Zenith D952X - Page 19

SECTION THREE
MULTIPLEX ALIGNMENT PROCEDURE
Before any attempt is made to align, or service, FM Multiplex
circuitry, the technician must be certain that the RF, IF, and
Ratio Detector alignment is correct, and that the receiver
functions normally on monaural signals.
Because of the wide band pass required in a FM multiplex
receiver, it is desirable to use a FM signal generator having a
deviation of at least 200 KHz with a sweep rate of 60 Hertz, 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 IF curve symmetry 10.6 and 10.8 megahertz
markers must be positioned symmetrically to each side of the
10.7 megahertz marker, which must be at the center of the
curve. When aligning the Ratio Detector, 10.5 and 10.9 megahertz
17
markers are desirable to achieve S curve symmetry. The
pattern illustrating marker use to obtain S curve symmetry
indicates it is most necessary to adjust for maximum gain and
at the same time maintain linearity and symmetry. 10.7
megahertz must be on the curve at the reference line. 10.5
megahertz and 10.9 megahertz 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 technician proceed to align the
multiplex portion of the receiver.
Preliminary Procedures
Before using a FM multiplex signal generator, it is recom 
mended that it be turned on, giving it a 10 to 20 minute
warmup period. This will allow ample time for the RF, audio,
and 19kHz oscillators to stabilize.
Although the 19kHz pilot generator oscillator is extremely
stable, there is always the possibility that it could shift from
its precisely assigned frequency. There is a very simple method
to check the 19kHz pilot frequency using a FM multiplex
receiver and a FM multiplex station as a frequency standard.
Proceed as follows:
1. Tune your FM multiplex receiver to a FM multiplex
station. When the pilot indicator lights up, it indicates the
19 kHz pilot amplifier is functioning. Since the 19kllz sine
wave is from the transmitter it must be on frequency and
can be used as a reference standard. Connect a cable
between the output of the 19kHz amplifier and the vertical
input of a good oscilloscope.
2. Connect the composite output terminal of the generator,
directly to the horizontal input of the oscilloscope. Apply a
10% pilot carrier. The oscilloscope will display an oval
Lissajous figure which should be motionless when the
19kHz output of the generator is synchronized with the
19kHz signal from the transmitter. Should the Lissajous
figure rotate, it will only be necessary to adjust the pilot
carrier frequency adjustment on the multiplex generator,
until the Lissajous figure ceases to rotate. Refer to
generator instruction book after the generator has been
adjusted to zero beat, disconnect all cables.
Multiplex generators provide a composite multiplex signal as
well as an RF signal, which is 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 recommend 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
alignment 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 carrier 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
at the maximum for this receiver.
The RF signal should be injected at a point in the FM band
where no other signal is present. If at all possible this should
be at a frequency near the middle of the FM band. Tune the
FM receiver to this point and adjust the RF frequency
adjustment on the generator to this same frequency. The AGC
voltage developed in the receiver should be maximum. AGC
voltage substantially less than this will indicate the RF
frequency adjustment is tuned to an image.