STAR*TECH Joumal
•
November 1991
Procedures for Repairing Boards with
Overvoltage Damage
Sam Cross
Godwin Distributing Company
North Little Rock, Arkansas
When a board is damaged by over-
voltage or some extreme condition
such as lightning, the first thing a
technician does when he figures
out this has happened is moan. He
knows that a quick fix is out of the
question, and if he must, it usu-
ally is a well-rounded test of his
troubleshooting skills and may
cause him to develop some new
ones.
After troubleshooting quite a few
boards like these, out of the swirl
of my thoughts a loose plan
formed that I use to tackle them.
Here they are presented in the
sequence I use.
If I don't have the schematics I
usually order them. It buys me
time to blow off that board and fix
easier fare. Although schematics
sometime seem to be drawn by
drafters with failing ink pens or
reproduced on bad copiers, it's
better than nothing, just is that
chip at position 3E or 8E?l
Really, let's be hone.st. You can set
the board to the side and wait for
the schematics, or you can find the
obviously bad chips and get them
ordered and put in before the
schematics arrive. How? By look-
ing for signs of obvious visual fail-
ure, feeling for hot chips, and by
substituting the socketed chips
into a known good board.
After troubleshoot-
ing quite a few
boards like these,
out of the swirl of my
thoughts a loose
plan formed that I
use to tackle them.
Here they are pre-
sented in the se-
quence I use.
When a board gets zapped by ov-
ervoltage, a close visual inspec-
tion may yield a couple or more
bad chips. Looking for chips with a
section blown out of the top are the
easiest to find, but chips can also
blow out the bottom as well as
split along the sides. Look for tell-
tale signs of what looks like tiny
bits of tar on the board around the
chips, revealing another less obvi-
ous bad chip. The closer you look,
the more you probably will find.
After you're sufficiently cross-
eyed from using this method and
can't find any other visually bad~
ones, the next step is to feel for hot
chips. Power the board up with a
power supply and set for the cor-
rect operating voltage ON THE
BOARD. Let the board stay on
long enough to find the first if any
chips that start heating up too
quickly, takingintoaccountchips
that normally run hot.
It is a point of my experience
though that chips that normally
run hot are the ones that get taken
out by overvoltage first. High
speed video rams, counters in the
clock generating section, PALS,
and other high speed versions of
chips that use a lot of power usu-
ally take the fall for the rest of the
bunch. They fail first and drag
down the power supply until it
shuts down or fails.
This is a good point to remember,
because most of the other chips
avoid failure because of these sac-
rificial lambs. When you find the
-