STAR*TECH Joumal
suspect. You are only testing the
diodes because they can be
checked very quickly. Also in this
category is the deflection yoki~. It
is rare for them to short on game
monitors, but hey, a plug in part
-pull the plug.
Most flybacks can be unsoldered
cleanly enough to check them
without even removing them
from the board. Take any short-
cuts that will work, but make
sure of each step. Dogs, for the
most part, tend to be technician
created.Poorprocedure,assump-
tions, installing wrong parts,
making problems instead of :fix-
ing them -we all have our mo-
ments. Let's try to keep them far-
ther apart.
Just think, if you can be sure that
part is bad what a satisfying thud
it will make as it hits your
trashcan! What confidence you
can exhibit while you wait for the
replacement! How much of that
questionable pile you can either
now use or throw out! What free-
dom, whatrest, whatanexcuseto
ask for a raise! Coffee break's
over, back to work.
I mentioned dart monitors a mo-
ment ago. There is another trick
your little oscillator can do, too.
When the monitor is fixed (you
hope) but you can't test it on your
RGB fixture and the machine is
on location, there is a way! Switch
the generator to square waves
and tune it to about 31 kHz. Feed
that into the input of the monitor
and turn the output down a bit to
give a good balance of light a:nd
dark. You will probably see a
tweedy, out of sync pattern. Rock
the frequency a bit and presto,
two vertical black bars and two
light bars! This is strictly a go/no
go test and can be useful for
July 1992
checking brightness and con-
trast, but not for setting size. Do
not try to set horizontal or verti-
cal holds by this, instead adjust
the frequency of the generator. If,
however, the monitor fails to
come up youjust saved yourself a
location trip, and your reputation
as the company that "gets 'em
fixed" is intact.
Just think, if
you can be
sure that part
is bad-what
a satisfying
thud it will
make as it
hits your
trashcan!
'fRANsISTORS
We have already looked at diode
testing. Closely related are tran-
sistors. This tester is of lesser
value here, but the probe in the
hand is often the fastest. A tran-
sistor is two diodes, the base be-
ing the middle of the sandwich.
Thus, between the base and the
other two legs one should get a
diode reading. C-E should read
open except for some of those
fancy transistors with an inter-
nal diode. Some Darlingtons also r--,
have an internal resistor E-B, so
suspect parts should be checked
against a known good one. Actu-
ally, in circuit resistance and es-
pecially capacitive coupling
tends to make many such mea-
surements involving transistors
"iffy". You'll still find some this
way, but watch your time. I have
better success here with a VOM .
One aspect of transistors may be
of interest. That is the tendency
oftheE-Bjunction tozenerat@6
- 7 volts (Figure 6). If your oscilla-
tor puts out this high of a voltage
you can see this happen. Aside
from helping you identify the
Emitter and the Collector it
doesn't help you much, but it is
interesting.
Other uses for this set up include
amplifiers and power supplies. !"8"',
Even when repairing other prob-
lems it can be re-assuring to give
them a quick cap-scan before
sending them out. The high fre-
quency used is especially compat-
ible with switching supplies. You
can also test the diode on a pin-
ball coil without desoldering it, at
the same time checking the coil
for short turns. Or run down the
row of drivers on a pinball CPU
and pick the shorted ones right
out. Or check the traces on a logic
board for continuity or shorts one
to another.
Another caution would seem to be
in order. If you are checking on a
logic board it is probably a good
idea to lower the oscillator volt-
age somewhat. I have never
caused any problems doing this,
but microprocessor parts get~
more sensitive all the time.
Happy hunting!