XT clone motherboard not accepting -12V rail \ VOGONS

XT clone motherboard not accepting -12V rail

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First post, by RetroCat

Posted on 2020-11-14, 19:46

RetroCat Offline

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Hi all,

I have a tightly integrated XT clone motherboard based around the Faraday FE2010A chipset. I can't recall if the original PSU that came with this computer always had the -12V pin disconnected in the P8 connector to the motherboard, but I dug this mobo out recently and it's shorting the power supply whenever this -12V pin is connected (constant clicking from the PSU and no POST on the motherboard). As soon as I disconnect the blue wire (-12V) from P8 the machine POSTs just fine.

Ordinarily this wouldn't bother me, but I believe I'm seeing some expansion cards not working quite right, especially sound cards - my thinking here is that these cards (that work fine in a different retro PC by the way) need this -12V from the ISA bus.

Thoughts? Anyone seen the same PSU shorting/clicking due to -12V problem ?

Reply 1 of 4, by adalbert

Posted on 2020-11-14, 19:51

adalbert Offline

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Maybe there are some shorted capacitors or other components on the -12V line?

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Reply 2 of 4, by RetroCat

Posted on 2020-11-15, 20:08

RetroCat Offline

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Thanks, I will check - there are four tantalum capacitors just beside the P8 and P9 connectors - they don't show any visible sign of damage, but worth putting them to the test.

Reply 3 of 4, by adalbert

Posted on 2020-11-15, 20:18

adalbert Offline

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Also if that motherboard has integrated serial port controller, it could be broken and shorting -12V (they usually use negative rails, especially -12V). EDIT: I also had a problem with Toshiba T3200SXC portable when connecting improperly wired mouse to serial port would cause massive voltage drop on other rails.

Last edited by adalbert on 2020-11-15, 20:21. Edited 2 times in total.

Reply 4 of 4, by maxtherabbit

Posted on 2020-11-15, 20:20

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RetroCat wrote on 2020-11-15, 20:08:

Thanks, I will check - there are four tantalum capacitors just beside the P8 and P9 connectors - they don't show any visible sign of damage, but worth putting them to the test.

I'd bet good money they are the culprit. The reason they explode is because they short internally, but if the PSU short circuit protection kicks in fast enough they will never pop

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