C64C Black Screen

[yellerjeep]

Hey fellow Commodorians!

I've taken up repairing C64s and have two successes under my belt but this one is confusing to me.

I'm using a well built Power Supply from Ray Carlsen for testing so for everything else assume that my power is good.

The unit powers up to a black screen, dead test cartridge does not produce any output either. The address and data lines show activity on the 8500R4 CPU and no chips are "hot" to the touch aside from the VIC and SID which are just moderately warm.

Other things checked:

Clock, nice solid signal.
Reset, normal high after power on.

The original owner said that it started to "hang up" with screen corruption. He'd reset the computer and it would be normal for a bit, but then crash again. Eventually it just got to this state. His power supply is a 5v brick and a 9vac brick with a cable he made.

My current working theory is that there's something wrong with the VIC. I plan to swap it with my known working 64c.

 

[Zippy Zapp]

My current working theory is that there's something wrong with the VIC. I plan to swap it with my known working 64c

A black screen alone does not immediately point to a bad VIC, though it is a possibility. Literally any faulty chip in the system could produce a black screen. I have had all sorts of chips fail that produced a black screen.

Does the black screen produce a signal on the monitor or is there no signal at all? If it produces an NTSC (or PAL if it is a PAL model) signal, even though it is a black screen, then that is an indication the VIC is doing something. If it produces no signal and you have a disk hooked up can you issue a disk command, for instance LOAD"$",8? If it accesses the drive then that might be an indicator. The 85xx chips in the 64c tend to be reliable so I would probably look at RAM or related chips. Did you check the activity of the 2 RAM chips?

 

[Ruud]

Nowadays IMHO the most likely culprit is the PLA.

 

[Zippy Zapp]

Nowadays IMHO the most likely culprit is the PLA.

For sure if it is a MOS one. But since he said it has an 8500 CPU I assume this is a shortboard 64C with the "monster PLA". Those very rarely go bad so it would be unlikely in this case. But if it is an early 64C that used the 1986 board with 2 RAM chips it still has a MOS PLA.

So I guess my first question should have been what board REV is in there?