REPLACING CHIPS IN COMMODORE EQUIPMENT    02-22-99

     For the small "glue logic" chips such as the RAM in a C64, cutting
the pins off is probably the best way. Replacements are cheap and you
don't risk damage to the board. The new chip can be tack-soldered onto
the stubs left from the old one or, for a neater job, you can remove the
old pins one at a time and solder in the new chip or install a socket.
     The bigger and harder-to-get proprietary CBM chips such as the PLA
and SID in the C64 require a more cautious approach. Unless you're
certain the chip to be removed is bad, it makes sense to desolder. After
doing that much work on the board, I always install a socket. If you're
careful how you remove it, the old IC should be OK. It's important to
use the proper equipment for chip removal so you do not damage the board
or chips. A vacuum type of desoldering iron works best for me. I found
one at an outlet called MCM Electronics (www.mcmelectronics.com. Phone:
1-800-543-4330). The desoldering iron has a small hollow metal tube that
fits over the IC pin, heats the connection, and then draws the molten
solder into a chamber when a button is pressed. The MCM number is 21-945
and it sells for $27.25... well worth the money! It is designed for
one-handed operation. The iron is pressed against the board for a few
seconds, the plunger released with the thumb, and then the waste solder
is forced out of the tube into a waste container by retriggering the
spring-loaded plunger. It is then ready for the next pin. That small
handheld unit is adequate for once-in-a-while use, but for production
work, the next step up is a temperature controlled unit with a motorized
vacuum pump that sells for about $300. I've seen commercial models for
over $3000. How serious are you about desoldering? ;-)
     The PC boards in Commodore computers and drives are double sided
(circuit traces on both sides) with plated-through holes. The main
problem with removal of an IC is that the pins just barely fit the tiny
holes. Each hole must be cleared of solder and all pins -must- be free
before you attempt to lift the chip out. Otherwise, the pin will break
off and/or the board will be damaged. A careful visual examination of
the board after IC removal is important. Unless you look closely, you
may not even notice a PC board trace is broken. The device will not work
when you put it back together, and you've just added a problem that's
hidden -under- the new chip! <shudder>
     Removing a chip is an art. After all these years, I'm still
learning. I like to set the PC board on end with the pins facing me. If
it's lying flat, waste solder can dribble from the iron onto the board.
Each IC pin needs to be heated for about two seconds, or until all the
solder is molten on both sides of the board. Some have suggested using
an additional iron on the opposite side of the board, but that obviously
requires another set of hands. Before drawing the molten solder out, I
move the tip of the iron (which fits over the pin) slightly, and then
press the button on the vacuum plunger to draw the molten solder away. I
move the pin and hit the button at the same time so the pin is moving in
the hole when the solder is being drawn off. If all goes well, the pin
will then move freely in the hole. If it's still stuck to the wall of
the hole and all the solder is gone, I free it by grasping with tweezers
and wiggling it back and forth a few times, or by pressing sideways on
the pin from the top of the board (inwards, towards the chip). If done
properly, I can hear and feel it pop loose. If any solder remains in the
hole, I add more fresh solder (with a regular iron) and try to desolder
it again. The worst places to desolder are the larger areas of the board
(power and ground points, usually at the ends of the chips). Those large
copper areas tend to quickly draw the heat away from the iron and it's
difficult to get them hot enough to get all the solder molten. For the
really stubborn ones, I use high quality solder wick and a larger
soldering iron tip to desolder those pins. In any case, I try not to
hold an iron on the board for more than a few seconds at a time. Too
much heat will lift the pads (solder holes) and traces. The tendency is
to press the iron into the solder, but too much force will likewise do
damage to the board. That's why it's wise to practice on an old surplus
board first.
     It's necessary to clean out the residue of old solder from the iron
every once in a while... after each chip removal is recommended. If you
let if build up, the vacuum will be reduced, causing some solder to
remain in the holes. My iron came with a wire probe to help clear the
tube of solder and flux that eventually tends to clog it up.
     If you are successful in getting the chip out of the board intact,
by all means install a good quality socket. That way, you will never
have to touch that area again with a soldering iron. It degrades it each
time, so it's best to avoid resoldering if you can. After extraction of
the old chip, examine the board on both sides with a strong light and a
magnifier. Look for broken traces near the holes and solder splashes
that may cause shorts between pins. Repair any damage and clean any
holes that have enough solder left to prevent installing a socket. The
easiest way is to poke a large pin in the hole and wiggle it around.
Solder is soft and will be pushed out of the way. Hold the board up with
a light behind it to see if all the holes are clear. If it looks OK,
install the socket and solder each pin carefully. If you intend to
reinstall a chip you have previously removed, clean all the pins by
squeezing each one with a small pair of needle nose pliers, then heat
each with the iron to remove the excess solder. Don't bend the pins any
more than you have to. They will break if flexed too much. Assuming the
worst happens and you break a pin, if there is any stub left on the body
of the IC, you can tack solder a small wire to the stub and cut it off
in line with the rest of the pins... not pretty, but it will work.

Ray Carlsen
CARLSEN ELECTRONICS... a leader in trailing-edge technology.
rrcc@u.washington.edu  or  rcarlsen@i-link-2.net