REPLACING CHIPS IN COMMODORE EQUIPMENT 7-5-05.


           REPLACING CHIPS IN COMMODORE EQUIPMENT    
                     updates and corrections 7-5-05


For the small "glue logic" chips such as the RAM in a C64, cutting
the pins off is probably the best way to remove them. Replacements are 
cheap and you don't risk damage to the board. The new chip can be 
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 
better yet, install a socket. Just make sure you clean the pins of a 
removed chip before you install it in a socket. If a blob of solder 
remains on a pin, it can damage the socket by forcing the contact areas
too far apart. Another thing: NEVER solder a socket in place with a 
chip installed in it... you'll never get it out again!
     The bigger and harder-to-get proprietary CBM chips such as the PLA
and SID in the C64 require a more cautious approach than casual snipping
of pins. Unless you're certain the chip to be removed is bad, it makes 
sense to desolder it intact. 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. I used to use a vacuum type of 
desoldering iron, but they tend to clog easily (requiring frequent 
cleaning) unless you have one of the very expensive models that has an 
electric vacuum pump. Now I use a separate iron and solder sucker, one
called a Edsyn SoldaPullt. MCM Electronics (www.mcminone.com) sells 
several equivalent types. I prefer the larger ones because of the higher 
vacuum they create. That type is easier to keep clean inside than the 
all-in-one desoldering iron.
     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 or socket! 
     Removing a chip is a "black art". After all these years, I'm still
learning. 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... nice if you've got 'em. Before 
drawing the molten solder out, I move the tip of the iron 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 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, so 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 copper board 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.
     Freeing up all the pins before extracting the chip is essential. I
wiggle each one with tweezers to make sure its free. It's sometimes 
helpful to push on the top of a chip to force all the pins one way, then
the other in the holes. You can hear them clicking as they break free
of the holes. When you're sure they are all loose, try prying up one
end of the chip with a small screwdriver. If any pins appear to be 
stuck, STOP. Free that pin and continue prying up one end, then the other.
     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 used 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 many times. 
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.

 

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