November 1991
•
STAR*TECH Joumal
Zener diodes used for overvoltage
are prime non chip candidates. If
you're lucky, it took the fall for the
whole board, although this is not
always the case.
first round of hot chips, replace
them and power the board up
again, checking and adjusting the
supply voltage again as it may
have risen higher because of the
reduced load.
You will probably find other chips
which weren't very hot before
start become so because they as-
sume the low resistance path and
thus dissipate more heat.
Some important notes. The back-
side of my index finger is much
more heat sensitive than other
parts of my hand, so that is what
I use. I usually do a double take if
I am in doubt, switching to the
front part ofmy index finger. If I
can't keep my finger on the chip
for an appreciable length of time,
I usually change it.
If the board has such a short that
it keeps shutting down your sup-
ply, you may have to use a brute
force supply consisting of variac,
bridge rectifier, and filter capaci-
tor. Bring up the supply slowly
with the variac while feeling for
hot chips, keeping in mind not to
exceed the normal operating volt-
age, and always turn the variac
down when you replace chips.
For when you take out chips that
have shorted, the brute force sup-
ply which has poor voltage regula-
tion will naturally rise. If you do
not turn down the supply every
sweep, you take the chance of sub-
jecting the remaining chips to a
higher than normal voltage.
Other non chip components that
are suspect are tantalum capaci-
tors and noise capacitors around
the board. Yet in troubleshooting
many different logic sets their
failure rate is very minimal.
I have several older
working
boards
that I keep around
the shop that I use
simply to check
chips. It tests them
under actual oper-
ating conditions un-
der full load.
Substitution is also an excellent
way to verify chips, such as
eproms and mpus, which are usu-
ally in sockets. Both these types
are prone to failure due to over-
voltage. Substitution should be
done of the bad chip into the good
board, not vice versa, for you prob-
ably have more than one bad chip.
One chip at a time is almost cer-
tainly the way to go, for if you sub
more than one at a time and it
doesn't work, you can't pinpoint a
particular one without going back
and doing them individually.
I have a chip checker on the com-
puter at the shop, but it has fooled
me into thinking the chip was
good when it actually wasn't be-
cause it wasn't tested under load
conditions or at the right speed. I
have several older working
boards that I keep around the
shop that I use simply to check
chips. It tests them under actual
operating conditions under full
load.
If you are going to use this
method, select a board with a wide
variety of chips and socket the
ones in the video section and mpu
section that you desire to use as
test sockets. Make sure from the
schematic that all of the gates are
used and if any of them are bed, it
will immediately show up on the
monitor. Whole board substitu-
tion on multiple board sets is a
quick way to verify the integrity of
it's operation. In general, proceed
from the general to the specific.