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I picked up this PET and trying to restore it. Screen is garbeled on boot and stays like that displaying random chars, some of them flashing. Tried to resocket the chips, but it didn't help. I don't have a logic analyzer, and ram chips are not socketed. What sort of checks I can I do to identify the fault? I have a multimeter and osciloscope, not very skilled at electronics, though.

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    If / when you get it working, it would be great if you updated this question with what the actual problem turns out to be.
    – Geo...
    Commented Oct 8, 2020 at 21:29

2 Answers 2

Reset to default
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The garbled screen is normal on startup, but should be replaced by the BASIC screen after a moment.

There's a rather detailed troubleshooting guide, including links, at http://www.dasarodesigns.com/projects/troubleshooting-common-problems-with-the-commodore-pet-2001/

I'd suggest that you ignore the spare parts sales pitch at the beginning and work your way through it.

You might also want to have a look at the Tynemouth Software PET repair blog entries at http://blog.tynemouthsoftware.co.uk/ -- they show fixing some interesting PET issues step by step.

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    I'm afraid that, other than the first sentence, this is a link-only answer. It might be helpful, but only as long as the links remain valid.
    – tofro
    Commented Oct 7, 2020 at 21:52
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    @tofro Fair enough, but then, the question is too broad to have a short, specific answer, so pointing the person asking to existing troubleshooting guides, be it on paper or online, is the best one can do. If you're concerned about link validity: The page in question has been archived by the Internet Archive, and I trust everyone on this Stack Exchange to be able to use that.
    – Michael Graf
    Commented Oct 8, 2020 at 5:38
  • In this case, your answer should have been a comment.
    – tofro
    Commented Oct 8, 2020 at 7:23
  • If the question is too broad, it should be closed as such so it can be fixed before it's answered.
    – Mast
    Commented Oct 8, 2020 at 8:55
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    The question isn't too broad, but there's a fairly lengthy set of diagnostics that you need to go through to find out what's wrong. Bad/failed RAM is most common, but ROM failures, PCB corrosion and many other causes plague older CBM machines. (As an aside, I'd heard rumours of PETs with drilled-out PCBs to prevent upgrading memory to force buying a new computer, but this photo shows it off clearly)
    – scruss
    Commented Oct 8, 2020 at 15:17
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This is anecdotal, so take with a big grain of salt...

I have a C-64 power supply that was home-brewed for me by my Dad when I was a kid from the remnants of the original. My Dad, never being one to do anything halfway, did a fantastic job and added a 'voltage adjust dial' and analog meter to the finished supply. This means I can actually dial-down the voltage to my C-64 until the computer does _weird things_™.

Not that I do this on purpose, but occasionally the dial gets bumped and the volts go low and the C-64 breaks out into a screen of gobbledy-gook that looks almost exactly as you describe.

I'd check the power supply first thing and make sure there are enough jolts going to the main board.

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  • I don't throw very much away in terms of tranformers. Now I have a pile of transformers with no inkling of what each one was originally for. Basically if the connector, voltage and ampage match, I use it.
    – cup
    Commented Oct 8, 2020 at 15:35
  • Yep. Always a good idea to keep the old wall warts. I have a tub of them myself. - But the OP question immediately made me think of what my 64 looks like when it's under-powered.
    – Geo...
    Commented Oct 8, 2020 at 16:35

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