Commodore 64 question - test-drb@ccmp.vtda.org

25 Feb 2013

...

 I have a similar problem; I've noticed that the IRQ line is
 constantly asserted, which it's not if I just pull the VIC
 (of course, that could be because the boot routine is
 crashing; the VIC otherwise looks like it's probably working
 fine, since it emits a proper NTSC black screen with color
 burst).

 Is there a relatively simple test that can be done with an
 analog scope (no storage) that can determine whether the
 PLA is bad?  Some of the outputs, at least, seem to toggle.
 I'd like to narrow it down before I put down the money for
 a replacement part.

A friend of mine asked me to repair his failed Commodore 64 many years ago.
He thoughtfully supplied a schematic for me to work from.

As far as I recall (and nobody ever accused me of having a good memory), I
wrote some code using the assembler on my BBC Micro, put it in an EPROM,
swapped that for the ROM in the Commodore and identified a RAM problem
by outputing status on the user port.

I'm not sure I should admit to this but rather than an EPROM, I think it was
actually my cheapo EPROM eliminator - a 2K static RAM equipped with two flying
to a bench power supply...

If I remember correctly, I replaced the suspect RAM chip and the machine
worked.

Regards,
Peter Coghlan.