More Compact XT fun

Well, I’ve gotten her to post but she won’t boot. The boot disks were new old stock, MS DOS 4.01. So old enough that the disks might have failed even sitting in the shrinkwrapped package. But it could be other things.

The odd part is it would POST one out of maybe 3 attempts, then stop doing even that. And then it needs to rest hours at least before it will POST again. No error tones, though I’m not sure if this BIOS has them. If it’s going to POST, it will beep once and proceed to do the memory test. Which it shut down during once at 384k out of 512.

Now, that’s odd. I’m thinking maybe unreliable power, so I pull out my meter. The +5V are solid. -12V is at like -11.9. +12v? Well that’s where it gets interesting, it reads 11.4v. Dead on the bottom of the tolerances for ATX. If my meter is reading even slightly high, not even enough to worry about normally, it’s out of spec. I’m not sure how tolerant the computer itself is, but if I’m getting values this low I’m wondering if it’s even reliably delivering 11.4. Is it delivering like 10 the times it doesn’t POST? Not sure. But it’s borderline at best on the +12v rail.

Oh, the failure to read the boot disks? While I’d have expected to see a disk drive related error message rather than non system disk, the floppy drive here requires +12v. Going too far out of tolerances(and I’m not sure how tight they are for this drive) and malfunctions will ensue, and there’s a chance it’s working well enough that the BIOS thinks it’s fine, but is still unable to read the disk. Not sure.

At any rate, given the system behavior looking like it’s power related, and the PSU testing as borderline *at best*, I think I need a new power supply, this one is junk. Once I get a new PSU, and the voltages delivered are more reasonable, I’ll resume working on this system.

Might research PSU repair and see if fixing it might be a viable learning project rather than throw the old one out. It’s not generally a component you’d fix(unless it’s an original PSU for a vintage system), but as a learning project it might be worth doing.

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