Play Meter

Servicing remotely controlled wa ll
games can be a harrowing experi-
ence for a field mechanic with
limited electronics background . Be-
cause the system includes a trans-
mitter, a receiver and a logic system
(with its associated drivers and
display units), fault diagnoses can
often be a painstaking process of
trial and error. Fortunately there are
quite a few "tricks" to diagnosing
and repairing the little devils, so let's
take a look at the system and how to
troubleshoot it.
The majority of wall game prob-
lems are in the transmitter box. The
box is subject to great abuse at the
hands of players, and failure can
often be attributed to a fumbled
lateral across the back bar. The
shock from the drop can break the
crystal, and so the transmitter fails.
Crystals are a major replacement
item for the transmitter. (Batteries
and push button switches are also
high failure items but their diagnosis
is simple, and I'll say no more than
to keep them on hand at all times for
field service.)
One frequent field problem lies in
determining if the transmitter is
working properly. As a result of this
many servicemen have been haunt-
ing Radio Shack for C.B. goodies
like field strength meters, crystal
checkers , and the like . There is,
however, a highly simplified test for
the transmitter that you may already
be equipped to perform .
Through the miracle of Harmonic
Radiation , the transmission from the
" box" can be picked up on the high
end of your standard FM radio! Just
tune in around 108 MHz and, if
transmitter is functioning properly,
you'll here a tone modulated signal.
This is possible because the funda-
mental frequency of the transmitter
is approximately 27M Hz . This gives
it a fourth harmonic frequency of
(27 x 4 = ) 108 MHz. (See box .)
Since many vehicles are equipped
with combination AM -FM radios,
you may already have a mobile test
station that can give you positive
results without a penny of invest-
ment.
It is important to note that the
transmitter radiates a tone modu-
lated signal. In this respect, the FM
radio performs a function more
valuable than all those C.B. gad-
gets . The radio allows you to
actually hear what is coming from
the transmitter and so you can
instantly verify the presence of the
tone and its frequency. (Transmitter
frequency adjustment will be cov-
ered in Part II of this series. Watch
for it in next month's Update!)
WHAT IS A HARMONIC?
Harmonics occur in nature when-
ever oscillation takes place. Simply
stated, a harmonic has a frequency
that is an integral multiple of its
fundamental frequency. The organ-
ist, who might be playing the
fundamental note "A" (440 cycles
per second), can also bring in a
second harmonic (880 cycles per
second) or a third harmonic (1320
cycles per second) to enrich the
total sound of the note being
played .
We can make use of harmonic
radiation to receive the 27 MHz
-being transmitted by the "box ." A
standard broadcast FM radio is all
that is required to pick up the fourth
harmonic , as shown below.
FUNDAMENTAL FREQUENCY
27 MHz
SECOND HARMONIC
54MHz
THIRD HARMONIC
81 MHz
FOURTH HARMONIC
108 MHz
The tone is the "key" that
unlocks a circuit in the receiver
board . This allows a hi or lo signal to
pass to the logic circuits of the
game . If the tone is missing or
considerably off frequency, the
game will work erratically or not at
all. In Gremlin's Playball game, for
example, preswinging faults can be
traced to a bad switch, bad
batteries, and / or lack of audio
modulation .
Loss of the modulating signal can
almost invariably be traced to a
broken wire on the audio coil. (See
Figure 2. ) This can be easily
checked with an OHM meter which
should show 90 OHMS between
test point A and test point B, and
130 OHMS between test point A
and test point C. A broken coil can
be repaired if you can find the end of
the wire and perform micro-surgery
on it. The break usually occurs at a
post where vibration has broken the
connection .
Part II of this series will include a
step by step system for field
diagnoses and repair of wall game
transmitters, all centered around the
use of your FM radio . Tune in next
month!
~
Test Point B
Test Point C
~
L3
(antenna)
I
Test Point A
(
xtal ]
C9
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