VOGONS


First post, by radiounix

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So I'm pretty sure the power supply in my Unisys 486 has failed. Within the span of a day, the machine went from instability in Windows, to locking up in DOS copying files or running a benchmark and sometimes failing to complete the boot, to not posting and not sending out a VGA signal. I've got it stripped down to just 1 hard disk, one stick of RAM, no cache, and had tried setting BIOS to defaults, disabling all onboard hardware and shadowing in BIOS. It was stable before and had passed memtest86 and Spinrite when I got it maybe two weeks ago. This is probably a power supply fault, right?

This machine is a weird, late PCI 486 DX2/66 with no 3.3v regulator onboard and a 1000UF 10v capacitor where I would expect the regulator. It has a third motherboard plug which supplies 3.3 volts, and the OEM power supply has a 14 amp(!) 3.3v rail. I saw posts where people were like, "you don't need the 3.3v" rail, but I think they had regulators onboard still.

I know I should probably just be recapping this, but I don't like the idea of me soldering and messing around on a circuit that will run live AC. What are my other options?

Last edited by radiounix on 2020-09-21, 15:27. Edited 2 times in total.

Reply 3 of 4, by radiounix

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Ok, so I figured what's going on. A small number of late AT power supplies included a third motherboard plug, named P10 or AUX. It does supply 3.3 volts, and was a thing on a few OEM systems from Compaq, Zeos, Unisys, and probably various other. I could not find such a power supply on Ebay, and there's hardly any mentions even on Usenet -- this must have been rare, even back in the day.

I found a site which supposedly still has an ATX to P8, P9, and P10 adapter in stock, and I also bought a 200 watt Seasonic power supply with the old -5V line since I don't have any other power supplies, ATX or otherwise.

Reply 4 of 4, by TheMobRules

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I have one of those P8, P9 and P10 adapters, basically the P10 takes 3 x 3.3V and 3 x grounds from the ATX power connector. I have a couple of boards with that type of connector, an Intel Batman's Revenge (Socket 4) and a Zeos Pantera. In the Intel board for example, that connector is there only to support 3.3V PCI cards, so it usually isn't necessary since most PCI cards from that era are 5V. The manual of the Zeos board on the other hand claims that the P10 needs to be plugged in when setting the 3.3V jumper for the CPU voltage.

One problem with the adapter I bought was that the cables were very short, which would cause problems if the P10 is located far from the P8-P9 on the board... so you may end up needing an "adapter for the adapter" in that case.