TECHNICAL TOPICS
BY Randy Fromm
Servicing wall game transmitters
is an easy, on -location job if you
follow a system of fault diagnosis
and repair . Once one determines
that it is the transmitter that is
probably faulty (if e.g., the game
gives credit but doesn't play or plays
only intermittently), step number
one is to tune the F.M. radio in your
service vehicle to the fourth har-
monic at 108 MHz . This will enable
you to determine the presence of
the carrier ( R. F.) transmission and
the one signal that modulates it .
If you cannot receive a signal
from the transmitter, your fault
probably lies in the R. F. (Radio
Frequency) section. Since the crys-
tal is the most subject to failure due
to shock, try replacing it first. The
crystals used are of standard
frequencies for radio controlled
models and can often be obtained
from hobby shops. This can come in
handy if you're stuck with a busted
transmitter and no spare crystals.
If a new crystal doesn' t fix the
problem, examine (and resolder if
necessary) the soldered connections
on the bottom of the transmitter's
printed circuit board . Pay particular-
ly close attention to the antenna
connections . Since the antenna also
carries the D.C. power for the final
R.F. amplifier, the connections
must be well soldered . Repeated
droppings can cause the antenna
connections to crack, and so power
to the final amplifier is lost.
If the R. F. section of the
transmitter is functioning normally
but the audio tone is missing, you
will hear a quieting effect on the
F.M . radio . This is because the
unmodulated signal from the "box"
is cutting out the background noise
without carrying any audio informa-
tion of its own . If your transmitter
has lost its voice, follow the test
procedure outlined in last month's
Update.
If you service a large number of
wall games, always make sure that
you carry the following replacement
parts. With just these four items,
you can repair at least 75 per cent of
your transmitters on location! The
list is in order of failure rate .
1.Batteries. Use alkaline or, even
better, mercury cells . The power
curve for mercury batteries is
excellent for use in wall game
transmitters. They provide steady
current and voltage to the very end
of their chemical energy and then
drop like a rock. Since service calls
cost money, don 't buy cheap
batteries!
2 . Crystal. As mentioned before,
the major cause of R. F. failure is this
little guy . If you carry spare crystals
with you , for heaven 's sake don 't
throw them in the box with your
heavy tools . Put them somewhere
that will protect them from the hard
knocks of field service.
3.Switch. The originally equipped
switch can be disassembled and
cleaned if no replacement is avail-
able. Some operators prefer to use
an enclosed pushbutton that mounts
from the front instead of from the
rear . This prevents the player from
unscrewing the nut on the top and
causing a service call when the
button drops inside the box . An
enclosed switch will also keep dust
and beer from interfering with its
operation . One suitable replacement
switch is a #914 by Switchcraft . You
can get these for about a quarter
each at electronics stores.
4. Audio Coil. If the broken wire
cannot be found and repaired,
unsolder the bad coil and replace it
with a new one. (You may have to
order this part through your dis-
tributor .) If you do replace the audio
coil, the new coil will not be tuned
and you will have to . . . tune the
transmitter!
The tuning procedure is a lot less
involved than it might sound . It is, in
fact, a lot of fun if you ' re at all
musically inclined . All that is
required is a working transmitter of
the same type , a hex alignment
tool - one is supplied with Motorola
monitors, so you surely have one
around somewhere - and your F.M.
radio.
Start by tuning in the working
transmitter on your radio . Note the
tone you hear . Next, energize the
other transmitter simultaneously
with the first . Using the hex
alignment tool, rotate the "slug"t in
the audio coil until the audio tone of
the transmitter being tuned equals
that of the working transmitter. This
point can be easily identified by an
effect known as " zero-beat."
As the two tones approach each
other in frequency, they will seem to
pulsate at a rate which decreases as
they get closer together . This is
because the tones "hetrodyne"
against each other and produce a
third tone which equals the dif-
ference of the two . As the dif-
ference decreases, this hetrodyne
beat slows down until a zero-beat is
achieved . At this point you will hear
one steady tone and your transmit-
ter will be tuned .
Verify that the two tones are the
same by energizing first one trans-
mitter and then the other while
listening to the radio .
(Note: You might want to try this
tuning method with two good
transmitters first to familiarize your-
self with the procedure . While the
process is quite simple, it helps to
know what to expect before you
start . )
Next month's Update Technical
Topics will include instructions for a
cheapo-cheapo field strength meter
that you can build to tune up the
R. F. section of the transmitter.
tWhen returning an old coil , melt
the wax with a soldering iron first .