General Category > Lisa Troubleshooting and Repair
Lisa 2 started throwing Error 45
dmark:
--- Quote from: sigma7 on May 07, 2022, 03:34:33 pm ---Thanks for documenting the process of bringing your Lisa back to life; always an interesting adventure!
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You're very welcome; thanks for reading it!
--- Quote ---Running your Lisa for a long time at a normal operating temperature shouldn't cause any problems, so AFAIK that doesn't change the approach to the troubleshooting task. Maybe you're looking for something that was thermal related, but since the problem now occurs hot and cold it seems it isn't any more.
Replacing damaged and particularly old caps in the PSU is recommended, but not elsewhere typically. Since the large ones on your video board were suspect I suppose those should be on the list too. Check all the PSU output voltages if you haven't already.
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I did replace the two large caps on the video board as a precaution, so that's done.
Do you have advice where good test points for each voltage rail is? I have it on my todo list to study the schematics to find out, but if you have suggestions off the top of your head that'll save me a little time. :)
--- Quote ---If there is another Lisa owner in your neighbourhood, swapping parts about will probably help to localize the problem quickly.
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As mentioned above, yes I do, although it is more heavily battery damaged than mine.
--- Quote ---Do you have any test equipment beyond a multimeter?
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I do have a digital scope! I was going to jump into looking at signals next...
dmark:
--- Quote from: AlexTheCat123 on May 07, 2022, 04:29:12 pm ---I had a similar problem a month or so ago where the floppy controller started acting weird and the system kept giving bus errors. The problem turned out to be the 8T97 tri-state buffer at U2F on the I/O board. Looking at the I/O board schematics, that 8T97 buffers the /DTACK signal that goes back to the CPU and the absence of this repsonse from the floppy controller could definitely be causing your problem.
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How did you narrow down the root cause? Any particular signals I should try measuring?
AlexTheCat123:
--- Quote ---How did you narrow down the root cause? Any particular signals I should try measuring?
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I started by checking the clock and reset signals on the 6504 microprocessor that controls the floppy drive to make sure that it was being properly clocked and reset, which it was. After that checked out, I probed all of the address and data lines between the 6504, its two 444C-3 RAM chips, and the I/O board ROM to make sure that there was activity on everything and that nothing was stuck high or low. I also checked a couple of the other signals feeding into the chips, like the R/W signals on the 6504 and the chip select signals on the RAM chips. By the way, you can find all of this stuff on the upper half of page 4 of the I/O board schematics.
When all of that looked good, I moved on to the bottom half of the schematic, particularly the circuity that generates /DTACK since I knew that the 68000 needs this signal in order to finish a bus cycle and will cause a bus error if it's never received. I didn't feel like going through all of the complicated flip flop stuff in the schematic, so I started at the end of the chain, just before the 8T97, to see if /DTACK was getting that far. It was, so I guessed that the 8T97 that buffers this signal to the processor might be the problem. I happened to have a spare 8T97 lying around from another project, so I figured that it would be quicker to swap it out than it would to take a deeper dive into what /DTACK was doing after it left the tri-state buffer and headed to the processor. And sure enough, that fixed my problem!
If you happen to have a chip tester of some kind, like the Retro Chip Tester Professional or the more limited TL866 programmer, you can save some time by throwing any suspicious chips in there to see if they're working properly instead of having to order replacements to see if the originals were faulty.
So clearly there was a lot of luck involved in my repair here (especially the part where I happened to have an 8T97, which in my experience isn't a very common chip, lying around), but hopefully you'll get lucky with your issue too!
rayarachelian:
--- Quote from: dmark on May 07, 2022, 07:08:07 pm ---
--- Quote from: rayarachelian on May 07, 2022, 02:51:12 pm ---So, indeed error 45 is a bus error, this happens when some device doesn't respond to the CPU in time. Question that's harder is which device. It might be the floppy controller since you've noticed issues there. 45 is also linked to the MMU as the MMU will throw bus errors if the CPU tries to access memory it hasn't mapped. Could be the adder/comparator chips or the RAM used as MMU registers or something messing with their signals or operation (dust? board corruption from an old battery leak? short? bad filter cap on the RAM used as MMU regs?)
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In what area of which board are there chips located? Are you talking about RAM on the RAM boards here, or is there RAM on other boards as well?
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They're 1Kx8 SRAMs on the CPU board.
see: https://lisaem.sunder.net/LisaSchem/CPU4.gif and https://lisaem.sunder.net/LisaSchem/CPU5.gif
They're around U8A, U9A, U10A They're the 214H8 SRAMs.
I'd also look at the adders at U8E, U9E, U10E - the F283s.
I'd also look at the latch at U4E.
dmark:
--- Quote from: rayarachelian on May 08, 2022, 12:50:30 am ---They're 1Kx8 SRAMs on the CPU board.
see: https://lisaem.sunder.net/LisaSchem/CPU4.gif and https://lisaem.sunder.net/LisaSchem/CPU5.gif
They're around U8A, U9A, U10A They're the 214H8 SRAMs.
I'd also look at the adders at U8E, U9E, U10E - the F283s.
I'd also look at the latch at U4E.
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Sorry if this is an obvious question, but how do you test the CPU board in operation? Is there something like an extender board to run the computer with the card cage outside the case?
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