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Introduction
I purchased this very nice Commodore 64 off eBay. It was listed as not working and sold for parts. It has a clean 250407 Revision B board and boots to a garbled screen as shown below.
Case Study
The first thing I did was check the voltages and feel around for hot chips. Everything seemed normal. I also did not see any obvious damage to the board. The heat shield was still attached so this board was likely not repaired at any point although someone did install heatsinks on the CPU, PLA, and SID.
The pictorial fault guide points to a number of possible causes of screen like this including a bad MMU, RAM, PLA, VIC-II, etc.
In poking around I noticed that pressing on the VIC-II changed the video pattern and sometimes got a clear or nearly clear screen. This allowed me to run the diagnostic test cartridge which seemed to pass. This was good news. I also ran Desmond’s RAM Test (DesTestMax) from cartridge, and it gave a grey border which indicates a problem with the VIC-II. This was further evidence pointing to the VIC chip.
Before replacing any suspect chip, I always reseat it with some contact cleaner (I usually buy WD-40 contact cleaner from Amazon or a local hardware store) to remove oxidation. This previously helped fix a kernel ROM that was not making good contact (Case Study #8). So, I sprayed the socket with contact cleaner and used a toothbrush to scrub it for a few seconds. I reseated the chip, and the garbled screen went away! I didn’t get a chance to probe the pins of the VIC with the oscilloscope. It would have been interesting to see which one was not making good contact.
Comments
This is another nice example of using contact cleaner to remove oxidation on a non-working chip. The DesTestMAX cartridge provided an important clue pointing to the VIC. I finished up this board by clipping the C1 and C2 capacitors in the RF module for a sharper image and placing heatsinks on the VIC-II.