Circuit board repairs

Replacing an IC

Locating a faulty chip on a circuit board is only halfway of fixing. It must be replaced with new one! Hopefully in such a way that no further damage occurs to the board. Double sided circuit boards used in pinballs demand skill and accuracy. Too much heat or too forceful handling may cause the PCB foils to tear off or the plate through holes to get damaged.

This board is not from a pinball, but anyway a good example board. Let's imagine that one of the logic chips on board needs to be replaced.

Before attempting repairs on a circuit board, check your workplace. Most chips are static sensitive. Do not use a plastic coated workbench. Touch the circuit board ground first, before touching other parts of the board. Avoid unnecessarily handling the components. Do not wear clothes that you find generating excessive static electricity. Do not allow your cat near the boards or chips.

The faulty IC must first be removed. First it comes to mind that solder wick or desoldering pump might be useful. Problem is, with those it is quite difficult to completely remove all solder from plated through holes. Chips legs remain partially soldered and when pulling the chip out, it rips some foils or the hole innards with it.

A desoldering pump is a good tool to remove components with lesser amount of legs, and also for cleaning the holes after component removal. And still better it gets, when you attach a piece of silicone hose to the tip. The hose is heat resistant and can be pushed over the soldering iron tip on the board, to get a tight seal for desoldering.

Because the IC we want to remove is already broken, it is OK to break its legs. With sharp pliers, cut every leg just at the chip case. This way you get the IC itself out of the way, after which it is easy to remove the legs one at a time with needle nose pliers and soldering iron.

Heat the leg with soldering iron and lift it away with needle nose pliers or tweezers.

Chip and its legs have been removed with no great force. All foils and plated through holes are fine, but the holes must still be cleaned from solder. Now the desoldering pump or solder wick will be useful.

Gently press on the solder wick over the hole with hot soldering iron. Solder melts and gets sucked into the wick. A desoldering pump works as well, especially with the above mentioned silicone hose.

After all holes are open, install a good machined socket on the board. Of course, if you are absolutely sure that the chip will never again fail, leave out the socket and solder the chip right in. But hey, it broke once, it can broke another time! Install a socket and next time replacing the chip will be much easier. Use a good machined socket instead of a cheap spring socket. Pinball machine is a hard environment for sockets, and you do not want the chip to shake out of its socket. Instead of keeping a supply of every possible socket size, I prefer socket strip. With that it is easy to fabricate every size of sockets as needed.

Sometimes the troubleshooting is so difficult that you can't be sure of the faulty chip. And especially if the chip is expensive or hard to find, it would be nice to have it removed in one piece so that it may be reused. Also, if you remove chips from trashed boards for spare parts, you obviously can't cut their legs when removing. A hot air gun is nice tool here. By heating the backside of circuit board, the chip will be loosened in a few seconds. Use a gun with a thermostat. Too hot air will make the board brown or burn it, and also damage the chips. Practice first with some junk boards.

Board fixing

What if despite all carefulness, the board itself gets damaged? Or, what if you must fix a board after Mr. Repairdude has worked on it? Foils are broken, plated through holes are damaged. Fortunately the situation is not always hopeless.

This Gottlieb / Premier System 80b board has two 6532 RIOT chips replaced. During replacing, Mr. Repairdude has broken some foils and then tried to replace them with the extra wiring showing in the picture. The board is still not working. Maybe the original fault was elsewhere, maybe some foil is still bad.

The chips are now removed with a hot air gun - they just might be OK, and are hard to find. The board does not look so bad after all.

By looking the board against light, the truth reveals itself. Many foils are broken, there are even quite long pieces missing here and there. But anyway the board is quite simple and I have the schematic.

I soldered thin wire onto board to fix the damaged foils. Some lacquer must be scraped off the board to enable soldering. And sockets for the IC's are now mandatory. The board definitely can't stand a third removal of these chips. One of the sockets is already in place. Because most of the holes had lost the plate through, the sockets must be soldered from both sides. That takes a small tipped soldering iron and steady hands.

Some hot melt glue is put onto the added wiring, to prevent them moving and getting loose. The chips are in place, and by looking at the schematic, I checked continuity from every RIOT chip leg to the place it should be connected to. That ensured my foil repairs were indeed succesful. Everything looks fine. Now, install a test prom and see what happens. Everything works! I am quite sure that the board will work even in a game. That will be seen when I give the board back to its owner.

Looking carefully you might notice that one of the chips has two stacked sockets! The chips legs were cut a little short, and I was unsure whether it would stay in a socket. So I soldered the chip onto one socket, that is then pushed into the circuit board socket.