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: 3, Interesting) by krishnoid on Monday April 14, @04:28PM (3 children)
It's good to understand how things work and how they fail. I should probably get my C64 out of storage.
(Score: 0) by Anonymous Coward on Monday April 14, @09:14PM (1 child)
Irrespective + regardless = irregardless
(Score: 3, Insightful) by DannyB on Tuesday April 15, @05:08PM
Regardless or irregardless of whether gasoline is flammable or inflammable, it burns, irrespective of fire codes.
The Centauri traded Earth jump gate technology in exchange for our superior hair mousse formulas.
(Score: 0) by Anonymous Coward on Tuesday April 15, @01:09AM
For a good time:
Mallow/khallow/s
(Score: 4, Informative) by VLM on Monday April 14, @05:03PM (3 children)
1) Reads like an Adrians Digital Basement video watches
2) Brave to smack the power as the first step to test the power supply; you can assume any RIFA brand caps in the RF filtering network have gone bad, plenty of electrolytics get questionable. He 'won' this time but I'd suggest testing the power supply separately first on any old equipment, computer or ham radio or anything old really.
3) People like to rip on shotgun troubleshooting but a fair first step is just look at every pin on the o-scope looking for weirdness and following up. Its fast and you can get pretty far with that. He saw there was a bus conflict and some chips were fighting making multiple voltage levels on the bus but didn't directly follow up on it and just tested all the chips; probably could have figured it out by reasoning about the o-scope waveforms.
4) He didn't explain why he knew his eprom programmer was to blame as opposed to the eraser; if I remember my days of playing with 27xxx eproms, they erased to bit high, all 0xFF so when he saw a 0x7A instead of a 0x72 he instantly knew it was a programmer failure not an eraser failure. Look a bit 3, it's still 1 and was supposed to be 0. He didn't necessarily need to use a different programmer, forced reprogramming probably would have set (cleared) that bit eventually in another attempt or two.
5) Note that he/they hit the logic analyzer; its an important step before hitting the analyzer to put the scope to each pin (see step 3) because the logic analyzer's voltage limits do not necessarily match the voltage limits of the mystery family chips onboard. They probably checked each pin with an o-scope before slapping the analyzer on. The fault could have just as well been electrical something on that pin being pulled up. Of course then step 112238 would not have had a 0 for bit 3 but whatever, you get what I mean in general. Kind of like OSI networking model the higher level stuff shouldn't be troubleshooted until the lower level stuff actually works.
That said troubleshooting stories are always cool and its hard to argue with success!
(Score: 0) by Anonymous Coward on Tuesday April 15, @03:01AM (2 children)
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.
("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
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
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.
(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.