exxos's Atari, Amiga & retro forum

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PhilC wrote: 14 Feb 2024 17:53 @rubber_jonnie thats for a function rom and is empty as standard.

Cool, thanks.

I tried the diag cart first thing in C128 mode, but there are definitely some issues, as it's telling me that most of the RAM is bad (I don't believe that for a minute) and can find different banks dead on different iterations of the tests.

Basically right now it's on iteration 14 and only shows LO RAM bank1 as good, and it's also showing the 8721 PLA as bad, but also reporting the PLA as OK. Strange, unless it means the C64 PLA is OK.

That suggests to me maybe a marginal component between the RAM and the rest of the machine, or the 8721 PLA is bad. C64 tests are similar, but things like the cassette port works on the C128 tests, but not for the C64 tests.

I thought I'd try cold spray on the PLA, and now it won't boot to any of the diags, C128 or C64, though I can get a BASIC prompt, but the wording at the top is multi colour.

I probably should do some more diags, but since it crashes when I type GO64 at the C128 BASIC prompt (I can get to C64 by holding the left Commodore key at boot) I really do feel like what's wrong is at the core of the machine, which means PLA.

I think I'll order one up just in case.

@rubber_jonnie my money would be on either a PLA or MMU.

I had a C64 with a bad PLA and that would boot to the prompt but with multi coloured text

And it is running diags again, and interestingly the C64 dead test is passing every time.

PLA is on order, and I've popped heatsinks on the critical ICs so I can run tests for longer.

PhilC wrote: 15 Feb 2024 11:31 @rubber_jonnie my money would be on either a PLA or MMU.

I had a C64 with a bad PLA and that would boot to the prompt but with multi coloured text

Yeah, I have ordered a 8721, so we'll see, but the cold spray results seem like they are point at that as the issue.

Just in case I also ordered an MMU, the parts are actually surprisingly cheap.

@rubber_jonnie and even if you don’t end up using them it’s a good idea to have the spares.

It’s a good idea to look after that 8502 cpu as they are around £30 from Germany

PhilC wrote: 15 Feb 2024 12:29 @rubber_jonnie and even if you don’t end up using them it’s a good idea to have the spares.

It’s a good idea to look after that 8502 cpu as they are around £30 from Germany

Indeed :)

Will have a look, see what I can find re the 8502, thanks, keeping my fingers crossed it's all good though :)

As it goes, it plays normal C64 carts just fine and dandy, so that is positive at least.

That's exactly what I found with it as well @rubber_jonnie which made me think MMU or PLA.

If you do want to change the ram then check out the memory board here, it works very well on my C128D

https://www.retroleum.co.uk/c128-chips

Does the PLA have any effect on memory access? Inf not, seeing the random memory issues then surely the MMU would be a better bet for the faulty chip?

PhilC wrote: 15 Feb 2024 13:40 That's exactly what I found with it as well @rubber_jonnie which made me think MMU or PLA.

If you do want to change the ram then check out the memory board here, it works very well on my C128D

https://www.retroleum.co.uk/c128-chips

I'm not convinced that the RAM is at fault, as it fails under the C128, fails under the C64, but then passes the dead test. I can run C64 cartridges for long periods with no issues, PLA or MMU seems much more likely.

I think the C64 side is more or less OK based on the results with the cartridges. Lets hope the spares for the C128 side arrive quickly!