Heathkit IG 37 Manual

Page 1

" 7 "° IG-37 FM Stereo Generator "

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INTRODUCTION

The Heathkit Model IG-3'7 FM Stereo Generator
provides "on-the-air-type" signals for aligning
monophonic and stereophonic FM receivers. It
is a compact, accurate instrument which will
produce the following output signals:

A composite stereo signal for either left or
right channel alignment.

- A phase test signal (left plus right channel)
for accurate adjustment of subcarrier
transformers.

. A pilot signal with adjustable output level for
checking the lock- in range of stereo receiv-
ers.

- A monophonic FM signal that maybe modu-
lated by any one of three modulation fre-
quencies.

. A variable RF oscillator signal with adjust-
able sweep width and anominal frequency of
100 MHz.

- Four marker frequencies for RF alignment
checks.

. Two SCA (Subsidiary communications au-
thorization) signal frequencies for SCA fil-
ter adjustments.

- Completely shielded and selective RF atten-
uation.

The pilot signal and marker frequencies are
crystal controlled for maximum accuraCY and
minimum frequency drift.

The 16-37 FM Stereo Generator is a precision
instrument for service or engineering personnel
and other individuals who need an accurate
source of signals for the complete alignment of
monophonic or stereophonic FM receiving equip-
ment. The Generator is attractive, small in size,
and ruggedly built, and it shouldprovide you with
many years of trouble-free operation.

Refer to the "Kit Builders Guide" for complete
information on unpacking, parts identification,
tools, wiring, soldering, and step-by-step as-
sembly procedures.

Page 25

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Page 51

Switching Type

The switching type circuit is shown in simplified
Block Diagram form in Figure 16. The signal
from the FM detector, (L+R) + (L-R) + (19 kHz
pilot signal), is applied to the switching detector
circuit and to the 19 kHz filter and 38 kHz
subcarrier regenerator circuit. The 19 kHz pilot
signal is used to synchronize the 38 kHz oscilla-
tor signal of the receiver with the 38 kHz sub-
carrier of the multiplex signal. The 19 kHz pilot

signal is then filtered out. The 38 kHz oscillator
signal is applied to the switching circuit. Here it
is used to synchronize the left and right channel
switching detectors with the incoming waveform.
This causes the switching detectors to alter-
nately turn on and off. The left channel detector
operates only when the incoming signal carries
left channel information and the right channel
detector operates only when the incoming signal
contains right channel information. This pro-
duces an R and L signal output.

CIRCUIT DESCRIPTION

The following description of the FM Stereo
Generator describes each circuit in detail.
Refer to Block Diagram, Figure 17, and to the
Schematic (fold-out from Page 59) while reading
this section.

19 kHz CIRCUITS

A Piercecrystal-controlled oscillator, consist-
ing of tube VZA and its associated circuitry, is
used to generate the 19 kHz signal. Coil L1,
in the plate circuit of VZA, is used to tune the
circuit to resonance. A small portion of the 19
kHz signal is coupled through capacitor C36
and isolation resistor R58 to the grid of the 19
kHz buffer stage, V4A, The buffer stage is used
to isolate the 19 kHz oscillator circuit from the
phase shift circuit and the 19 kHz pilot ampli-

fier. The plate circuit of V4A is tuned by coil
L4. The output of the 19 kHz buffer stage is
coupled through capacitor C46 to a phase shift
network in the grid circuit of 19kHz pilot ampli-
fier VSB. The phase shift circuit is made up of
capacitors C9 and C10 and resistors R25, R26,
R27, and R28. To simulate a left channel and a
right channel signal, it is necessary to shift the
phase of the 19 kHz pilot signal by 45 degrees
to each side of zero phase. This is done by
selecting different combinations of resistors
and capacitors in the phase shift circuit with
the Function switch. The 19 kHz pilot amplifier
increases the pilot signal level and applies it to
the Function switch through R24, C11, and C12
to the grid of V5A, the composite audio ampli-
fier. Here it will be mixed with the output of the
balanced modulator circuit.

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F Igure 17