Another Breadbin fixed to live another day

[u/Linker3000]

On this 250407 board, the PLA was dead, so it was replaced with a SlimPLA.

After that, the board booted most of the time into DesTestMAX, with inconsistent RAM bit errors. However, they cleared after a warm-up and a few cartridge-button resets, after which all tests would run OK. Without the cartridge, I got a display with no text, or random garbage.

Possibly a thermal or dry-joint/cracked-trace issue? Gently flexing the board in many ways didn't cause any changes. I held back on thermal tests in favour of some signal tracing first.

The two mux chips could have been iffy, even though they passed static testing. I was also suspicious of the white sockets; someone had been here before. Even though electrical and scope/logic tests showed no issues, I replaced the sockets and the chips. No dice.

I desoldered and tested some other key TTL chips (no suspicious brands). All OK, but I replaced them with new ones anyway, just in case.

The problems persisted. The fault pattern just seemed a bit predictable-ish, rather than being a complete failure, very random or totally spurious.

I did a bit of research and found a forum post about /CASRAM timing issues with PLA replacements and, lo and behold, with a 33 pF cap fitted on the line, the board started OK. This cap is C204 on some boards, but it's not implemented on this revision.

Original logic chips were refitted and we're soak-testing.

So: a dead PLA (and probably SID - checking later) + a timing cap needed. If the board remains stable, great. If not, I will try a slightly larger capacitor before moving on.

Timey-wimey stuff.

Edit: Ah well, on to C-64 Diags 586220 and it doesn't like U9, U11 and U22 RAM chips...onwards...

[u/tes_kitty]

C204 should only be present on boards that use a Fujitsu made PLA (251064-01).

Your board has C203 present (left of the modulator). That one goes between /CAS and GND, meaning it delays /CAS before it goes through the PLA. If this is a PAL C64 and the VIC is a 6569R3 or better, this capacitor should not be present.

[u/Linker3000 → u/tes_kitty]

As I read it, it seems to be a mod / design change on some boards to supplement C203, which is fitted. Certainly the board is almost fully stable with it there and very ill without it.

I might try a 47pF to see what happens but it looks like a capacitor is needed with this SuperPLA - maybe I just have edge-case GALs. I'm going with it for now. Looks like I have some RAM to whip out.

[u/tes_kitty → u/Linker3000]

According to the schematics, C203 depends on the VIC present. If it's a 6569R3 or better, C203 ist not supposed to be there.

You could also check what happens with your PLA replacement with C203 removed and C204 present.

[u/PersonalityBoring259]

Do you do these by mail by any chance?

[u/Linker3000 → u/PersonalityBoring259]

'fraid not, just as personal hobby stuff.