Another Commodore 64 Black Screen Repair

(This was originally a Twitter thread on September 23, 2018)

September 23, 2018

On the bench today is a Commodore 64 picked up from @Recta_Pete at Atari Party East 2018. This revision is the cost reduced long board because of the sparse components around the VIC-II. I already spot several problems.

1. The SID and PLA locations are swapped. SID should be U18 and PLA should be U17.

2. There’s a character ROM (901225) where the missing kernal (901227) should be (middle, U4)

3. The VIC-II has some bent pins. These didn’t bend back nicely. I may try to solder some legs to the remaining stubs if it’s still good.

4. This board features the prone-to-failure MOS 7709 which is a 74LS258.

September 24, 2018

I have some 74LS258s on the way. Meanwhile I scoped around the board and found the /RES signal to the CPU never goes high so the #c64 will never start. It should go high after 0.5 seconds.

The 556 timer IC is the basis for the long board #c64 reset circuit (along with a capacitor that checked out ok). I removed the old 556 and swapped in a new one.

Progress! The #c64 diagnostic cart is able to start now and shows some bad RAM (or possibly RAM mux ICs).

September 26, 2018

I replaced the three MOS logic chips (2×7708 and a 7709) but that didn’t fix the problem. They’ll probably still fail at some point but are now socketed.

Also replaced RAM at U11 with no change. So far that’s 1 for 5.

Decided to try the piggyback RAM trick at U24 and look at that— it’s booting now. 2 for 6. Ignore BAD as there’s no harness connected yet.

With the C64 diagnostic harness attached, it’s working 100%! Final score: PLA/SID swapped, bad PLA, missing kernal, bad 556 timer, bad U24 RAM. VIC-II ok with new legs, SID ok.

That was a fun repair! Thanks for joining me!

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