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A Tricky Commodore PET Repair: Tracking Down 6 1/2 Bad Chips

posted by janrinok on Monday April 14, @03:39PM   Printer-friendly

owl writes:

http://www.righto.com/2025/04/commodore-pet-repair.html

In 1977, Commodore released the PET computer, a quirky home computer that combined the processor, a tiny keyboard, a cassette drive for storage, and a trapezoidal screen in a metal unit. The Commodore PET, the Apple II, and Radio Shack's TRS-80 started the home computer market with ready-to-run computers, systems that were called in retrospect the 1977 Trinity. I did much of my early programming on the PET, so when someone offered me a non-working PET a few years ago, I took it for nostalgic reasons.

You'd think that a home computer would be easy to repair, but it turned out to be a challenge. The chips in early PETs are notorious for failures and, sure enough, we found multiple bad chips. Moreover, these RAM and ROM chips were special designs that are mostly unobtainable now. In this post, I'll summarize how we repaired the system, in case it helps anyone else.

Original Submission

 
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  • (Score: 0) by Anonymous Coward on Tuesday April 15, @03:01AM (2 children)

    by Anonymous Coward on Tuesday April 15, @03:01AM (#1400252)

    Informative and insightful. But a few points / augmentations / disagreements:

    1) (huh? means something to someone, so that's fine)

    2) Absolutely agree. I'd even dummy-load test the PS before connecting it to those fragile old chips, check for anything getting hot, check waveforms, give it some time to run, etc. I didn't know about RIFA caps, so thank you for that.

    3) Well, the problem chips were all sharing a bus, so you pretty much have to isolate them to figure out which one is (or ones are) causing the problem (any one or several can load down a signal).

    4) I guess you missed it- he said the first pass programming the EPROM was on a programmer that didn't provide enough programming voltage for the particular chip they used.

    The 2716 EPROM requires a bit more voltage to program than Marc's programmer supported, but the chips seemed to have the right contents (foreshadowing).

    ("foreshadowing" meant that there would be problems later due to the insufficient programming voltage.)

    5) Absolutely agree. Makes me wonder: do any logic analyzers auto-sense the input voltage ranges? Or allow you to adjust the thresholds?

    Absolutely agree, really really good article. I learned a lot, a lot. Not sure I would have figured it all out. Certainly would have taken me longer, and my lack of knowledge of those older machines might have demotivated me.

  • (Score: 2) by KritonK on Tuesday April 15, @05:45AM

    by KritonK (465) on Tuesday April 15, @05:45AM (#1400267)

    Adrian's Digital Basement [youtube.com] is a youtube channel where said Adrian restores old computers and peripherals. He's even restored a PET or two. The rest of the comments are comments that Adrian or frequent viewers of his channel would have made.

    If you are interested in retro computing, this channel is really worth watching.

  • (Score: 2) by ChrisMaple on Tuesday April 15, @02:32PM

    by ChrisMaple (6964) on Tuesday April 15, @02:32PM (#1400303)

    There's a good chance that reprogramming (without erasing) at the below-spec voltage would shift enough charge to correct the bad bits. The risk is that they'd still be marginal; that they'd be erratic and the first to fail again as time passed.