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Compaq Portable III power supply with 7.5 V instead of 5V

mattg1974

New Member
Joined
Feb 5, 2023
Messages
3
Hi,
I am currently restoring and upgrading my portable III 286. The machine generally work's but while I added a gotek and wanted to measure some signals i slipped and shortened the 5V rail. That caused the portable to reset and blew one of the fuses on the motherboard. That was pretty stupid of me. After that the system still boots up but the 5V rail now carries 7.5V. i am not really sure if that was before as well but it can't be healthy for the CPU and other components. So i don't want to turn it on again until I have that fixed sadly my knowledge about power supply's is rather limited. I opened the power supply I couldn't see any obvious issues...
Anybody has a clue what I could do?
 
What board are you talking about? And you need to post some pictures.

There are no fuses on the motherboard as far as I'm aware. If you shorted the 5v rail to something that wasn't expecting 5v, you probably caused a lot of damage that's going to be very difficult to fix.
 
Thanks for the advice. I shot some pictures of the fuse on board. I never seen a fuse like that but it's clearly marked with f2 and yes there is a f1 on the board too. Right besides the plug for the keyboard above the modem/extension Slot. Another picture is of the open PSU. Everything looks still neat. The thing is it boots up and counts the memory... Well I did turn it off once I saw that we have 7.5v. if I bridge the fuse the drive work again too. It could even be that there where always 7.5v cause I only started to measure voltages after my very stupid mistake. The 12v rail reads about 12.3 v which is within spec.
Maybe that might help someone giving me at least a starting point... Sadly I don't have a multimeter that is able to measure capacitors....
 

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Fuses don't blow for no reason, there was a significant fault. It could be the high voltage on the 5v rail.

You can start with the low hanging fruit and check for leaking/failed capacitors. In that era, you'll want to remove the capacitors from the board and check under them, because the rubber plugs on the bottom shrink and let the electrolyte leak out.

For a capacitance tester, just get one of those cheapish chineseium component testers, they're good enough for the hobbyist.


Just be warned that these testers have zero protections on the input pins. Make absolutely sure the capacitor is discharged before you test it, or it will fry the microcontroller and brick the unit.
 
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