VOGONS

Socket 370 board - low +3.3 and -12v values

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First post, by bananaman

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I'm working with a self-contained data acquisition device built around a Pentium 3 board.

These are the voltage readings in the bios:

Processor (Expected) =  1.75v
Processor (Measured) =  1.76v
Backup Supply        =  2.82v
+1.5v Supply         =  1.46v
+1.8v Supply         =  1.80v
+2.5v Supply         =  2.48v
+3.3v Supply         =  3.18v
+3.3v Standby        =  3.13v
+5v Supply           =  4.98v
+12v Supply          =  12.04v
-12v Supply          = -10.89v

The readings for the -12 and +3.3 rails are technically within the general tolerances for the ATX specification...
but I work as a calibration tech in a metrology laboratory, and I am aware of the implications of things being "technically" acceptable and the uncertainty it can add to a measured system.

I guess my primary question is:
What is the -12v line used for? is it integral to the PCI bus or power architecture?
I'm aware that an older ISA-capable system has a -3.3v line that is used as a reference for some ISA devices, so I wondered if this negative 12v line in the ATX standard perhaps served a similar purpose.

( context: )

these units can no longer be adjusted/calibrated by outside labs and are no longer serviced by the manufacturer.
They have slowly been drifting out of spec over the years and we have been limiting their use accordingly, but I tend to find myself breathing temporary life back into a lot of things that I put my hands on, so I was weighing whether it might be worthwhile to try and refresh part of the power supply to bring that -12v closer to its nominal value.
The 16 channel input card is just a giant logic board that gets its power, and is controlled, via PCI slot; when verifying with voltage and frequency standards, upper and lower values in certain ranges fall short toward the furthest ends of a range... ie: 20v scale is -10 to +10, and once you get to and beyond |8v| in either direction, the value may be ~7.8 instead of 8, ~9.7 instead of 10, etc.
I have seen multiple instances of assorted instrumentation failing to meet their own specs if they aren't receiving ideal supply/excitation, so I figured similar logic might apply in this instance... and it is effectively the only variable that I have control over since there's no physical adjustments available on the acquisition board, and no software calibration options available as an end-user.

You might be missing some of the benefits that stereo can provide.

Reply 1 of 4, by akimmet

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The -12V rail is mostly used for the serial ports, and by sound cards. Otherwise it isn't used on most PCs.

I don't consider -10V alarming, I have measured many power supplys with similar weak negative rails.

Do not assume that the built-in motherboard voltage monitoring is accurate. I have witnessed readings 10% off compared to a calibrated multimeter.

Reply 2 of 4, by bananaman

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Yeah my next step was to test the PSU outputs with one of our 3458's, but between yesterday and today the thing has decided not to POST, so I have other issues to solve now.

honestly I wouldn't be upset if this thing accidentally found its way off my bench and onto the floor; we have half a dozen of these units and they're all 20 years old...
we even have a few of the newer models that have superseded them but customers and lab engineers can't be bothered to update their test procedures and fold the new equipment into how they do things.

I appreciate the insight on the -12v rail though; I'm surprised I didn't come across any mention of that when I was poking around online yesterday. No serial activity on this thing so at least i can write that off.

You might be missing some of the benefits that stereo can provide.

Reply 3 of 4, by luckybob

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Do not trust the values the motherboard reports. These voltages are being measured by the CHEAPEST 90's equipment. Would you trust a $1 volt meter from today? From the 90's?

That said, I would expect -12v to be closer to -12. you might have a loose ATX connection. (or a slightly faulty PSU)

Once men turned their thinking over to machines in the hope that this would set them free. But that only permitted other men with machines to enslave them. - Reverend Mother Gaius Helen Mohiam

Reply 4 of 4, by rasz_pl

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You can safely ignore -12V ATX supply, its rated for very small loads and pretty much only used by some serial ports. Same goes for PCI spec https://archive.org/details/bitsavers_pciSIGP … ge/136/mode/2up

>expansion card must limit its total power consumption to 25 watts (from all power rails)

Power Rail
5 V ±5% 5 A max. (system dependent)
3.3 V ±0.3 V 7.6 A max. (system dependent)
12 V ±5% 500 mA
-12 V ±10% 100 mA

Your Aquisition board looks too big and packed to consume only 25W ergo it has to have some power connector and produces its power rails on board from external 12V.

>when verifying with voltage and frequency standards, upper and lower values in certain ranges fall short toward the furthest ends of a range

You will need to find a lab willing to service your Aquisition boards, starting with all electrolytic caps (not that I see any, maybe somewhere deep hidden next to dc/dc converters).

> but between yesterday and today the thing has decided not to POST, so I have other issues to solve now.

Recap whole box then 😀 socket 370 era was prime time for bad caps.

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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