A tricky Commodore PET repair: tracking down 6 1/2 bad chips
25 comments
·April 13, 2025kmoser
TacticalCoder
[dead]
userbinator
How could the computer be operating well for the most part, yet also completely wrong?
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.
cshokie
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.
micheljansen
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.
qiqitori
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.)
micheljansen
Thanks I will see if I can get my hands on an oscilloscope and look at the 12v and 5v rails.
kens
Author here if anyone has questions.
MBCook
Can you explain what’s odd about the ROM chips of the PET necessitating the adapters for replacement with more common parts?
philiplu
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.
dep_b
That keyboard doesn't look like fun to type on. The one on the VIC-20 and C-64 were pretty okay!
classichasclass
It's not. The chiclet PET keyboards are uniquely horrid. The later PET keyboards, however, aren't too bad.
dep_b
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.
jgrahamc
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.
SoftTalker
If it works, don’t mess with it.
RetroTechie
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.
cesaref
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...
genter
To add to this, the electrolyte will leak out and damage the circuit board.
MegaDeKay
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.
jgrahamc
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.
morphle
I have one in pristine condition for sale (in Europe, willing to ship worldwide).
> I typed in a simple program to generate an animated graphical pattern, a program I remembered from when I was about 13 [...]
I'm astounded that anyone would be able to remember such a complicated program after several decades. About all I remember from those days is: