Star Tech Journal

•
STAR* TECH Joumal
November 1991
When you have the advantage of
schematics, then you can apply
another troubleshooting tech-
nique, which is to use an ohmme-
ter set to beep continuity.
Place the black lead to ground,
touch every leg on every chip on
the logic, looking and listening for
low resistance readings to ground.
Use the knowledge that most legs
on chips, with the exception of
those connected straight to power
and ground, are going to yield a
reading higher than beep continu-
ity, which for my meter- is 330
ohms.
If you should get a reading of 10 to
70 ohms or thereabouts, that chip
and possibly whatever is con-
nected to the leg of that chip
should be removed until the short
disappears. Failures tend to
travel along certain lines and all
the chips should be noted, using
the schematic, that share that
line.
For even if the short should disap-
pear, it doesn't necessarily mean
the rest of the associated chips are
good, for it could have opened up
the input leg on one of them. Not-
ing them is for later when you pull
out the scope and the logic probe.
When you have completed the
above steps to your satisfaction,
power up the board and see if it
works. If it doesn't, and it prob-
ably won't, let the board stay on a
while and feel for chips that are
abnormally cool, i.e., dead.
For as chips can short, and most
do in this case, others can open up
and become completely inopera-
tive. This is where the scope
comes in handy, for most of the
output legs on a dead chip will be ~.
floating. For TTL, a scope signal
will be in the 2.5vdc range. There
may be some kind of a signal, but
it won't have good 5vdc to ground
swings.
If you should get a
reading of l O to 70
ohms
or
there-
abouts, that chip
and possibly what-
ever is connected
to the leg of that
This will show on most logic
probes as neither high nor low
lights illuminated even when the
input legs are active. Most TTL
chips that are not tri-state or open
collector type will have either a
high, low or active signal on it's
outputs, not floating, unless they
drive transistors or resistor lad-
ders and other designs. This must
also be taken into account.
Probably this step could be taken
before the previous step or per-
formed in conjunction with it, but
what you are up against now is to
apply standard logic board
troubleshooting procedures to get
the board up and running.
This includes using the scope to
check the integrity of the address,
data, control, input, and output
lines, The board may partially
work, but there may be problems
in the video, sound, input and au-
dio sections to be repaired.
moved until the
In a failure this drastic, checking
all sections to your satisfaction
should be done. I will not go into
standard repair techniques, be-
cause that is beyond the scope of
this article.
short disappears.
I have used this procedure for
years and it works well for me. I
hope it does for you too.
chip should be re-
* * *