15
STAR*TECH JOURNAL/AUGUST 1982
MIDWAY'S
COLOR TV MONITOR REPAIR (PART 3)
We conclude our three-part article with Troubleshooting,
Differences Between Monitors, and Parts Interchangeability
Parts 1 and 2 appeared in our June and July issues respectively
Part 1 - Introduction and Symptom Diagnosis
Part 2 - Theory of Operation
Part 3 - Troubleshooting, Differences Between Monitors, Parts
Interchangeability
TROUBLESHOOT! NG
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
inspected 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 18,000 volts,
CAN BE KILLERS!! Avoid handling power
transistors ( usually output transistors), yoke
termials, and other high power components
when the monitor is on.
WARNING: That picture tube is a bomb!
When it breaks, first it implodes, then it
explodes. Large pieces of glass have been
known to fly in excess of20 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. Use 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 screw-
driver. Using ONE HAND ONLY (put the
other in your pocket) and touching ONLY the
plastic handle of the screwdriver (DO NOT
TOUCH THE METAL 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 the wrong
color output transistor may give a funny color
to the picture. Service your monitor on a
nonconductive firm table like wood, NOT
MET AL, 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.
infinite resistance. Any lower reading, such as
50 ohms (which is typical on a bad transistor),
indicates a short.
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.
Ifin doubt, change it. I tis 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 ohmmeter reading.
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.
NOTE: DO NOT leave soldering equipment
on the leads too long since all semiconductors,
especially integrated circuits, are easily
destroyed by heat.
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.
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.
Tum 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 10% to 15%
of the voltage specified on the schematic indi-
cates 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.
To test a transistor, one lead of the ohmmeter is
placed on the base; and the other lead placed
just on the emitter, then 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 leads should be switched, one
remaining on the base, and the other switched
from the emitter to the collector. Now the
opposite condition should result: the resistance
should be infinite if it was lower when the other
lead was on the base. Consistently infinite
readings indicate an open, and a short is
demonstrated by 0-30 ohms on most of these
test readings. Finally, place one lead on the
collector, then the other on the emitter. No
matter which lead is used, there should be
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 suspects. Substitute a new one and keep
your fmgers crossed.
DIFFERENCES BETWEEN
MONITORS
The easiest way to identify the brand of monitor
you are working with, assuming you can't find
the brand name written on it anywhere, is to see
if there are two circuit boards rising up from the
chassis toward the picture tube neck. In other
words, they stand up, or are perpendicular to
the chassis, with a black plastic bracket holding
them in place. This is a description of a Wells-
Gardner monitor. They use separate boards for
main chunks of circuitry. Therefore, you have a