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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.
(Score: 5, Interesting) by bzipitidoo on Monday April 14, @05:59PM (2 children)
When I was breadboarding ICs (7400 series, for the most part) I had a 5V power supply that was supposed to be safe from short circuits. It had a fuse, of course. So when I accidentally wired in a short circuit, and powered up without first checking that there was resistance between the high and low sides, found that nothing worked, and then found I had put a wire one spot off thus creating a dead short, I undid my mistake and tried again. But that second time, the power supply pumped out 25V, and fried all the chips. I found that out the hard way too, first by seeing that still nothing was working, then by noticing that the chips were almost burning hot to the touch. Then I checked with the volt meter, and learned that the 5V rail was now putting out 25V. Evidently, the power supply was not as safe as had been advertised, and had sustained damage when I accidentally wired in a dead short and powered up. I might have fried still another good power supply by hooking a 2nd one up only to find that the fried chips were themselves now acting as dead shorts, but I had had quite enough of destruction for that day.
Hot swapping is another thing. Modern PCs, yes, that's possible. But on those 1980s computers, no way. The Apple II disk card is notorious for instantly frying itself if it is removed from the expansion slot while the computer is powered on. The Apple II has a lot of IC sockets, and after many years, the electrical connections can get a little uncertain. My Apple II+ started having problems reading the ROMs. Machine code would still work fine, but BASIC code, for which the interpreter is on those ROMs, would not. By ill fortune, one of those times the corrupted ROM code generated a jump to the DOS routine for formatting a disk. I heard the characteristic noise that formatting makes a little too late to save the data on the disk, and sadly, didn't have write protection set.
(Score: 5, Informative) by Mojibake Tengu on Monday April 14, @06:26PM
You should always use a true lab-grade PSU with its own metering and current limitation regulator. Especially on first power-up of a new build.
In electronics, experience is measured by total number of contraptions destroyed.
Rust programming language offends both my Intelligence and my Spirit.
(Score: 1, Interesting) by Anonymous Coward on Tuesday April 15, @03:12AM
Oh, all those chips dying, I felt a great disturbance in the Force, as if millions of voices suddenly cried out in terror and were suddenly silenced.
(ha ha!)
Still hurts that a very long time ago I killed 5 or so antique vacuum tubes in an antique radio. Very accidentally swapped the 1.5V heater supply with 75V plate supply (or whatever the voltages were).
Once I had to repair a small TTL logic board. 20 or so chips. 5V was being drawn down. Simple linear regulator which had overload / short-circuit protection. IIRC chips were soldered in.
So, I connected a high-current 5V supply and very quickly the dead chip got very hot, probably blew out some smoke. Changed it and the rest worked.