Sometimes repairing a retro computer is as simple as replacing a leaking capacitor or a broken chip. It only takes a few minutes. But there are also tough cases where you can spend several evenings digging around the board, diagnosing, testing, and all to no avail. Usually I go back to such cases from time to time and sometimes I end up finding the culprit and fixing the damn thing. But some cases are so hopeless that it is not worth wasting time on them. This post will be about such a case… and the Commodore 64 necromancy
It all started with one of the dirtiest C64 boards I’ve ever seen. I bought it for around 10 euros. It was complete (except one transistor) with VIC and SID, but sold as broken. Both RAM and one ROM chip were socketed so someone definitely worked on this board in the past. When it finally arrived, after a short inspection I noticed that whoever was repairing this board didn’t have much soldering skills. The sockets were raised, there was an awful lot of tin under them, an awful lot of flux on the bottom, two CPU legs were shorted, and some of the legs of other chips also had a lot of tin on them. It wasn’t looking good.
I started by checking why there is tin on some chips. It turned out that some of the legs were replaced with wire, although it was not visible that the chips were desoldered in the past. Maybe the legs were eaten by corrosion or something? I’ll never know. Then I desoldered the badly seated sockets and found some horrible looking pads that were hidden under tons of tin. There was also one raised pad and a trace between the ROM chips, as well as some broken traces near the CPU. I’m pretty sure someone was using too high a temperature while soldering.
Of course I tried to repair this board. After cleaning, adding new sockets, checking / fixing pins of all chips, fixing broken traces with wire… C64 has turned on and showed well known blue basic screen! Unfortunately, the diagnostics showed problems with the user port and one CIA. I replaced this CIA with known working one, but the problems remained. I checked all the circuitry related to the CIA and the user port and everything was fine as well. I decided to swap all the chips one by one to see if any of them were broken. And after three I gave up … The board was literally falling apart in my hands. I don’t know where it was stored, but it was so damaged (possibly by temperature difference) that it couldn’t be repaired. Even at the lowest possible melting point of the tin, almost all of the pads were lifting with the tracks. But hey! That C64 was turning on and almost working! It deserved to be saved! So I dug up the this board, which I got for free some time ago when I bought other C64 parts.
This is where necromancy begins on C64. I checked this nearly empty board for broken traces, and indeed there were several of them. Then I took the components out of the damaged board and put them in that dead “body”. I only skipped the desoldering of the RF modulator box as I had a spare. After a while of soldering, I had a fully equipped C64 board.
To complete the resurrection of the C64, I took the chips from the damaged board and put them in a new one. The only exception was the 64-pin SuperPLA which differs between the board 250469 revisons A and B. Luckily I had a spare. I checked everything again and then tried turning on the C64. It went on but the video was black and white, some characters were incorrect and the diagnostics revealed a few issues.
One of the 4066 chips was damaged and I found one short circuit due to damaged isolation in one of the wires used to repair the traces between rom chips. After replacing them, the diagnostic was fine and the C64 was successfully resurrected! The black and white screen was due to my spare RF box so I had to desolder the original one anyway (CT 1 adjustments didn’t help and I was too lazy to check and repair the RF box). However, after additional several hours of diagnostics, it turned out that the board sometimes reports user port errors related to CIA at U2. I replaced this chip and now it’s fine even after a few hours. The weird thing is that this “broken” CIA works perfectly fine in another C64. Well, maybe it was just a bad connection or something. Here it is, the C64, alive again and in all its glory – minus the case