November 1995
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0 Check the flyback transformer
for cracks.
0 Replace any open fuses AF-
TER checking with a volt meter
for the presence of any charged
caps in the power supply.
0 Connect the monitor power in-
put to the Variac. I made up a
few cables to match the variety
of connectors, clip leads are an
alternative.
0 Slowly bring up the voltage
from zero.
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CD
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down level and so the monitor,
well, shuts down.
0 Check the main power supply.
Experience shows that if the
monitor shuts down, the 130V de
(in this example) will rise to 160
V de or so. Of course a shorted
voltage regulator or pass transis-
tor will cause the same symptom.
0 Tum down the AC until the
160V de falls to the desired
130Vdc.
In my example we will find a
large current draw. Power down.
At this point we would look for
shorted horizontal output transis-
tors, shorted rectifiers in the main
power supply, shorts across the
power supply, and don't forget
the degaussing coil.
Unplug the degausser until you
are done. After replacing any-
thing we find bad, slowly power
up. Use your scope to monitor
the power supplies for excess
ripple or other signs of trouble.
When you are done with this
step, you can bring the line all
the way up to 120V. Of course,
when servicing an older
Nintendo cabinet, you stop at
100V because that is all it wants.
Handy, isn't it?
START/STOP
-..
0 K, now if we have solved the
excess current draw, the unit
should be able to hold a fuse. Uh
oh, my mythical unit no longer
blows fuses, but now it starts up
for a second and then quits. That
static sound then nothing. I know
that a bad filter cap can allow
voltage peaks to exceed the shut-
UAscrapped
vid-game
cabinet should
provide an isola-
tion transformer;
a power cord, ...
maybe even a
loose quarter or
two.,,
0 Power down for a second us-
ing the switch, leaving the AV
voltage at its lower setting. This
will reset the shut down circuit.
The unit now functions, though
poorly, allowing you to continue
troubleshooting.
0 Now you can check for proper
action of filter caps, the most
common failure. A bad filter af-
ter the regulator will allow hash
from the flyback to add itself to
the main power supply. This ex-
ceeds the shut down threshold.
At reduced line voltage, this
causes severe distortion of the
STAR*TEcH
Journal
. . .. . . . . . . .. . ... . .... .
picture. A shorted or failed regu-
lator will result in hum bars in
the picture at reduced line volt-
age.
Fuu. SERVICE
At this point, we can continue the
repair beyond voltage related
problems. In my shop we would
check and rejuvenate the CRT,
make any adjustments, and send
it back out. The Variac/iso setup
on the bench allowed us to eas-
ily repair the monitor without
wasting parts and fuses, and with-
out resorting to guess work.
Try it ... you might like it!
PARTS/ SOURCE HOUSES
MCM ELECTRONICS -800-543-
4330 - sells a Variac in their house
brand for $89.98. Rated at ten
amps, includes current meter,
switch, outlet, and power cord.
Ready to use with your own iso.
FAIR RADIO SALES - 419-223-
2196/419-227-1313FAX - This
fine old surplus outlet is great
source of transformers, Variacs,
meters, isolation transformers.
Everything you need to build a
neat piece of test gear and lots
more neat stuff too.
TIP
All the big general parts houses
like ALLIED, MousER, NEWARK,
RS, etc. carry variable autotrans-
formers starting at about $50 if
you want to build your own. Best
to consider a five amp rating as a
minimum.
Tell 'em Star Techjournal
& 'Enzo' sent you!