LTV G-187 User manual

INSTRUCTION BOOK
FOR
MODEL G-187 SPECIAL PURPOSE RECEIVER
LTV - Electrosystems, Inc.
(A Subsidiary of Ling-Temco-Vought)
TEMCO Aerosystems Division
P.O. Box 1056
Greenville, Texas 75402

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WARNING
This equipment employs voltages which are dangerous and may be fatal if
contacted by operating personnel. Extreme caution should be exercised
while working with this equipment with any of the protective covers removed.
NOTICE
This is not the original Istruction Manual of the LTV G-187 Receiver,
it is only an attempt to write one from scratch (but for sure it is very
close to the original manual however). It is based upon visual
inspection and visual circuit tracing on an existing and unmodified
LTV G-187 Receiver that has survived and upon the Instruction Manuals
of some Nems-Clarke units using similar components.
The work is dated 2010-2011 and the author is Paolo Viappiani
of La Spezia, Italy ([email protected]), a radio collector whose
only purpose was to share information on this particular unit.
It is a no-profit work and consequently in no way this manual may be sold
and/or made object of commercial transactions.
The work is still in progress, so any contribution and suggestion
is very welcome. The author also apologizes for eventual ad unwanted
mistakes, misprints or omissions of which he cannot be held as responsible.
La Spezia, Italy, March 2011

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TABLE OF CONTENTS
Paragraph Page
WARNING & NOTICE………………………………………………………. 2
SECTION 1 - GENERAL DESCRIPTION
1. PURPOSE OF EQUIPMENT............................................................……… 11
2. DESCRIPTION OF EQUIPMENT...................................................……… 11
SECTION 2 - THEORY OF OPERATION
1. ANALYSIS, MODEL G-187 RECEIVER……………………………………15
2. “MASTER” AND “SLAVE” RF TUNERS.......................................………...16
a. Antenna Inputs ..................... ...................................................... ………..16
b. First RF Stage ("Master or "Slave" Tuner)……………………………….16
c. Second RF Stage ("Master" or "Slave" Tuner)…………………………...17
d. Mixer ("Master " or "Slave" Tuner)............................................ ………..18
e. Local Oscillator ("Master" Tuner Only) ..................................... ………..18
f. Oscillator Buffer ("Slave" Tuner Only)...................................... ………...19
3. IF STAGES .................................................................................……..…19
a. Function Switch in 200 Kc. FM Position, Master IF Channel……………19
b. Function Switch in 200 Kc. AM Position, Master IF Channel…………...20
c. Function Switch in 200 Kc. FM or AM Position, Slave IF Channel……..20
d. Function Switch in 40 Kc. FM Position, Master IF Channel…………….20
e. Function Switch in 40 Kc. AM Position, Master IF Channel......……….21
f. Function Switch in 40 Kc. FM or AM Position, Slave IF Channel………21
4. OTHER CIRCUITS .....................................................................……….22
a. Video............................................................................................……….22
b. Squelch.........................................................................................……….22
c. Audio Amplifier...........................................................................……….24
d. Carrier Operated Relay (COR).....................................................……….24
e. Power Supply ............................................................................................26
f. Signal Strength and Tuning Meters............................................................27
SECTION 3 - OPERATION
1. INTRODUCTION ............................................................................. ………..28

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2. CONTROL SETTINGS ...................... ..............................................………..28
a. Power .........................................................................................………....28
b. Function Switch...........................................................................................28
c. Squelch.........................................................................................................28
d. Audio Gain....................................................................................................28
SECTION 4 - MAINTENANCE
1. INTRODUCTION........................................................................... .................29.
2. IF ALIGNMENT PROCEDURE (Sweep Method)
a. General Instructions........ .................. .......................................................30
b. Equipment Required . ................................................................………..30
c. Control Settings ............................... .......................................... ………..30
"Master" 200 Kc. IF Channel Alignment...........................................................31
d. Second Limiter, Adjustment of T-507........................................………..31
e. Discriminator, Adjustment of T-508 ..........................................………..31
f. First Limiter, Adjustment of T-505 and T-506 .........................………..31
g. Second IF Amplifier, Adjustment of T-503 and T-504 ............. ………..31
h. First IF Amplifier, Adjustment of T-501 and T-502 ..................………..31
"Slave" 200 Kc. IF Channel Alignment..............................................................32
d. Third IF amplifier, Adjustment of T-307 and T-308..................………..32
e. Third-Second IF amplifier, Adjustment of T-305 and T-306.... ………..32
f. Second IF Amplifier, Adjustment of T-303 and T-304 .............………..32
g. First IF Amplifier, Adjustment of T-301 and T-302 ..................………..32
"Master" 40 Kc. IF Channel Alignment.............................................................33
d. 40 Kc. Band-Pass Filter, Adjustment of L-606 and L-607.......................33.
e. 2.5 Mc. IF Amplifier...................................................................………..33
"Slave" 40 Kc. IF Channel Alignment................................................................34.
d. 40 Kc. Band-Pass Filter, Adjustment of L-602 and L-603........………..34
e. 2.5 Mc. IF Amplifier...................................................................………..34
3. LOCAL OSCILLATOR ADJUSTMENTS ........................................………..34
a. Local Oscillator Adjustment, "Master" RF Tuner only..............………..34
b. Mechanical Adjustments ............................................................………..34
4. RF AMPLIFIER ALIGNMENT………………………………………….…....35
a. RF Amplifier Alignment............................................................. ………..35
5. IF ALIGNMENT (CW Method) ......................................................... ………..36
a. Introduction................................................................................. ………..36
"Master" 200 Kc. IF Channel Alignment............................................. ………..36
b. Second Limiter Alignment - (CW Method)................................ ………..36

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c. Discriminator Alignment - (CW Method) .................................. ………..36
d. IF Amplifiers - (CW Method)......................................................………..36
"Slave" 200 Kc. IF Channel Alignment................................................ ………..39
b. Third IF amplifier, Adjustment of T-307 and T-308.................. ………..39
c. IF amplifiers.......................................…………………………………...39
"Master" 40 Kc. IF Channel Alignment.............................................................40
b. 40 Kc. Band-Pass Filter, Adjustment of L-606 and L-607.......................40
c. 2.5 Mc. IF Amplifier................................................................... ………..40
"Slave" 40 Kc. IF Channel Alignment................................................................41
b. 40 Kc. Band-Pass Filter, Adjustment of L-602 and L-603........ ………..41
c. 2.5 Mc. IF Amplifier................................................................... ………..41
SECTION 5 - PARTS LIST
a. Parts List...................................................................(currently under construction)
DISCLAIMER…………………………………………………………………… 75
LIST OF ILLUSTRATIONS
Figure Title Page
1-1 Model G-187 Special Purpose Receiver, Front View……………………. 7
2-1 Block Diagram, Model G-187 Receiver ........................................……… 14
2-2 Receiver Squelch Circuit, Simplified Schematic........................... ……… 23
2-3 Receiver COR Circuit, Simplified Schematic................................ ……… 24
4-1 Model G-187 Receiver, Top View (Dust Cover Removed)........... ……… 44
4-2 Model G-187 Receiver, Bottom View (Dust Cover Removed) .... ……… 45
4-3 Model G-187 Receiver, Rear View ............................................... ……… 46
4-4 Model G-187 Receiver, Panoramic Top View (Cover Removed)………... 47
4-5 Model G-187 Receiver, Panoramic Top View Left Side (C. R.)…………. 48
4-6 Model G-187 Receiver, Panoramic Top View Right Side (C. R.)………… 49
4-7 Model G-187 Receiver, Panoramic Bottom View (C. R.)………………… 50
4-8 Model G-187 Receiver, Panoramic Bottom View Left Side (C. R.)……… 51
4-9 Model G-187 Receiver, Panoramic Bott. View Right Side (C. R.)……… 52
4-10 Model G-187 Receiver, Large Component Board……………………….. 54
4-11 Model G-187 Receiver, Small Component Board………………………. 55
4-12 Model G-187 Receiver, Auxiliary & Output Circuits (Top View)……… 55
4-13 Model G-187 Receiver, Auxiliary & Output Circuits (Bott. View)…….. 55
4-14 Model G-187 Receiver, Auxiliary & Output Circuits (Bott. View)……. 56

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4-15 Master & Slave RF Tuners, Bottom View (Covers Removed)…………. 57
4-16 Master & Slave RF Tuners, Panoramic Bottom View (C. R.)…………. 58
4-17 Master RF Tuner, Bottom View (Cover Removed)………………………. . 59
4-18 Slave RF Tuner, Bottom View (Cover Removed)………………………….. 60
4-19 21.4 Mc - 200 Kc. BW IF Strip, Top View…………………………………. 61
4-20 21.4 Mc - 200 Kc. BW IF Strip, Bottom View……………………………… 61
4-21 21.4 Mc - 200 Kc. BW IF Strip, Bottom, Input Side…………………………62
4-22 21.4 Mc - 200 Kc. BW IF Strip Bottom, Output Side………………………. 62
4-23 2.5 Mc - 40 Kc. BW IF Strip, Top View……………………………………. 63
4-24 2.5 Mc - 40 Kc. BW IF Strip, CEI Label…………………………………… 63
4-25 2.5 Mc - 40 Kc. BW IF Strip,B.P.Filter Details……………………………. 63
4-26 2.5 Mc - 40 Kc. BW IF Master Channel Bottom View……………………. 64
4-27 2.5 Mc - 40 Kc. BW IF Slave Channel Bottom View……………............... 64
4-28 Controls in the Front Panel (Bottom Side)………………………………… 65
4-29 AGC “MON/DF” and Function Switches, Front Panel……………………. 65
4-30 Front Panel Function Switch, Rear View………………………………….. 65
4-31 AGC “MON/DF” Rotary Switch, rear View………………………………. 66
4-32 Power Supply Section, Top View………………………………………….. 66
4-33 Power Supply Section, Bottom View………………………………………. 66
5-1. Schematic Diagram: Master RF Tuner……………………………………… 67
5-2 Schematic Diagram: Slave RF Tuner………………………………………. 68
5-3 Schematic Diagram: 21.4 Mc.- 200 Kc. BW IF Strip………………………. 69
5-4 Schematic Diagram: 2.5 Mc.- 40 Kc. BW IF Strip………………................. 70
5-5 Schematic Diagram: Main Chassis Circuits………………………………… 71
5-6 Schematic Diagram: Mainframe…………………………………………….. 72
5-7 Schematic Diagram: Power Supply Circuits………………………………… 73
5-8 Schematic Diagram: Various Details…………………………………………74
LIST OF CHARTS AND TABLES
Table Title Pages
1-1 Performance Specifications ............................................................ ………. 8
1-2 Semiconductor and Tube Complement ......................................... …… 9, 10
4-1 Voltage Measurements, Model G-187 Receiver..............................…… 42, 43
5 Parts List............................................................(currently under construction)

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Figure 1-1. Model G-187 Special Purpose Receiver, Front View

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PERFORMANCE SPECIFICATIONS
Model G-187 Receiver
Tuning Range 55-260 Mc.
IF Rejection 70 dB, minimum
Image Rejection 58 dB, minimum
Noise Figure 6 dB, maximum
Absolute Sensitivity 4 uV produces at least 23 db S/N with 100 kc deviation, 1 kc
modulation frequency.
IF Frequencies 21.4 Mc. and 2.5 Mc.
IF Bandwidth 200 Kc and 40 Kc
FM Output 0.10 Volt per Kc, approximately
AM Outputs,
200 Kc and 40 Kc strips Approx. 10V RMS for 500 uV input modulated 50% at 1 Kc.
FM Output Stability Varies less than 2 dB for inputs above 1 uV.
AM Output Stability
200 kc and 40 kc strips 7 dB maximum variation for 40 dB variation in input.
Input (Ant.) Impedance Approx. 50 Ohms.
Video Response 10 Cps to 300 Kc
Power Input 115 VAC, 50-60 Cps (400 Cps on special order);
28 V (DC or AC) for front panel lamps.
Power Consumption 170 Watts
Weight 40 lbs.
Power Input connector: 12-pole round male DEUTSCH, “DS” Series (12-0), J-106;
Output connector: 19-pole round female DEUTSCH, “DS” Series (19-0), J-108.
Table 1-1. Performance Specifications

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Table 1-2. Semiconductor and Tube Complement.

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Table 1-2. Semiconductor and Tube Complement (continued).

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SECTION 1
GENERAL DESCRIPTION
1. PURPOSE OF EQUIPMENT
The Model G-187 Special Purpose Receiver has been specifically designed to meet the
requirements of a highly stable, extremely sensitive AM-FM receiver for critical application
in the 55 to 260 Mc. range and for Direction Finding purposes specifically.
The receiver has a self-contained power supply and is capable of operation from a power
source of 115 Volts +/- 10%, 50 to 60 cycles +/- 5%, single phase, alternating current.
The particular features of the G-187 receiver include the following: a dual-tuner and dual-IF
channel system; a carrier operated relay (COR) which may be used to control auxiliary
equipment; an audio squelch with adjustable threshold; FM and AM reception with very low
distortion with selective IF bandwidths of 200 or 40 Kc; a separate high-quality 600 ohm
audio output; a video output and a special control signal output (for direction-finding
purposes).
For further details concerning the capabilities and special features of the Model G-187
receiver, see table 1-1, Performance Specifications.
2. DESCRIPTION OF EQUIPMENT
The Model G-187 receiver is 7 inches high by 19 inches wide by 19-7/8 inches deep and it
weighs approximately 40 pounds.
Panel and chassis are of aluminum construction, and the front panel has a black plexiglass
outer covering with aeronautical-style grazing light lamps mounted on it.
The panel is designed for standard 19 inch relay rack mounting, although the receiver is
equipped with dust covers and side panels, and may be used independently on a shelf or table.
The receiver uses single-conversion (with an IF value of 21.4 Mc) for the 200 Kc. bandwidth
(AM or FM modes) and double-conversion (IF values: 21.4 Mc. and 2.5 Mc.) for the 40 Kc.
bandwidth (AM of FM modes).

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The two bandwidth values are obtained through separate IF channels (the 21.4 Mc. - 200 KHz
one using conventional miniature vacuum tubes, the 2.5 Mc. - 40 Kc. one using nuvistors
instead), by properly switching the H.V. supply to the involved subchassis.
Both the IF amplifiers are "doubled units" (one section is fed by the "Master" RF Tuner and
the other one by the "Slave" RF Tuner, so there are four IF channels total in the receiver).
For each mode and bandwidth, the "Master" RF-IF channel provides to the signal treatment
and intelligence, while the "Slave" RF-IF Channel produces special outputs (suitable for
direction finding purposes) and is also used for driving the signal strength meter and for AGC.
The two RF Tuners are identical each other but as for the oscillator circuit (a common
oscillator stage is employed for both the mixers and it is allocated in the "Master Tuner");
both of them make use of a 4-Section Mallory "Inductuner" each.
The RF Tuners employed in the G-187 receiver derive directly from the ones that Nems-
Clarke Co. used in their "1302" and "1502" receivers.
Inductuners are mechanically tuned variable inductors for VHF and UHF applications that are
tuned using a common rotary shaft moving contacts along single turn or spiral silver coils.
This is a product Mallory has been making since the 1940s and it is still used in military
communications equipment and precision test instruments.
The advantage of the Inductuner over other variable inductance devices is its stability and
repeatability; it operates on the principle that inductance of two lines, shorted at one end, is
proportional to the area that these lines enclose. Two such lines are laid concentrically on a
coil form and a shorting contactor, connected to the tuning shaft, is allowed to be moved
along their length, changing the inductance.
The tuning dial on the G-187 receiver series is a spiral scale printed on 1/8" thick translucent
plastic disk illuminated from behind. A small crescent-shaped window mask, sliding
vertically, tracks each spiral showing on the relevant part as the radio is tuned. This dial
mechanism is a common item in some Nems-Clarke and CEI equipments. All the receiver
circuits are built as completely shielded subassemblies with most of the audio and video

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components mounted on a single terminal board on the underside of the main chassis.
The G-187 receiver is not intended for CW reception and so it is not equipped with a BFO.
It is also worth to say that the audio output of the G-187 receiver is at a very low level; for
sure this receiver requires an external audio amplifier associated to a good speaker for a
comfortable listening.
Figure 1-1 shows a front view of the Model G-187 receiver, and table 1-2 shows the
semiconductor and tube complement.
As shown by the various pictures of the manual, the two RF Tuners are located in the center
of the G-187 chassis, while the 200 Kc. "Double- Channel" 21.4 Mc. strip is placed near the
left side of the receiver. The 40 Kc. "Double-Channel" nuvistor 2.5 Mc. IF subchassis is
directly fastened just to the left side panel by three 1/2"-turn screws.
Near the rear panel of the unit are located the video, audio, squelch and COR circuits, while
the power supply circuits are located in the right side of the main chassis instead.
All the connections between subchassis are made by coaxial cables or color-coded single
wires.
It's also useful to notice that the components employed in the various sections of the receiver
are identified by different "prefixes" according to the section itself, i.e.:
Prefix: Receiver Section:
1xx Main Chassis;
2xx Both the RF Tuners;
3xx 200 Kc. - 21.4 Mc. IF "Slave Channel"
5xx 200 Kc. - 21.4 Mc. IF "Master Channel"
6xx 40 Kc. - 2.5 Mc. IF Channels (Both "Master" and "Slave").
This means that, as an example, R-101 identifies a resistor located in the Main Chassis, L-
203 an inductor placed in one of the two RF Tuners, T-305 a trasformer located in the 200
Kc. - 21.4 Mc. IF subchassis ("Slave Channel") and so on.

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Fig. 2-1. Block Diagram, Model G-187 Receiver.

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SECTION 2
THEORY OF OPERATION
1. ANALYSIS, MODEL G-187 RECEIVER
a. A block diagram of the Model G-187 receiver is shown in Figure 2-1.
The circuit, with the function switch in the 200 Kc AM or FM position is a single
superheterodyne with an IF of 21.4 Mc. With the function switch in the 40 Kc AM or FM
position, a dual-conversion circuit is used, with a 21.4 Mc first IF, followed by a 2.5 Mc
second IF.
Each of the separate IF channels used for the available bandwidths has been built in a two-
section arrangement: one section is fed from the “Master Tuner” output and the other from the
“Slave Tuner” output. The “Master” section is used for FM and AM signal demodulation,
while the “Slave” section produces special outputs for AGC and Direction Finding purposes
and is not directly involved with signal intelligence.
The 2.5 Mc IF strips and double-conversion are used for the 40 Kc bandwidth only and make
use of a special kind of tubes (called “Nuvistor”) in a separate subchassis that is fastened to
the left side of the receiver. Such a subchassis is the “FM-75J-100 IF strip” that was built by
CEI.
The two RF tuners are designed to produce the lowest possible noise figure consistent with
the type tube used (a 416B first RF amplifier} and a practical tuning structure capable of
tuning 55 to 260 mc, with reasonably uniform performance over the band.
The basic tuning element in both the tuners is a Mallory type S-4 spiral inductuner. The input
circuit is broadly tuned by the first section of the inductuner. The second and third sections
double tune a bandpass filter to produce a high gain with a minimum response to spurious
signals. Adjustable coupling is provided by a capacitive "tee" section between the input and
output sections of the bandpass filter. The remaining inductuner section is used as the
oscillator tank inductance in the “Master” tuner and as a simple tuning device in the plate
circuit of the oscillator buffer in the “Slave” tuner.

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The IF amplifiers, with the function switch in the 200 Kc. FM position, use two stages of
amplification, cascade limiters and a phase-shift discriminator. With the function switch in the
200 Kc. AM position, AGC voltage is applied to the first two stages, and the second limiter
becomes the AM detector. With the function switch in the 40 Kc. FM or AM position, plate
voltage is removed from the 21.4 Mc IF amplifiers and applied to the 2.5 Mc dual conversion
mixer and IF amplifier channels.
The output signal of the two “Master” IF strips (200 Kc. AM and FM or 40 Kc. AM and FM)
drives a two-stage direct-coupled video amplifier with cathode follower output. A portion of
the follower output drives a four-stage squelch-audio amplifier circuit whose output is fed to
the J-108 connector (a 19-pole round female connector of the Deutsch “DS” Series) placed in
the rear apron of the receiver.
During either AM or FM operation the carrier operated relay (COR) is actuated by AGC
voltage from the stage which functions as either a detector or second limiter. The COR
controls a light and two sets of DT contacts (in the above mentioned J-108 connector) to
which auxiliary equipment can be connected.
To the same connector is also fed the output signal of the two “Slave” IF strips (200 Kc. AM
and FM or 40 Kc. AM and FM); it can be used for direction-finding purposes and is also
employed for the AGC circuits and for driving the signal strength meter.
2. "MASTER" AND "SLAVE" RF TUNERS
a. ANTENNA INPUTS
The main and auxiliary antenna inputs are two N-type connectors mounted on the chassis rear
apron, from which the signal is applied to each RF Tuner (“Master” and “Slave”).
The input impedance of each tuner is approximately 50 ohms over the frequency
range of 55 to 260 Mc.
b. FIRST RF STAGE (“MASTER” OR “SLAVE” TUNER)
The input signal is applied to the cathode of the 416B low noise planar triode.

17
To prevent loss of input signal due to cathode-to-filament capacity, the filaments are kept
above RF ground with broad-band chokes. The input resistance of the 416B is approximately
equal to 2/Gm when RL = RP, and in this case is 40 Ohms.
The cathode circuit is not tuned, due to the extreme bandwidth produced by the 40-Ohm shunt
load and the low cathode-to-ground capacity.
The plate tank circuit takes the form of a modified pi network and is used to couple the high
impedance plate circuit of the 416B tube to the low input impedance of the 6J4 grounded grid
second RF amplifier.
c. SECOND RF STAGE (“MASTER” OR “SLAVE” TUNER)
The output of the pi-network drives the cathode of the 6J4 grounded-grid second RF
amplifier. A low-noise second stage is used so that the system noise figure (first RF,
second RF, and mixer) is essentially that of the first stage.
The plate of the 6J4 is coupled to the grid of the 6AK5 pentode mixer by a double-tuned over-
coupled band-pass filter. A capacity "T" is used to provide coupling between the primary and
secondary tuned circuits. The shunt element of the "T" is adjustable, thus providing a control
over the interstage bandwidth.
A small iron-core inductor across the shunt element of the "T" network approaches parallel
resonance at 55 Mc, thus increasing the coupling at the low end and providing a more uniform
coupling over the tuning range of 55 to 260 Mc.
The single-tuned high Q plate circuit of the 416B tube is used to "fill in" the dip in the over-
coupled interstage network. The overall RF response when viewed at the mixer grid test point
is essentially flat over the band.
A convenient means for measuring the plate current of the 416B tube is made possible by TP-
201 at the junction of R-201 and R-202, the cathode bias resistors. A VTVM at TP-201 will
read the voltage drop across 100 Ohms. Thus 2V equals 20 mA, 5V equals 30 mA, etc.
The filament of the 416B is operated from a 12.6 volt winding of T-101 through a total series
dropping resistor of 5.1 Ohms. This produces a self-regulating effect, which extends the tube
life.

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A blower motor mounted on the front end assembly is used to cool the 416B tube. The motor
plugs into the main chassis with a Jones plug. A jumper between two of the pins of the Jones
plug removes plate voltage from the 416B when the motor is disconnected, thus protecting the
tube.
Positive grid bias of 8 Volts is applied from a voltage divider from the 150 Volt regulated B+.
This voltage is necessary to cancel the cathode self-bias voitage of 8.2 Volts so that the tube
will operate with approximately 0.2 Volts bias. The D-C degeneration due to the large
cathode resistor has a considerable stabilizing effect on the 416B tube and minimizes
performance variations from one tube to the next if replacement becomes necessary.
If, for any reason, the grid bias voltage is shorted or removed, the plate current is reduced and
the tube will not be damaged.
d. MIXER (“MASTER” OR “SLAVE” TUNER)
A 6AK5 pentode is used as a converter. The oscillator signal is injected into the grid circuit,
developing an operating bias proportional to the amplitude of the local oscillator signal. This
causes a minimum effect on the receiver operation due to variations in local oscillator
amplitude. A de-coupled test point (TP-202) from a tap on the mixer grid resistors provides a
convenient means for observing the response of the RF circuits.
The output signals from the mixers (IF signals) have a frequency of 21.4 Mc.
e. LOCAL OSCILLATOR (“MASTER” TUNER ONLY)
The local oscillator utilizes a 6AF4A tube in a modified Colpitts configuration. The end
inductors are made of heavy straps to insure frequency stability. The frequency stability of the
oscillator is very high due to the use of a high Gm tube which is loosely coupled to the high
Q tank circuit.
The Local Oscillator is present in the “Master” Tuner only and its signal is fed to the mixer
stages of both the tuners (directly in the “Master” Tuner and through a buffer stage in the
“Slave” Tuner). The oscillator signal is also fed (via the J-204 socket) to J-103, a BNC socket
in the rear apron of the receiver, so that the oscillator frequency can be precisely measured or
the signal used for other purposes.

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f. OSCILLATOR BUFFER (“SLAVE” TUNER ONLY)
A 6AK5 pentode is used in this stage as “oscillator buffer”, in order to avoid any loading
effect to the oscillator itself.
3. IF STAGES
The first 21.4 Mc IF transformers, located on the 200 Kc. IF subassembly (T-302 in the
“Master” channel and T-301 in the “Slave” channel) are connected to the respective mixer
plates through short lengths of RG-62/U coaxial cable that also carry the regulated voltage
(+150 V DC) to the mixer plates.
In addition the 21.4 Mc signal from the mixers is applied to the two channels of the
“Nuvistor” IF strip (via coaxial cables plugged into proper SMA sockets in the subchassis)
and reachs the 2.5 Mc mixer tubes (V-606/V-607 and V-601/V-602 for the “Master” and the
“Slave” channels respectively, all 6CW4’s).
The four-position “Function” switch also provides to apply the H.V. to the proper pair of IF
subchassis (200 Kc or 40 Kc IF subassemblies) depending upon the selected mode and
bandwidth.
a. FUNCTION SWITCH IN 200 Kc FM POSITION, MASTER IF CHANNEL
Two high gain stages (V-501, V-502) using 6DC6 tubes are followed by a 6CB6 (V-503) first
limiter and a 6AK5 (V-504) second limiter. A 6AL5 (V-505) is used in a discriminator circuit
of the phase-shift type. Accurate balance is obtained by using a bifilar winding for the
secondary. An automatic gain control voltage is derived from the first limiter grid circuit and
applied to the first IF amplifier stage. The first IF amplifier and the first limiter do not have
their cathode resistors bypassed, thus causing cathode degeneration which practically
eliminates the detuning caused by changes in tube input capacitance resulting from a change
in the bias voltage. Such circuitry is not necessary in the second limiter due to the different
tube structure and smaller change in grid bias. A minimum of approximately one volt bias is
on the second limiter due to grid rectification of noise signals, A self-resonant choke is
connected in the output lead of the discriminator to prevent IF signals from leaving the IF
subassembly.

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b. FUNCTION SWITCH IN 200 Kc AM POSITION, MASTER IF CHANNEL
The first two high-gain 6DC6 remote cutoff amplifiers (V-501, V-502) receive an AGC
voltage developed at the grid of the 6AK5 (V-504) AM detector. The third IF amplifier (V-
503) which drives the AM detector is not gain controlled but its signal handling capabilities
have been improved by increasing the screen voltage. A self-resonant choke is connected in
the AM output lead from the 6AK5 AM detector (V-504) to prevent IF signals from leaving
the IF subassembly.
With the AGC switch in the manual position the AGC voltage is shorted to ground, and the IF
gain control in the cathode circuit of the two 6DC6 IF amplifiers is unshorted and becomes
operative. The gain-controlled stages use cathode compensation of input capacity variation
with bias change. The zero center tuning meter operates only in the FM position. Correct
tuning of an AM signal may be accomplished by first tuning in the signal with the selector
switch in the FM position and then switching to the AM 200kc position.
c. FUNCTION SWITCH IN 200 Kc FM OR AM POSITION, SLAVE IF CHANNEL
Also the “Slave” 21.54 Mc IF channel has two high-gain stages (V-301, V-302) using 6DC6
remote cutoff amplifiers that can receive an AGC voltage The third IF amplifier (V-303)
drives a solid-state AM detector (CR-301) whose output is used for various purposes (AGC,
signal strength meter M-101, direction finding) .
The M-101 meter has a logarithmic characteristic and it is not calibrated, though it may be
used for a relative indication of signal strength.
d. FUNCTION SWITCH IN 40 Kc FM POSITION, MASTER IF CHANNEL
Plate and screen voltage are removed from both the channels of the 21.4 Mc IF amplifier
subchassis and a 21.4 Mc signal is coupled from T-602 to the grid of V-606 (6CW4), that
together with V-607 (6CW4) forms a 2.5 Mc cascode mixer. V-608 (6CW4) is the 18.9 Mc
crystal controlled 2nd conversion oscillator, that is capacitively coupled to the above
mentioned cascode circuit.
The output of the 2.5 Mc mixer is coupled to V-609 (7587), a 2.5 Mc amplifier, and then is
fed to V-610 (7587) through L-606 and L-607, which, together, comprise a 2.5 Mc double-
tuned 40 Kc. Pass-Band filter.