motherboard repair, ISA pin B05 voltage \ VOGONS

motherboard repair, ISA pin B05 voltage

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First post, by pshipkov

Posted on 2023-07-10, 18:55

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After extensive repair of a motherboard with a lot of damage it came back to life, kind of.
AMI BIOS beeps 8 times - graphics card related error code.
Voltage on ISA pin B05 should be -5V, but is +1.9V. Suspicious. Unable to figure out where this discrepancy comes from.
Hesitant to just connect it to -5V line.
Any opinion on the issue ?

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Reply 1 of 10, by majestyk

Posted on 2023-07-10, 19:13

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I assume the graphics card doesn´t need -5V?

AT or ATX PSU? Tried a different PSU?

Reply 2 of 10, by pshipkov

Posted on 2023-07-10, 19:32

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The rest of the components are fine. PSU serves 4.6V-4.8V, not ideal, but ok.

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Reply 3 of 10, by matze79

Posted on 2023-07-10, 20:27

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Did you check (Tantalum) Capacitors in -5V Path ?

Reply 4 of 10, by pshipkov

Posted on 2023-07-10, 22:55

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Not yet, just not sure if -5V plays a role for VGA, does it ?

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Reply 5 of 10, by maxtherabbit

Posted on 2023-07-11, 04:24

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pshipkov wrote on 2023-07-10, 22:55:

Not yet, just not sure if -5V plays a role for VGA, does it ?

-5V is not used for *anything* except old soundcards and maybe obscure industrial interfaced

Reply 6 of 10, by pshipkov

Posted on 2023-07-11, 05:21

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Thanks.

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Reply 7 of 10, by maxtherabbit

Posted on 2023-07-11, 13:52

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It's original purpose was a bias voltage for ancient 4116 DRAM on the first model of 5150. Almost nothing used it thereafter but it nevertheless stuck in the spec until ATX was revised in the early 2000s

Reply 8 of 10, by pentiumspeed

Posted on 2023-07-11, 16:31

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Not only that, some sound cards and true hard core serial cards.

Cheers,

Great Northern aka Canada.

Reply 9 of 10, by pshipkov

Posted on 2023-07-11, 17:56

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Thanks for the clarification. I will keep looking then for other issues.

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Reply 10 of 10, by rasz_pl

Posted on 2023-07-11, 19:28

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I would trace where that positive voltage is coming from, can be blown onboard serial transceiver

https://github.com/raszpl/sigrok-disk FM/MFM/RLL decoder
https://github.com/raszpl/FIC-486-GAC-2-Cache-Module (AT&T Globalyst)
https://github.com/raszpl/386RC-16 ram board
https://github.com/raszpl/440BX Reference Design adapted to Kicad

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