REPLACING IC "CHIPS" IN COMMODORE EQUIPMENT updates and corrections 12-19-2012 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, clean the holes, then 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... any solder on the chip pins will adhere to the socket contacts, and you'll never get that IC 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 thus requiring frequent cleaning unless you have one of the very expensive models that has an electric vacuum pump and a chamber that holds the excess solder. 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 vacuum desoldering iron. PLAESE READ, THIS IS IMPORTANT! 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 in the board. Each hole must be cleared of solder and all pins -must- be free before you attempt to lift the chip out. Otherwise, the stuck pin(s) will break off and/or the board will be damaged. A careful visual examination of the board and each IC after 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! That damage is VERY hard to find! The worst kind of board damage is destruction of the plated-through holes. If you ever see a copper colored blob on a chip pin after removal, that is the through-hole plating that has been torn off. That damage breaks the circuit from top to bottom of the hole and it must be repaired. Most important is the connection at the top of the board... it will not be connected since the solder is done on the other side. The fix is easy. I use a single strand from a section of stranded hookup wire, poke it through the hole and solder it on both sides. It may be necessary to scrape away some of the paint and coating to get down to bare copper. After that repair is done, install the socket but don't apply any more heat than necessary to that repaired hole or it will lift the repair you just did. Removing a chip is a "black art". After all these years, I'm still learning and still make mistakes, especially if I've been at the bench for several hours. 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 to press each pin closer to the center of the hole, then press the button on the vacuum plunger to draw the molten solder away. If all goes well, the pin will then move freely in the hole. If it's still stuck to the wall of the hole (it happens frequently) 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 at the 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 on both sides of the board. 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 the 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 it's free. It's sometimes helpful to push sideways on the top of a chip to force all the pins one way, then push the other way, back and forth a few times. You can hear pins 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... gently! 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 again. 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 to do that is to touch each hole with a clean iron for half a second each. That causes any solder in the holes to mirgrate to the outer surface of the hole. As an alternative, you can poke a large pin in the hole (gently) 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 flat needle nose pliers. 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. It's not pretty, but it will work. Ray Carlsen CARLSEN ELECTRONICS... a leader in trailing-edge technology.