Zenith Y504B - Page 12

GENERAL INFORMATION
MULTIPLEX THEORY
For all theory regarding the operation of the multiplex
circuits, refer to service manuals HF- 14 and HF- 14
Supplement.
EXTENSION SPEAKER SYSTEMS
It is recommended that only Zenith extension speakers
be used in conjunction with Zenith receivers. However,
if the technician should desire to use other speakers
it is imperative the total impedance be not less than
that impedance indicated on the back of each receiver
adjacent to the speaker terminal board.
MUTING CONTROL
The muting control is factory adjusted, and should not
require readjustment. However, if the receiver is oper 
ated in an extremely noisy area, there is a possibility
that there may be noise bursts of sufficient magnitude
to overcome this mute voltage ... when this occurs, the
Stereophonic FM Indicator will light up. To further cut
off the 19KC amplifier, carefully rotate the muting
control to cut off the light. This should only be done
when a stereo signal is on the air since the mute control
must only be advanced to a point where the Stereo Indi 
cator does not light up on noise, but it should not be
advanced to a point where the desired stereo signal is
cut off.
MULTIPLEX ALIGNMENT
These receivers have been properly aligned at the
factory and will not require further adjustment. As a
result, it is not recommended that any attempt be made
to alter the multiplex stages. However, should any ma 
jor components in these circuits require replacement or
should anyone tamper with the multiplex adjustments
then, of course, realignment will be necessary.
Zenith has designed and manufactured an SPTE-1
Multiplex Generator that can be used to properly align
the multiplex portion of these receivers. The multiplex
alignment procedure is included in later pages of this
manual. The SPTE-1 Multiplex Generator is available
at your Zenith Distributor.
ANTENNAS FOR STEREO FM
Due to the characteristics of the stereo FM system, it
will require more signal for proper performance than
does monaural FM. As a result, it may be necessary to
operate the stereo FM'receiver with an external antenna.
The necessity for an external antenna will be deter 
mined by the signal conditions at each individual in-
stallation.
EXTERNAL FM ANTENNA
If the receiver is operated in an area of either low sig 
nal strength, high noise, or where multipath (FM ghosts)
signals are present, a good external FM antenna will
be required. The necessity of an external antenna as a
result of weak signal or noise, will be quite evident
since the set will not limit, and/or noise will be quite
evident. It is extremely difficult to determine if multi 
path (FM ghosts) signals are present, however, should
the program material be distorted, the best manner to
decide if multipath signals are the cause of the problem,
is to connect an external FM antenna to the receiver.
Usually a TV antenna may be available for trial, but
even then the results can be misleading, since many
TV antennas are of low gain on FM frequencies.
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AUTOMATIC FREQUENCY CONTROL AFC
These receivers feature an automatic frequency con 
trol which automatically keeps your receiver on the
exact station frequency when you are tuned to an FM
station. To utilize this feature tune the receiver as
i A n ^^ ucted and tllen move the selector or mode switch to
AFC position.
When the desired FM station is a weak station, adja 
cent in frequency to a strong station, the AFC may
pull the tuning into the stronger station. Under these
conditions, place the switch in FM position and tune
the receiver as instructed.
FM CABINET ANTENNA
All console models contain an FM antenna built into
the cabinet. This antenna consists of a Folded Dipole
cut to the desired frequency, and attached to the inter 
nal periphery of the cabinet.
SIGNAL STRENGTH CHART
There are certain minimum voltages necessary for
proper stereo FM reception. To help determine if there
is sufficient signal available, the following developed
AGC voltage versus microvolt input voltage charts
have been compiled. Since the desired FM Station may
not always be operating in the stereo mode when an
installation is made, these AGC voltage measurements
have been taken with a monaural FM signal. The point
* of minimum AGC voltage necessary for good
stereo FM reception has been indicated on these charts.
For chassis 18YT20 & 20YT20 connect a V.T.V.M. to
the junction of 10K, 4.7 K and C30 5 mfd (test point B).
For chassis 23YT124, 26YT20 and 26YT21 connect a
V.T.V.M. to the forward AGC input feed-thru terminal
at the rear of the FM-RF tuner. (Test point B)
Chassis 18YT20 and 20YT20
Micro-volts
AGC Voltage
Input
at RF Coil
0
-1.4
25
-1.4
100
-1.52
200
-1.70
500
-1.78
1 K
-1.83
5 K
*-1.98
50 K
-2.22
100 K
-2.28
Chassis 23YT124,
26YT20 and 26YT21
Forward
AGC Voltage
At Tuner Forward
Micro-volts
AGC Input
Input
Feed-Thru Terminal
0
-1.0
25
-1.01
100
-1.12
200
-1.50
500
-1.81
1 K
*-1.98
5 K
-2.28
50 K
-3.25
100 K
-3.41