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Socket7 mATX Motherboard - Wouldn't boot - now PSU fan stays on after shut down?

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First post, by justin1985

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I just got a BCM IN5598 / Packard Bell 850 micro ATX motherboard from eBay. It was described as having been working before it was put away for several years, and arrived looking absolutely perfect - clean, no sign of any damaged components, and no signs of any scratches and in an anti-static bag. It came with 4 x SIMMs and a Cyrix MII already fitted.

IMG20240521215723.jpg

I loosely fitted it to a case (3 or 4 screws) and tried powering it up with a modern ATX PSU (which I'd tested with a PSU tester). Initially there was nothing apart from the CPU and PSU fans spinning up and the keyboard LEDs flashing, but then the CPU fan spun back down, but the PSU fan stayed on.

I tested the coin cell battery, which was dead, replaced it, and sure enough it booted! I power cycled it a few times, saving the date time etc., powered down and added a mouse and floppy drive, and successfully booted DOS. Then I powered it down, and noticed the PSU fan was still on for long enough to realise it wasn't just cooling down. I powered it off at the wall, tried again, but then it went back to just spinning up the CPU fan then back down, with keyboard LEDs flashing but no POST beeps.

After a minute or two, I noticed an electrical crackle sound (that actually kind of seemed to come from the wall socket rather than the PSU or motherboard!) Obviously yanked the mains cord from the wall within seconds. I briefly wondered if maybe something had been shorting against the case, but checked carefully and that seems really unlikely (foam spacer blocks between many of the mounting holes and no mounting spacers in wrong places). I moved it out to the open workbench now.

Since then I've tried testing the PSU again, swapping to the oldest ATX PSU I have (P4 era), removing the battery for a few minutes to reset the CMOS, and giving the SIMM slots some contact cleaner + cleaning the SIMM contacts with an eraser. Still the same behaviour - CPU fan spins up, keyboard lights flash, CPU fan spins back down, PSU fan stays on. In fact, most recently the PSU has powered up and the CPU fan spun up immediately on connecting to the mains - which it didn't do before.

It feels like maybe something related to voltage regulation was intermittent, but has now failed? Any ideas where to start?

Last edited by justin1985 on 2024-05-23, 20:05. Edited 1 time in total.

Reply 1 of 3, by justin1985

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For the record, I've made some progress! The board now boots again, but the PSU fan still stays on after the motherboard is turned off - which seems weird, and still leaves me uncomfortable ...

I did some extra testing: no shorts on any pins on the ATX pins; no shorts on the voltage regulator or the schottky diode package next to it; none of the capacitors test as shorted. The CPU definitely getting warm. I desoldered one of the green capacitors (1500uf, 6.3v, Sanyo branded) and testing it in diode mode it seemed to charge nicely in the correct direction, so I soldered it back.

I also tested the voltages on the voltage regulator and found the VSS pin on the CPU socket - the core voltage seemed to come out slightly high compared to what it was set to - with the jumpers set to the 2.9V specified for the Cyrix CPU, it was testing as ~2.98v. With the jumpers set to 2.8v, it actually tested as ~2.9v. I left it set to that, but it didn't in itself seem to make any difference. Might it be testing high because there is no load? Or something to be concerned about?

The final things I tried were turning down the multiplier from the specified x3.5 to x3, and also removing the BIOS EPROM chip and applying contact cleaner to the socket, and cleaning off its pins with an eraser. Finally it POSTs!

Still though, the PSU fan stays on even after the board has been shut down using the power button. The board seems to power down, the CPU fan stops, but the PSU fan keeps running. It does seem to give three beeps on shut down, which I've never noticed before, and seems weird? This happens with both a P4 era 'Ac-Bel' PSU and a (relatively) more modern FSP TFX PSU.

Any ideas what is happening with the PSU fan staying on, and whether I should be worried?

Reply 2 of 3, by Dan386DX

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Just looking at your picture of the board, it looks like the PWR_SW is not bridged but the SLEEP header is.

I'm sure you've already noticed/thought of this, are you sure you're actually shutting off the PC?

90s PC: IBM 6x86 MX 233MHz. TNT2 M64. 256MB/1GB.
Boring modern PC: i7-12700, RX 7800XT. 32GB/1TB.
Fixer upper project: NEC Powermate 486SX/25. 16MB/400MB.

Reply 3 of 3, by justin1985

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Dan386DX wrote on 2024-05-23, 20:28:

Just looking at your picture of the board, it looks like the PWR_SW is not bridged but the SLEEP header is.

I'm sure you've already noticed/thought of this, are you sure you're actually shutting off the PC?

You're right in that photo! But I think that was just an experiment. It's now definitely on the PWR_SW pins:

IMG20240523213326.jpg

The three beeps does definitely feel like a sleep mode kind of thing though doesn't it? Pressing the power switch again does seem to just start up a normal boot though.

Could it be a weird Packard Bell customised BIOS thing to work with a custom case or media centre type setup or something? I have tried setting every Power Management option in the BIOS to disabled but doesn't make any difference.

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