Star Tech Journal

.,
STAR*TECH Joumal
July 1992
TROUBLESHOOTING MONITORS
WITH..A.N AUDIO GENERATOR (PART
2)
Gary A. Hughes
General Coin Distributors
Portland, Oregon
Part 1 discussed the universal problem of monitors piling up in the repair shop corner-taking up space
and costing time and money. Using this simple, easy-to-understand method of troubleshooting-using an
Audio Function Generator~ these monitors can be made well and useful again. Part 1 troubleshooting
included: The Setup, Connection of Probes and Testing Electrolytic Capacitors. This issue we push on
with: Checking Flybacks, The Horizontal Circuit, Diodes and Transistors.
FLYBACK TRANSFORMERS
Ah yes, the fly back! You've got a
pile of those in the corner too,
don't you? Some of them were
replaced on a hunch, then you
found another problem, and it
just ain't worth it to solder it back
in for testing purposes, not to
mention wear and tear on the
circuit board. So here you are,
stuck with junk that's too expen-
sive to throw away!
Most flybacks can be easily be
tested on our little unit here,
again with high accuracy. Refer-
ringtofigure IV, The input termi-
nals are marked with the X's.
These are the terminals to hook
your probes to. If you do not know
which these are, say for that u:sed
part on your shelf with the ques-
tion mark on the side, then do an
ohm check for continuity. Ring
each set of pins that has continu-
ity. A good reading anywhere in
the unit would tend to indicate a
good part.
To test the flyback, sweep the
frequency between 10 and 100
kHz. Somewhere there should be
a strong resonant peak, where
the amplitude goes very hi 1 gh.
This indicates a good trans-
former. At this point you can take
that fearsome high voltage wire
and short the clip to the pins on
the bottom and see the waveform
fall as you load it down. Due to the
controlled voltage and current
used, you can even short the volt-
age through your hand, but don't
let that become a habit when the
unit is operational.
This test is most effective on line
voltage monitors where the
transformer input runs at about
120 volts. It will work on those
little 12v dart monitors too, but
the resonant peak will not be as
high. In this case, it is a good idea
to compare your suspect to a
known good one before calling it
good or bad.
Last week I replaced a flyback in
Matsushita monitor. It had a
small crater in one side corre-
sponding to a black cloud on a
nearby bracket. Just for fun I
gave it a ringing test: perfectly
good! Evidently the breakdown
was only in the plastic case.
That's enough, though. Trashola!
Good servicing is keeping an eye
on all the signs.
HORIZONTAL CIRCUIT
This test can be performed in-
circuit by connecting between
chassis ground and the collector
of the horizontal output transis-
tor.
In this case you are not only
checking the flyback, but the en-
tire section, including:
• Output transistor (C-E shorts)
• Damper diode (often this is in-
ternal within the transistor)
• Scan-derived voltages, associ-
ated diodes
• Deflection yoke
There are three main differences
between checking this section as
a whole and just ringing the
flyback. First, some of the coils
and capacitors are there to tune
the flyback, and so the resonant
frequency will change. The text-
book norm is 4 7 kHz, but it is very
common to see them anywhere
from 15-70 kHz. Secondly, there
is now more "stuff" loading down
the circuit, so the peak will have
a lower amplitude. Nevertheless,
there should be a definite peak.
Thirdly, there is now a damper
diode involved, so we should only
~