16
STAR•TECH Journal APRIL 1982
MIDWAY'S
X-Y MONITOR TROUBLESHOOTING GUIDE (PART 3)
We conclude this 3-part article with TROUBLESHOOTING.
Part 1 - Introduction, Symptom Diagnosis, appeared in S•TJ's February '82 issue.
Part 2 -Theory of Operation, appeared in S•TJ March '82.
TROUBLESHOOTING
Troubleshooting monitors requires experience,
patience, and luck. The first step is to match the
symptom the monitor displays to the diagnosis
next to it in the "SYMPTOM-DIAGNOSIS"
subsection. This will pinpoint the circuit the
problem is probably in, and often the parts to
check. Next, the circuit should be visually in-
spected to see if there are any parts broken,
burned, or if something is there that shouldn't be,
like a loose screw, etc. Some parts go bad before
others and should be checked first. In fact,
following is the general order in which parts
usually go bad:
1. Semiconductors (like transistors, diodes, and
integrated circuits).
2. Fusible resistors.
3. Electrolytic capacitors.
4. Resistors.
5. Capacitors and coils.
Always remember that a monitor can bite like a
snake. Even when it is turned off, capacitors hold
voltage and will discharge it to you should you be
touching chassis ground. The picture tube or
CRT, itself, is a giant capacitor, so avoid the
flyback anode plug hole. With the monitor on,
the power supply circuit and/or the flyback,
which puts out at least 12,000 volts, CAN BE
KILLERS!! Avoid handling power transistors
(usually output transistors), yoke terminals, and
other high power components when the monitor
is on.
WARNING: That picture tube is a bombl
When it breaks, first it implodes, then it explodes.
Large pieces of glass have been known to fly in
excess of 20 feet in all directions. DO NOT
carry it by the long, thin neck. Discharge its
voltage to ground by shorting the anode hole to
ground. Using a plastic handled screwdriver,
connect one end of a wire with an alligator clip at
each end to chassis ground and the other end to
the metal shaft of the screwdriver. Using ONE
HAND ONLY (put the other in your pocket)
and touching ONLY the plastic handle of the
screwdriver (DO NOT TOUCH THE MET AL
SHAFT), stick the blade of the screwdriver into
the anode hole. Be prepared for a fairly loud pop
and a flash. The longer the monitor has been
turned off, the smaller the pop and dimmer the
flash. But BE CAREFUL. Picture tubes will
hold a very healthy charge for at least a week if
not longer. Even after you've discharged it once,
it may still carry a residual charge. It's better to
be too careful than dead, which is why electronic
equipment always carries stickers referring
servicing to qualified personnel. Handle the side
with the viewing screen against your chest when
changing it. ALWAYS wear safety goggles
when handling the picture tube.
To maintain the safety and performance of the
monitor, always use exact replacement parts.
For instance, the wrong components in the
power supply can cause a fire, or picture distor-
tion may result from the wrong transistor being
placed in the deflection circuitry. Component
manufacturers offer specification sheets which
are useful for "mixing and matching", but why go
through all the trouble? Order exact replacement
parts! Service your monitor on a nonconductive
firm table like wood, NOT METAL, and take
off all of your jewelry just in case. With all this in
mind, you are ready to begin troubleshooting.
Observe the picture carefully. Try to vary the
appropriate control that would most likely affect
your particular symptom. For example, if there
is poor brightness or no picture, try turning up the
brightness or contrast control. If the controls
have no effect at all, chances are there is trouble
with the control itself, the circuit it controls, or a
nearby circuit that may be upsetting voltages. Go
to the list of symptoms and determine with the
schematic where the bad circuit is.
CAUTION: KEEP IN MIND THAT
CAPACITORS HOLD A CHARGE AS CAN
THE PICTURE TUBE (FOR AT LEAST A
WEEK AND USUALLY LONGER), AND
COULD SHOCK YOU.
First, check for obvious visual defects such as
broken or frayed wires, solder where it is not
supposed to be, missing components, burned
components, or cracked printed circuit boards. If
everything looks good up to this point, make sure
that diodes, electrolytic capacitors, and transistors
have their leads connected in the right polarity as
shown on the schematic and the circuit board.
Turn on the power and measure the voltages at
the leads of the active devices such as tubes,
transistors, or integrated circuits. Any voltage
that does not come within at least I 0% to 15% of
the voltage specified on the schematic indicates
either a problem with that device or a component
connected with it in the circuit. The next step is to
use the ohmmeter to narrow down the field of
possible offenders.
This all sounds pretty confusing, but a little
experience on a good transistor will make you an
expert in no time. Usually, the lowest ohmmeter
setting is used for testing transistors. Once in a
great while a transistor may check out good on
this test, but may actually be "leaky" or break
down only on higher voltages. If in doubt, change
it. It is also wise to check the transistor out of the
circuit just in case some component in the circuit
is affecting the ohmmeter reading.
A diode is tested like a transistor except it only
has two leads. Again, there should be high
resistance one way and little resistance the other.
If it tests bad, take one lead out of the circuit in
case some component is messing up the ohm-
meter reading.
NOTE: DO NOT leave soldering equipment on
the leads too long since all semiconductors,
especially integrated circuits, are easily destroyed
by heat.
Without special equipment, integrated circuits
are checked by verifying the proper DC voltage
on the pins and the correct AC wave form using
an oscilloscope. BE CAREFUL: shorting their
pins can easily destroy them.
Resistors are checked with an ohmmeter and
should usually be within ten percent of the value
stated on them and on the schematic. You may
have to desolder one lead from the printed circuit
board. If you wreck the foil on the board,
carefully solder a small wire over the break to
reconnect the conductive foil.
Capacitors are tricky. Their resistance goes up
when checked with an ohmmeter which shows a
charging action. As they suck up current from the
meter, the voltage goes up and so does the
resistance. If you are sure a particular circuit is
giving you a problem and everything else checks
out O.K., electrolytic capacitors are prime sus-
pects. Substitute a new one and keep your fingers
crossed.
What to do when you don't know what to do-
and other tips
If you are totally confused about where to begin
to hunt for a problem, and can't find the problem
in the "SYMPTOM DIAGNOSIS" subsection,
there may be another way to proceed.
To test a transistor, one lead of the ohmmeter is
placed on the base; and the other lead placed just
on the emitter, than on the collector. A normal
transistor will read either high resistance (infinite),
or little resistance ( 400 to 900 ohms), depending
on the polarity of this type transistor. Then the Take a VOLTMETER and (if possible) an
leads should be switched, one remaining on the oscilloscope and begin probing the jacks. You
base, and the other switched from the emitter to can start with the input jack to the monitor. Using
the collector. Now the opposite condition should the oscilloscope, make sure both the "X" and
result: the resistance should be infinite if it was "Y" information is present ( which it isn't during
lower when the other lead was on the base. the "SOUND" test).
Consistantly infinite readings indicate an open,
and a short is demonstrated by 0-30 ohms on NOTE: It is advisable to use one of the game's
most of these test readings. Finally, place one test patterns ( obtained when you put the game in
lead on the collector, then the other on the the Self-Test mode) when using the oscilloscope.
emitter. No matter which lead is used, there The simple diamond one is a good choice. This
should be infinite resistance. Any lower reading, way the "X" and "Y" information at the above
such as 50 ohms (which is typical on a bad jack isn't changing and a recognizable wave form
is easy to see if it's there. The DC voltages tend
transistor), indicates a short.