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Introduction
I purchased this working Commodore 64 326298 Revision A board (1982) with a 5-pin video connector for $100 from a friend. It did indeed work fine when I got it home. However, after it was left on for about 3-5 minutes it would develop a wavy screen exactly like image the shown below that I got from the c64 Pictorial Fault Guide (I did not get a photo before I fixed it). The Pictorial Fault Guide indicates that this symptom is the result of a failed 74LS629 (U31) oscillator chip.
Case Study
I installed a socket and replaced the 74LS629 chip with a new one and that did not fix the issue.
I knew the problem must be in that same circuit, so I did quite a bit of probing with the multimeter and oscilloscope. However, all I could see is that after a few minutes the voltages in that circuit would start to drop. Then I noticed that the 5V voltage regulator on the right side of the board was missing the typical black heatsink. I grabbed it with my pliers and the screen immediately went back to normal. I added a few heatsinks (I buy these ARM heatsinks on Amazon) on the front and back of the regulator and that fixed the issue. Since this happened, I have seen at least one other board missing this heatsink. I think they get loose over time and fall off. Those regulators get really hot and definitely need heatsinks.
Comments
This issue drove me nuts for several days. Fortunately, I spotted the regulator and noticed it was missing the usual black heatsink. A great example of a simple problem and simple fix that can be easily interpreted as a bigger or more mysterious issue that can lead you down some rabbit holes. The later revision boards have this regulator bolted to a big silver heatsink mounted to the board. I went ahead and added heatsinks to the ceramic Revision 56A VIC-II as well. Note that I labeled the SID with an “M” because the sound is marginal. This will need to be replaced at some point if this board is to be used regularly.