7/8 pounds.
TYPE 1231-P4 ADJUSTABLE ATTENUATOR
Source Impedance: 30 kfl, about equal
to output impedance of crystal detector in slotted
line.
Lood Impedance: At
least 1 megohm.
Insertion Loss: 3 db.
Attenuation Range: 80 db,
readable to nearest tenth db.
Attenuation Accuracy: ±0.3 db when operated between rated source and load impedances.
Additional errors caused by source between 14 kfl and 60 kfl are less than ±0.3 db.
Frequency Error: Negligible below 2 kc.
Maximum Input Power:
1/2 watt.
Terminals: Input, Type 938-W Binding Posts; output, shielded cable with Type 274-NK
shielded plug to fit Type 1231-B input terminals.
Accessories Required: One Type 874-R34 Patch Cord for connections between slotted
Iine and attenuator.
Dimensions: Height,
4-1/2 in.; width, 5-1/2 in.; depth, 5-1/2 in.; over-all.
Weight: 2 pounds,
11 ounces.

Figuro I, Typo 1 23 1 -B Amp lilior and Nu ll Ootoctar.
SPEC I FICATIONS <COHT)
TYPE 1231.P5 ADJUSTABLE Fll TER
Fr•queney Collbratlon: 50, 100, 200, 500 cps, 1, 2, 5, 10, 20, SO , 100 kc. %2% ot minimum
voltogu.
lnurtlon Gain: .7 to +15 db, depending upon ftoquoncy. Soeond·hormonic rojoction: 28
to 46 db.
Termlnols: Shielded cord and plug for connection to Typo 1231 ·6 jock·top terminals for
external capac it or s.
Acceuorln Availabl e: Type 219 Decode Copoeitou for tuning filter betwoon frequencies
provided.
Di me nsions: Front P anel: h eig ht, 7 in.; width, 6-1/8 in,,
Cabinet: depth, 9-3/4 in.
Inte rn al shiold box: hoight, 6-1/A in .; width, 4-1/2 in .; depth, 9 in.
W•lght: 9 poun ds, 12 ounces.

TYPE
1231-B AMPLIFIER
AND
NULL
DETECTOR
Section 1
INTRODUCTION
1.1 PURPOSE. The Type 1231-B Amplifier and Null Detector (Figure 1)
is designed for use asa high-gainamplifier, as asensitive nullindicator
for bridge measurements, and as a standing-wave indicator for measure-
ments with the Type 87 4- LBA Slotted Line. Used with a suitable crystal
detector, it will indicate amplitude-modulated high-frequency voltages,
and is also useful as a preamplifier for crystal microphones, vibration
pickups, and oscilloscopes.
1.2 DESCRIPTION.
1.2.1 GENERAL. The Type1231-BAmplifierand Null Detector is avail-
able in cabinet and relay- rack models, operated by either a battery
(Burgess 6TA60, Army Signal Corps BA48) or an a-c power supply (Type
1261-A
Power Supply). Type numbers used todesignatethevarious mod-
els are indicated below:
1231-BM
Cabinet model, battery operated
1231-BMA Cabinet model, a-c operated
1231-BR Relay-rack model, battery operated
1231-BRA Relay-rack model, a-c operated
1231-BRF Relay-rack model, battery operated with Type 1231-PS
Filter
1231-BRFA Relay-rack model, a-c operated with Type 1231-PS
Filter
1.2.2 CONTROLS. The following controls are on the panel of the Type
1231- B Amplifier and Null Detector:
1

GENERAL RADIO COMPANY
Name
Type FuncUon
GAIN
Continuous
Varies gain of amplifier or
rotary control
sensitivity of null detector.
ZERO
Cap-covered Makes the zero of the SENS
COINCIDENCE
screw-driver
meter range coincide with the
adjustment
zeros of the NORM meter
ranges and the null detector.
ZERO SET
Continuous
Sets electrical zero at meter
rotary control
zero.
AMPL METER
Push buttons (2)
Provide choice of range for
linear amplifier operation:
NORM range for monitoring
output during amplifier oper-
ation, SENS range for input
voltages too low for NORM
range or NULL DETECTOR.
NULL
Push button
Causes output stage to function
DETECTOR
as a logarithmic amplifier for
null-detector operation.
BATT CHECK
Push buttons (2)
Check condition of the plate
and filament batteries.
OFF
Push button
Turns off battery power supply.
INPUT
Push buttons (2) ~0.03 v - sets approximate
maximum no-load gain at 85
decibels with maximum input
voltage of less than 0. 03 volt.
<1 v - sets approximate
maximum no-load gain at 55
decibels with maximum in-
put voltage of less than 1 volt.
1MO- 10MO
Toggle switch
Selects input impedance of
the amplifier.
1.2.3 CONNECTIONS. The following connections are on the pane1of the
Type 1231-B Amplifier and Null Detector:
Name Type
Function
INPUT Jack-top binding posts (2) Input
terminals.
FILTER
Telephone jack
Provides for connecting a
filter to the output stage of
the instrument.
OUTPUT
Jack-top binding posts (2) Output terminals.
2

TYPE 1231-B AMPLIFIER AND NULL DETECTOR
1.2.4 ACCESSORIES. The following table lists accessories supplied
with the various models of the Type 1231-B Amplifier and Null Detector:
Type
Accessories Supplied
1231-BM
Cabinet
1231-BMA Cabinet, Type 1261-A Power Supply
1231-BR
Type 1231- P7 Connecting Panel
1231-BRA
Type 1261-A Power Supply, Type 1231-P7
Connecting Panel
1231-BRF
Type 1231-P5R Filter
1231-BRFA
Type 1261-A Power Supply, Type 1231-P5 Filter,
Type 1231- P6 Connecting Panel*
*for Type 1610-A Capacitance Measuring Assembly,
use Type 1231-P6-2 Connecting Panel
Section 2
PRINCIPLES OF OPERATION
2.1 GENERAL. The simplified circuit diagram (Figure 7) shows that
the amplifier has three stages and a built-in vacuum-tube voltmeter,
which consists of a diode rectifier, a d-e amplifier, and the panel meter.
2.2 LINEAR AMPLIFIER. During linear-amplifier operation, there is
a choice of maximum no-load gain of 55 or 85 decibels, as selected by
the input attenuator buttons at <1 volt or < 0.03 volt respectively. A
tapered wire-wound gain control allows continuous variation of the gain
over a wide range.
2. 3 SEMILOGARITHMIC NULL DETECTOR. During semilogarithmic
null-detector operation, rectified de from the vacuum -tube voltmeter
diode is applied as gain-controlling bias to the last amplifier stage to
produce the desired characteristics. An attenuator in the grid circuit
allows meter current to be zero with no signal at the instrument ter-
minals.
3

GENERAL RADIO COMPANY
2.4 USE WITH TYPE 874-LBA SLOTTED LINE. The Type 1231-B Am-
plifier can be used with the Type 1231-P2 Tuned Circuit and the Type
1231-
P4 Attenuator as a standing-wave indicator for the Type 874- LBA
Slotted Line. Since the amplifier should be tuned to the frequency of the
oscillator, choose a filter that selects the modulating frequency used
(refer to Section 5). Wit.'1 the amplifier gain setat maximum and theat-
tenuator set at 30 db, full-scale deflection on the amplifier meter extends
to r-f inputs with 50-percent modulation. For most accurate results,
the ratio of the inputs obtained at a maximum and at a minimum on the
line should be measured on the Type 1231- P4 Attenuator, rather than on
the meter scale. To determine this ratio, measure the difference in at-
tenuation required to produce the same meter reading for a voltage min-
imum as for a voltage maximum. If the crystal is operating in the
square-law region, the actual decibel difference in r-f voltage is half
the decibel difference measured by the attenuator or meter.
2.5 FREQUENCY RESPONSE. The maximum open-circuit voltage gain
is over 70 decibels at 10 cycles, over 45 decibels at 100 kilocycles, and
greater than 83 decibels at midband. The frequency characteristic is
shown in Figure 2.
~ 0
I
lll-1
0/
~-20
"'
a:
OJ•3
>
j::
5·40
II!
·5 0
.....-
IOc 20c SOc
-~---~
.........
""-
"""",
""'
IOOc 200c 500c Ike 2kc 5kc
IOkc 20kc 50kc IOOkc
FREQUENCY
Figure 2. Frequency Response Curve.
Section 3
INSTALLATION
3.1 BATTERY. Battery-operated models are shipped with the battery
in place. To install or replace battery (Burgess 6TA60, Army Signal
Corps BA48, or equivalent), first remove the instrument from the cab-
inet (refer to Section 6.2). Install the battery on the hangers at the rear
of the chassis and plug the battery cable into the socket.
3.2 A-C POWER SUPPLY. For a-c operation the Type 1261-A Power
Supply is provided in place of the battery. The 115-volt socket is in-
stalled above the hole in the base of the cabinet, where the line cord is
4

TYPE 1231-B AMPLIFIER AND NULL DETECTOR
inserted. The panel switch of the instrument must be left ON and the
switch in the line cord used to turn the power supply ON and OFF. With
both the panel switch and line cord switch ON, measure the filament volt·
age by pushing the F IL button. Note the meter reading. Push the button
on top
of the power supply and again note the meter reading. Tum the
screw - driver adjustment on top of the power supply until both meter
readings are equal. To keep the hum level low, do not ground the case
of the power supply to the instrument shield.
3.3 RELAY-RACK MODEL. The Type 1231 ·P7 Connecting Panel is in-
stalled to fit the standard 19-ln. relay-rack . Whenthe Type 1231-PSAd-
justable Filter is included, the Type 1231-P6 Connecting Panel Is used.
(See F lgure 3.)
Figure 3o. Typo 1231-P7 Connecting Panel.
Figure
3b. Type 1231-P6 Connecti ng Panel
and Type 1231-PS Filter.
5

GENERAL RADIO COMPANY
Section 4
OPERATING PROCEDURE
4.1 GENERAL. The following procedure should be followed when the
Type 1231-B is first set up.
a. Turn the power on by pushing either the NULL DET button or
one of the AMPL METER buttons.
b. Zero the meter by turning the ZERO SET knob with the INPUT
terminals short-circuited. If the zero of the SENS range does not coin-
cide with the zero of the NORM and NULL DET ranges, remove the
ZERO COINCIDENCE cap and set the screw- driver adjustment until
there is no change in meter indication as the meter is switched between
the SENS and NORM ranges.
c. To check the battery, push the BATT CHECK PL and FIL but-
tons, and note the meter indications. If the indication with either button
pushed is below the BATT mark on the meter, replace the battery.
d. Select the input impedance by means of the 1 MQ -10 MQ switch.
4.2 USE AS AN AMPLIFIER. For ordinary use, operate with the NORM
button pushed; then meter deflection is approximately linear, and full
scale corresponds to 20 volts. When the input voltage is too low to give
satisfactory meter indications, operate with the SENS button pushed.
The meter deflection is approximately linear, and full scale corresponds
to 2 volts.
4.3 USE AS A NULL DETECTOR. When using the Type 1231-B as a
null
detector first remove all load from the OUTPUT terminals. Then
select maximum no-loadgain by means of the INPUT attenuator buttons.
With the <: 1 v button pushed, maximum no-load gain is 55 db. With the
< 0.03 v button pushed, maximum no-load gain is 85 db and the meter
is more sensitive. Increasing the gain setting also increases sensitivity.
Use the NULL DET button for preliminary balances, the SENS but-
ton for final balance.
4.4 OPERATION AS STANDING-WAVEINDICATOR. (Refer to Operating
Instructions for Type 87 4- LBA Slotted Line.)
4.4.1 FREQUENCY SELECTION. Tune the amplifier to the modulation
frequency with an appropriate filter (refer to Section 5).
6

TYPE 1231-B AMPLIFIER AND NULL DETECTOR
4.4.2 TEST FOR ADEQUATE R-F INPUT FOR LOW VSWR MEASURE-
MENT ASSUMING 50% MODULATION.
a. Find the voltage maximum on the line.
b. Set the Type 1231-B for maximum sensitivity and set the Type
1231- P4 Attenuator for attenuation less than 30 db.
c. Find the voltage minimum on the line.
d. If the meter reading at the minimum is greater than one-fourth
of full scale, the r-f input to the line is adequate. If the meter reading
is less than one-fourth of full-scale, increase the r-f input or the probe
coupling.
4.4.3 MEASURING STANDING-WAVE RATIO. For most accurate re-
sults, the ratio of the outputs obtained at a maximum and at a minimum
on the line should be measured on the Type 1231-P4 Attenuator, rather
than on the meter scale.
a. At the voltage maximum, note the attenuation and the ampli-
fier meter reading.
b. At the voltage minimum, vary the attenuation of the Type 1231-
P4 Attenuator until the amplifier meter reading is the same as that noted
in step a.
c. If the crystal is operating in the linear region, the difference
in attenuation at these two points is the standing-wave ratio (SWR =20
E
log
10
Em~x) expressed in decibels.
m1n
d.
If the crystal is operating in the square-law region, the actual
db difference in r-f voltage is half of the db difference measured.
Section 5
ACCESSORIES
5.1 TUNED CffiCUITS. Types 1231-P2 (400 and 1000 cycles) and -P3
(60
cycles) Tuned Circuits (Figure 4) can be used as interstage filters
with the Type 1231-B for suppressing harmonics, noise, and hum in sin-
gle-frequency measurements. A telephone plug on each filter plugs into
a panel jack on the Type 1231-B Amplifier and Null Detector. Filters
thus connected are isolated by amplifier stages from the effects of vary-
ing input and output impedances and at the same time are at a high
7

0
1/1 I0
..J
llj
0
~ 20
!':
i5 30
Si
~
1- 4 0
!<i
50
/
GENERAL RADIO COMPANY
Figure 4. Ty pe 123 l.P3 Tuned Circuil
(ldenli ca l in A pp earance ro Typo 123l.P2).
TVI'E
TVPE
1231- P3 1231 · P2
J \
~
\
I 1/ -\
1/ v /
'\
1'\.
1- v
r-/
"'
""'
v /
""'
1'.::::
-
K
10 20 50 1 00 200 500 1 ,000 2POO 5poo 10,000
FREQUENCY IN CYCLES PER SECOND
FigureS . Attenuation Curve, T yp es 12 31 -P2
and -P3 Tunod Circ uit s.
enou gh voltage l evel along the amplifyin g chain to withstand effects of
external fields. Figure 5 shows the attenuation vs frequency for the
Types 1 23l ·P2 and ·P3 TUned Circuits.
5.2 ADJUSTABLE ATIENUATOR. When used with the Typel231-B am·
p ll fier and Null Detector and the Type 874- LBA Slotted Line to measure
standing -wave ratio, the Type 1231-P4 (Figure 6) Adjustable Attenuator
8

TYPE 1231-B AMPLIFIER AND NULL DETECTOR
Fi
;U<o 6. Ty.,- 1231-P4 Adjuuablo Allonuarar.
Increases both the range and accuracy of measurement over thatobtain-
able with the ampll!ler alone. Thls high-Impedance resistive voltage
divider covers a range of 80 decibels, with three 20-db steps and a 20-
db potentiometer.
5.3 ADJUSTABLE FILTER.
The Type 1231-PS Adjustable F ilt er ( Fig-
ure 3) Is a paro Jiol -reso n ant, sh ielded !liter tor reducing ha rmon ics
and background noise In bridge measurements. Designed pr im ar il y to
plug Into the Type 1231-B
Ampll!ler, the Type1231-PS Filter can be used
as an ant i resonant L-C element In the grid circuit of any Class-A am-
plifier where It Is not subjected to direct current. The Adjustable Filter
can be set to any one of 11 frequencies: 50, I00, 200, and 500 cycles, and
1, 2, 5, 10, 20, 50, and 100 kc. In
order to obtain high selectivity with
low lnsenlon loss, !our sepsrate Inductors are used:
L1 (20 henrys) for 50 and 100 cycles, centrlcore ;
L2 (2
henrys) for 200 and 500 cycles, duet core;
L3 (300 millihenrys) for 1, 2, and 5 kc, dust core;
L4 {IS millihenrys) for 10, 20, 50, and 100 kc, dust core.
The selector switch has eleven frequency positions, four Induct or posl-
tlons, andan OFF position. Maximum accuracy of the Indicated resonant
frequencies wi ll be attained only when the CU t er Is u sed with the Type
1231-B Ampl liler and Null Detecto r, and only when operating at vanish-
Ingly low l eve ls In balancing bridges or oth er null circ uits.
By the addition of external capacitance, this filter can also be tuned
to any other frequency from 20 cycles to 1 00 kc. Both terminals of the
9

GENERAL RADIO COMPANY
filter are available on the front panel for attaching external capacitors.
To choose the proper inductor for any operating frequency, take a com-
promise point of tolerable insertion loss and desirable discrimination
against the second harmonic. Data for determining this point for any
frequency are given in the graphs of Figure 7.
For convenience, the ranges of external capacitance needed to tune
over the indicated ranges of frequency are given in the following table.
In most cases these values could be obtained with the Type 1419-A Dec-
ade Capacitor. Resonance for any specific frequency is then indicated by
a maximum of observed gain.
To Tune Down
Set Switch And Add External
From
To
At
Capacitance (!Jf)
50 c 20 c 50 c 0 to 2.66
100c
50 c
100 c 0 to 0.381
200 c
100 c
200 c 0 to 0.951
500 c 200 c 500 c 0 to
0.266
1
kc
500 c 1
kc
0 to 0.238
2
kc
1 kc 2 kc
0 to
0.0634
5 kc 2 kc
5kc
0
to 0.0177
10 kc
5 kc 10 kc
0 to
0.0507
20
kc 10 kc
20 kc
0
to 0.0127
50
kc 20kc 50 kc
0
to 0.00354
100
kc 50 kc
100 kc
0
to 0.00051
Typical low-level characteristics of the Type 1231-P5 Filter, when
used with the Type 1231-B Amplifier and Null Detector, are given in
Table 1.
TABLE 1.
Inductor Resonant Resonant Insertion.
Discrimination
Frequency
Used ZMn
Q
db
vs Second Har-
monic db
50 c L1 0.28 45 6.4 loss
33
100 c L1 1.02
81
2.7 loss 31
200 c L2 0.070
27.6
15.0 loss 30
500 c
L2 0.47 75 7.3 loss 32
1
kc
L3 0.26 138
7.8 loss 42
2
kc
L3 0.86 228 3.0 loss
40
5kc
L3
2.70 287
1.9 gain 34
10 kc
L4
0.17 175 3.8
loss 46
20 kc
L4
0:50 262 2.2 gain 44
50 kc
L4 1.06
214 5.6
gain 37
100 kc
L4
2.13 192 7.4 gain
28
10

TYPE 1231-B AMPLIFIER AND NULL DETECTOR
Figure 7a.
Insertion Loss,
Type 1231-PS Filter.
48
0 44
~
~ 40
"0
tG 36
i5 32
!;i
~ 28
~
0 24
..c
"0
20
I
/
.......
"\i
v
/\
1\
\
~
z
0
i=
8
0
4
8
ffi 12
(/)
~
,g 16
20
/
/
I
I
/LI
/ -
I
L2
v
I I
)'
/ /
/ v
/ I
/ v v
L
I I I
I
/_
/L3
i/L4
I
02 .05 0.1 0.2 0.5 I 2 5 10 20 00 100
/
/ ~
1\
\
'\
L2
\
RESONANT FREQl£NCY- KILOCYCLES
v-
"'
~
~3
\
\ \\
L4\
Figure 7b.
Second Harmonic
Discrimination,
Type 1231-PS Filter.
02 .05 .10 .20 .50 I 2 5 10 20 50 100
RESONANT FREQUENCY KILOC'ICLES
11

GENERAL RADIO COMPANY
Section 6
SERVICE AND MAINTENANCE
6.1 GENERAL. The two-year warranty given with every General Radio
instrument attests the quality of materials and workmanship in our prod-
ucts. When difficulties do occur, our service engineers will assist in
any way possible.
In case of difficulties that cannot be eliminated bythe use of these
service instructions, please write or phone our Service Department,
giving full information of the trouble and of steps taken to remedy it. Be
sure to mention the serial and type numbers of the instrument.
Before returning an instrument to General Radio for service,
please write to our Service Department or nearest district office (see
back cover), requesting a Returned Material Tag. Use of this tag will
insure proper handling and identification. For instruments not covered
by the warranty, a purchase order should be forwarded to avoid unne-
cessary delay.
6.2 CABINET REMOVAL. To remove the instrument from the cabinet,
remove the black thumbscrew from each comer of the panel and slide
the instrument out of the cabinet.
6.3 TUBE VOLTAGES AND RESISTANCE MEASUREMENTS. Table 2
gives the normal d-e voltages and d-e resistances from various tube-
socket pins to ground. A deviation of 10 percent from any of these val-
ues is not necessarily abnormal.
6.4 ZERO COINCIDENCE. If either the battery, V3, V4, or the Type
1N56-A diode is replaced, or as the battery ages, the ZERO COINCI-
DENCE
control should be readjusted to make the electrical zero of the
AMPL METER SENS range coincide with the zeros of the NORM and
NULL DET ranges. The ZERO COI1'JCIDENCE control is a screw-driver
adjustment accessible through a ~ble in the front panel behind a snap
button cover. With the instrument' INPUT terminals short-circuited or
with the GAIN control turned to sero, adjust the ZERO COINCIDENCE
potentiometer until there is no change in meter indication as the instru-
ment is switched between the SENS and NORM ranges.
6.5 METER SENSITMTY. R19 is a factory-adjusted control, which
normally does not require readjustment. If its setting has been disturbed
it may be reset as follows:
a. Push the AMPL METER NORM button. Turn the GAIN control
to zero and adjust the ZERO SET control for zero reading on the NORM
meter scale.
12

TYPE 1231-B AMPLIFIER AND NULL DETECTOR
b. Reset the GAIN control to maximum (10). Apply a 1-kc signal
to the INPUT of the amplifier and increase the input voltage to give ex-
actly 20 volts at the amplifier OUTPUT terminals, measured with a vacu-
um-tube voltmeter.
c. Adjust potentiometer R19 (mounted above ZERO COINCIDENCE
potentiometer) until the amplifier meter reads exactly full scale for a
20-volt reading on the voltmeter.
TABLE 2
TEST VOLTAGES AND RESISTANCES
TUBE
(TYPE)
V1
(1U4)
V2
(1U4)
V3
(1U4)
V4
(1U4)
NOTES:
Batteries: A +1.5 v
B
+90v
PIN
1
2
3
6
7
1
2
3
6
7
1
2
3
6
7
1
2
3
6
7
No
batteries for resis-
tance measurements.
Panel controls and switch
settings: GAIN = 0
VOLTS RESISTANCE
TO GROUND TO GROUND
0
+38
+37
0
+1.5
0
+42.5
+26
0
+1.5
0
+53
+90
-1.1
+1.5
0
+90
+90
-1.1
+1.5
0
160 k
350 k
30 k
3.5
0
660 k
2.3 M
0
3.5
0
50 k
0
2.2 M
3.5
0
1.6k
1.6k
4.7 M
3.5
For resistance measurements,
short terminal 2 to 4 and 1 to 5
on
PLl.
INPUT =1 v, 1 MU
AMPL =NORM (20 v)
Voltmeter input resistance
should be at least 10 times the
value given in the RESISTANCE
column.
13

GENERAL RADIO COMPANY
PARTS LIST
PART NO. (NOTE A)
RESISTORS. k = kilohms,
M =Megohms
Rl
1M
± 5%
1/2 w
REG-20BF
RIA
10M
± 5%
1/2 w
REC-20BF
R2 30 k ±
5%
1/2 w
REC-20BF
R2A 300 k ±
5%
1/2 w
REC-20BF
R3 270 k ±10%
1/2 w
REC-20BF
R4 56 k ±10%
1/2 w
REC-20BF
RS
47 k ±
5%
1/2 w
REC-20BF
R6 100 k ±
2%
976-400
R7 2.2 M ±10%
1/2 w
REC-20BF
R8
lOOk
±10%
1/2 w
REC-20BF
R9 560 k ±10%
1/2 w
REC-20BF
Rl2
2.2 M ±10%
1/2 w
REC-20BF
Rl3
2.2 M ±10%
1/2 w
REC-20BF
Rl4
47 M ±
5%
1/2 w
REC-20BF
Rl5
47 M ±10%
1/2 w
REC-20BF
Rl6
4.7 M ±10% 1/2 w
REC-20BF
Rl7
50 k POSC-11
Rl8
l.SM ±
5%
1/2 w
REC-20BF
Rl9
50 POSC-11
R20 9.1 k ± 1%
1/2 w
REF-70
R21 680 k ± 1%
1/2 w
REF-70
R22
Sk
± 5%
973-410
R23 1 k ±
5%
1/2 w
REC-20BF
R24 4.7 k ±10%
1/2 w
REC-20BF
R25
12 k ±
5% 1/2 w
REC-20BF
R26 82 k ±
5%
1/2 w
REC-20BF
R27 47 M ±
5%
1/2 w
REC-20BF
R28 47 M ±
5% 1/2 w
REC-20BF
R29 2.7 ±10%
1/2 w
REW-3C
CAPACITORS. All capacitances are
in fJf unless otherwise indicated.
Cl
0.047
±10% 600 dcwv
COL-71
C2
0.2 Part of COWB-6
C3 2
Part of COWB-6
C4
0.25
±10% 600 dcwv
COL-43
cs
0.2 Part of COWB-6
C6
1
Part of COWB-6
C7
0.05
±10% 600 dcwv COM-SOB
14

TYPE 1231-B AMPLIFIER AND NULL DETECTOR
PARTS LIST (CONT)
CAPACITORS (Continued)
C8 o.os
±10% 600 dcwv COM-SOB
ClO 0.01 ±10% 600 dcwv COM-4SB
Cll
0.1 ±10% 200 dcwv COW-16
Cl2
0.8 ±10% 300 dcwv COW-4
Cl3
2
Part of COWB- 6
Cl4
soo fJfJf
±10% 300 dcwv COM-20B
CIS
16
ISO dcwv COE-4
MISCELLANEOUS
BATTERY
Burgess 6TA60
Bl BATTERY,
1-1/2 v 2BAC-4
Dl
DIODE 1NS6-A
Jl
JACK CDSJ-820
Ml
METER, 200 f.la
MEDS-36
PLl
PLUG CDMP-1344
Sl SWITCH SWPM-14
S2 SWITCH SWT-33SNP
Vl TUBE 1U4
V2 TUBE 1U4
V3 TUBE 1U4
V4 TUBE 1U4
vs
REGULATOR
NE-2
NOTES
(A) Type designations for resistors and capacitors
are as follows:
COE - Capacitor, electrolytic
COL - Capacitor, oil
COM - Capacitor, mica
COW - Capacitor, wax
POSC - Potentiometer, composition
REC - Resistor, composition
REF - Resistor, film