FilterHN

Anyone who has done any overclocking knows about that. When signals or timing are marginal, intermittentness appears. In the PC world, programs like Memtest86+ and Linpack are great at stress-testing to find marginality, but I'm not sure if such software was as common in the 8-bit era.

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cshokie

6 days ago

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I don’t personally have the time or inclination to do any retrocomputing, but I love seeing the repairs that people do for them. It’s interesting in a way I wouldn’t have expected. Thank you for sharing.

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micheljansen

6 days ago

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Amazing that they were able to figure this out. I have a PET that also sometimes boots up into a garbled screen similar to the one pictured in the article. I am usually able to get past it with a few hard resets. I don’t have access to (or much knowledge of) logic analysers, but this almost makes it look doable to figure out the problem.

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qiqitori

6 days ago

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If you want to go that route, you may find it easier to check your logic analyzer dump side-by-side with the debugger in an emulator (e.g. VICE). Break on the first instruction and then go step-by-step. (Just need to make sure you have the same ROM version, but that shouldn't be too hard.)

However, your problem kind of sounds like a power supply problem. So using a logic analyzer will maybe just produce a different result every time. So maybe check the 12V and 5V rails on an oscilloscope while turning on the computer. (Or maybe it's a problem with the reset circuit, etc.)

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micheljansen

6 days ago

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Thanks I will see if I can get my hands on an oscilloscope and look at the 12v and 5v rails.

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kens

7 days ago

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Author here if anyone has questions.

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MBCook

6 days ago

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Can you explain what’s odd about the ROM chips of the PET necessitating the adapters for replacement with more common parts?

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kens

6 days ago

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The MOS Technology 6540 ROM included some on-chip address decoding. The adapter board has a TTL chip to perform this decoding, and uses a standard 2716 EPROM.

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MBCook

6 days ago

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Oh interesting. I’ve never heard of a chip like that.

Thanks.

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philiplu

6 days ago

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That PET is in amazingly good physical shape. I think the keyboard on my 8K PET back in 1978 looked that bright for at most a year or so, before the plastic overlays started fraying.

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dep_b

6 days ago

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That keyboard doesn't look like fun to type on. The one on the VIC-20 and C-64 were pretty okay!

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classichasclass

6 days ago

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It's not. The chiclet PET keyboards are uniquely horrid. The later PET keyboards, however, aren't too bad.

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dep_b

6 days ago

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Why would they skimp on such an important item? The enclosure itself looks quite expensive. Most terminal keyboards were pretty OK from what I've seen.

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jgrahamc

6 days ago

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This is wonderful to read. I'm about to embark on getting my Sharp MZ-80K into good shape for the next 40 years. It still works fine but I bet there are a bunch of capacitors just waiting to die, and solder joints that are waiting for the perfect day to fail.

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SoftTalker

6 days ago

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If it works, don’t mess with it.

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RetroTechie

5 days ago

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Usually that's good advice. But not always. If it's still working, you can take preventive steps before an easy-to-fix issue causes impossible-to-fix damage.

Silicon does degrade (and fails). But in my experience, most issues fall in 2 categories:

1) "Mechanical" problems. Connector problems, loose solder joints, corrosion, cracks in pcb traces, etc.

2) Power supply issues. Electrolytic capacitors are suspect #1 there (and they may not respond well to being powered after a looong time in storage).

Rules I apply:

a) If unsure how a machine looks inside (condition, mods etc): inspect internals before powering up.

b) Check that a machine is working before modifying it in any way. If not working: minimize steps to get to a working state.

c) Do mods in small incremental steps.

That way you can always backtrack to last working state.

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cesaref

6 days ago

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The problem is that electrolytic capacitors can leak, even if not used. So, you can store a perfectly good computer and retrieve a ruined motherboard a number of years later. SMD capacitors are bad for this, the MZ80K predates all of that though, and I think only has a few through hole capacitors on the mainboard.

The power supply should also be checked, and in this case, it's safety capacitors which are the risk there, since some of the older ones (RIFAs for example) tend to be hygroscopic, and if they crack they end up blowing next time mains is applied across them. I've not had any problems with power supplies killing the machine with an over voltage, but i've heard this is also a risk.

Anyhow, 45 years or so is time for a recap, and it'll do another 30 years before it's time again...

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genter

6 days ago

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To add to this, the electrolyte will leak out and damage the circuit board.

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MegaDeKay

6 days ago

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Better advice with these older machines is "mess with it while it is still working". Even significantly more recent machines like the OG XBox pre-V1.6 can be badly damaged when the clock capacitor leaks onto the circuit board.

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jgrahamc

6 days ago

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The problem with not doing some preventative stuff on these old machines is that in the event of a power supply failure (IIRC the MZ-80K has +12V, +5V and -5V) you can get overvoltage and cause damage to the chips.

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morphle

6 days ago

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I have one in pristine condition for sale (in Europe, willing to ship worldwide).