Semi-faulty C64

Hi,

I have a partially working C64 - it won't run some programs/games. It has had this problem for years now without any change in symptoms. I've narrowed the problem down to the C64 itself (Disk drive, Power Supply and the files have been tested and found working). I've also tested swapping the socketed chips: CIA1, CIA2, SID, VIC. No change in symptoms.

The resulting errors depend on the program, usually a frozen garbage screen. The errors are NOT intermittent, meaning if I run a game and it causes a frozen screen, it will produce that exact same frozen screen every time I run it. It's interesting to note that when I use a fastloader called "Star Turbo", it usually makes a loaded program work at least a bit better, sometimes perfectly. The fastloader never seems to make a game work worse (unless the game simply isn't compatible with fastloaders).

Does anyone know what exactly does a C64 fastloader change in the loading process, how could it make such a big difference. Which chips inside the C64 could be most affected by the fast-loading. I was able to dump the C64 ROMs and they were exact copies of the ROMs bundled with a C64 emulator. Does this mean that the ROMs are OK, or could there still be a problem with them? If they are OK, then I would suspect that the faulty chip is one of the following: PLA, RAM or CPU. The RAM chips I haven't been able to swap, but I've tested them in many different ways:

-Piggybacking a working RAM chip on top of each chip
-2 different software diagnostic tests (both passed)
-Startup screen has never displayed more or less than 38911 bytes free
-Loaded a "non-working" program to memory, then verified it. Result: Verify OK.

Because of these tests I'm starting to think the RAM might be OK, although there is still a slight chance that it's not. I don't have diagnostic cartridges so I used disk-based diagnostic tools, which can only test about 80% of the available memory. I've also heard that some people have had problems just like mine and they were caused by faulty RAM.

If the RAM is ok and it can be assumed that the problem is with one of the bigger chips, then that leaves only the PLA and CPU as possible suspects. I haven't really been able to test these two chips in any way. I've read that PLA failures are very common so a semi-faulty PLA would be my best guess. Could fastloaders have an effect on how these chips work? Both chips run hot, but cooler than the SID and VIC. Are there any more tests that I could do to find out which chip is at fault here?

Thanks in advance.

[TCD]

Hi drevin,

I think the guys over at http://www.lemon64.com/ can help you better. But then again someone could prove me wrong