Zenith Y504B - Page 19

MULTIPLEX ALIGNMENT PROCEDURE
CHASSIS 23YT124, 26YT20 AND 26YT21
Using the Zenith FM multiplex signal generator, the
multiplex portion of Zenith or any FM multiplex re 
ceiver can be aligned, but first before any attempt is
made to do this it is necessary that the technician be
certain that the RF, IF, and ratio detector alignment
is correct, and that the receiver operates normally on
monaural signals.
to obtain S curve symmetry indicates it is most neces 
sary to adjust for maximum gain and at the same time
maintain linearity and symmetry. 10.7 megacycles
must be on the curve at the reference line. 10.5 mega 
cycles and 10.9 megacycles must be at the lower and
upper turn of the S curve respectively. Only when the
I. F. and ratio detector circuitry have been aligned in
accordance with these specifications should the tech 
nician proceed to align the multiplex portion of the
receiver.
PRELIMINARY PROCEDURES
Before using the Zenith FM multiplex signal genera 
tor it is recommended that it be connected to the
power source and turned on giving it a 10 to 20 min 
ute warmup period. This will allow ample time for
the RF audio and 19 KC oscillators to stabilize.
The following procedure is only necessary when the
generator has been received from the factory, or has
been subjected to a great deal of handling or trans 
portation vibration. Although the 19KC pilot genera-*
tor oscillator is extremely stable, there is always
the possibility that it could shift from its precisely'
assigned frequency. As a result we have a very
simple method to check the 19KC pilot frequency
using an FM multiplex receiver and FM multiplex
station as a frequency standard. Proceed as follows:
1. Tune your FM multiplex receiver to an FM multi
plex station and when the pilot lights up, this
indicates the 19KC pilot amplifier is functioning.
Since the 19KC sine wave is from the transmitter
it must be on frequency and can be used as a
reference standard. With a cable connect the
collector output of the 19KC amplifier to the
vertical input of a good oscilloscope.
2. On the multiplex generator set the pilot carrier
amplitude control to 10%. Place L-R, L+R and
67KC switches in OFF position and connect the
composite output terminal directly to the hori
zontal input of the oscilloscope. On the oscillo 
scope you will see an oval Lissajous figure
which should be motionless when the 19KC out 
put of the generator is synchronized with the
19KC signal from the transmitter. Should the
Lissajous figure rotate it will only be necessary
to adjust the pilot carrier frequency trimmer on
the multiplex generator with an IF alignment
wrench until the Lissajous figure ceases to ro 
tate. After the generator has been adjusted to
zero beat, disconnect all cables.
The multiplex generator provides a composite multi 
plex signal as well as an RF signal, FM modulated
by the composite multiplex signal.
The composite signal is very useful since it is an
excellent tool that can be used in signal tracing the
multiplex portion of the receiver. We do not recom 
mend that multiplex alignment be made using only the
composite signal injected at the output terminal of
the ratio detector tertiary winding, since there is
always some phase shift occurring in the RF, IF or
ratio detector circuits. As a result, multiplex align 
ment made by a signal injected at the ratio detector
would not be correct. For proper multiplex alignment
the composite signal must FM modulate the RF car 
rier and then be fed into the FM antenna terminals.
With the signal injected in this manner the multiplex
alignment would then be the best that could possibly
be obtained and separation would be the maximum for
this receiver.
RATIO DETECTOR CURVE
Because of the wide band pass required in the multi 
plex FM receiver, it is desirable to use an FM signal
generator having a deviation of at least 200 KC with
a sweep rate of 60 cycles, as well as an oscilloscope.
During the IF and ratio detector alignment it is not
only necessary to obtain maximum gain, but also
extremely important to maintain symmetry.
To help achieve this IF curve symmetry 10.6 and 10.8
megacycle markers must be symmetrically positioned
and the 10.7 megacycle marker must be at the center
of the curve; When aligning the ratio detector 10.5
and 10.9 megacycle markers are desirable to achieve
S curve symmetry. The pattern illustrating marker use