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Introduction
This Commodore 64 with a 250407 Revision B board came in a lot six nonworking computers that I purchased from eBay. This one seemed to boot but was flaky and would sometimes crash or boot to a black screen. Further, the keyboard was not working. The board as a ceramic R56A Vic-II video chip. The only other main IC that was socketed was the 6510 CPU.
Case Study
I first tackled the keyboard to see if I could get that working. My other c64s with nonworking keyboards simply needed a good cleaning of the contacts. So, I disassembled the Mitsumi keyboard and cleaned the contacts with 99% IPA (from Amazon) as I previously described in Case Study #4. This got the keyboard working. However, some of the keys (e.g. F1 – F4) were giving some characters that they weren’t supposed to. This, and the next symptom, led me to believe one of the CIAs was faulty.
Next, I wanted to track down the cause of the intermittent black screen. As I poked around pressing on chips it seemed as though pressing on the CIAs (upper left in circuit board image above) would remedy or cause the problem. Given the unexpected characters from the F keys, I strongly suspected one of the CIAs might be bad. So, I desoldered both of them and installed sockets so I could swap in working ones. I cleaned the pins of residual solder, sprayed contact cleaner in the new sockets, and reseated them for testing. This seemed to solve unexpected characters. My suspicion is that there was a cold solder joint that I did not see on inspection. However, this did not fix the black screen issue.
After further testing I noticed the power jack was a little loose and would cause a black screen when I jiggled the power cable. Inspection of the solder joints revealed they were coming loose. I reflowed those joints with fresh solder and that fixed the black screen issue. I also reflowed the power switch joints for good measure even though they looked fine. You can see the failing solder joints in the second image below.
Comments
This is another case study in problems that are not necessarily failing chips despite symptoms that suggest so. Also, a reminder to inspect all solder joints carefully and reflow any that look suspicious with fresh solder. This repair also reminded me how many c64s have nonworking keyboards that can be fixed by cleaning the contacts. I have another like this I fixed with cleaning that I will not post about given Case Study #4 that covers that fault.
To wrap up this repair, I also clipped the C1 and C2 capacitors in the RF circuit to improve the sharpness of the video. I also added heatsinks to the VIC-II.