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Old 03-19-2020, 10:58 AM   #1
dicky96
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Default Anyone have schematic/boardview MS-6178?

Hi guys
Does anyone have shematics for this old Pentium III motherboard MS-6178?

They are available on a paysite but I was wondering first in anyone could share them here. Otherwise I have to pay a few euros.

The motherboard was dead, there was 5V across the ATX Power switch pins but the PSU would not start.

Also there is something drawing more power from the CMOS battery than I normally expect. I am reading off the scale on uA (so it's above 200uA) on my meter. Normally I get about 2uA - 10uA tops for a motherboard, unless these old boards did draw more power from the battery.

I decided to replace the SIO W83627F-AW so I ordered a few from aliexpress

Now I changed that the motherboard powers up when pressing the ATX power switch, the CPU fan runs and the CPU gets hot but my Post analyzer reads 0000 in any of the PCI slots. And I still have that over 200uA drawn from the CMOS battery

I will check if I have a corrupted BIOS or possibly the BIOS revision does not support my CPU but I think I still have some hardware problem. Does anyone have the schematic or boardview so I can figure out what else is connected to cmos battery +ve

Thanks
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Last edited by dicky96; 03-19-2020 at 11:00 AM..
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Old 03-20-2020, 07:32 AM   #2
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Default Re: Anyone have schematic/boardview MS-6178?

I checked on my other meter and the drain from the cmos battery was 40mA. Way too much. I checked the RTC crystal and found it is not oscillating (nor is there any bias voltage to the crystal) so I removed the south bridge thinking that was probably the problem.

That came off OK, no damage to the PCB but when I checked the current from the CMOS battery it is not 27mA, still way too much. Having said thet it dropped by 13mA so the south bridge seems to be part of the problem, which is a step in the right direction!

I can get a south bridge for 5 euros inc post, but that isn't going to fix it yet. What else should I try? A schematic would be really helpful

Last edited by dicky96; 03-20-2020 at 07:34 AM..
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Old 03-20-2020, 06:06 PM   #3
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Default Re: Anyone have schematic/boardview MS-6178?

Leaky dual diode between coin cell battery/3VSB power rail and RTC power rail? Use continuity mode to find it with no schematics.
Or short to ground on the RTC power rail due to another component.
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Old 03-21-2020, 06:32 AM   #4
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Default Re: Anyone have schematic/boardview MS-6178?

Hi piernov. Thanks for the advice

Something to do with a diode could be the problem as when I test on diode mode with meter +ve to battery +'ve terminal (no battery fitted of course) and meter -ve to ground I read 0.6xxV, or basically a forward biased diode junction.

I am sure that should not happen .

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Old 03-21-2020, 06:41 AM   #5
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Default Re: Anyone have schematic/boardview MS-6178?

Just to clarify, did you mean I will have something like this circuit and the two diodes could be discrete or a three pin dual diode component probably looking like a SOT-23?

I have direct connection 0 ohms from cmos battery + to the SIO, I checked that much.

The RTC is in the southbridge.
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Old 03-21-2020, 09:39 AM   #6
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Default Re: Anyone have schematic/boardview MS-6178?

Yes that's usually how it looks like, so that RTC power rail is fed from the power supply (5VSB converted to 3VSB) when plugged in to the mains, and backed up by the coin cell battery when disconnected from the mains. I've already seen cases where the dual diode becomes leaky between coin cell battery and 3VSB, effectively discharging the coin cell battery into 3VSB.
Or the usual case of southbridge going short on RTC power rail…
The coin cell battery itself can be connected directly to one of the SIO pin to monitor its voltage. But it's not supposed to actually supply power to it, as power should be taken from 3VSB directly, SIO shouldn't need a battery backup.
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Old 03-21-2020, 11:39 AM   #7
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Default Re: Anyone have schematic/boardview MS-6178?

Quote:
Originally Posted by piernov View Post
Yes that's usually how it looks like, so that RTC power rail is fed from the power supply (5VSB converted to 3VSB) when plugged in to the mains, and backed up by the coin cell battery when disconnected from the mains. I've already seen cases where the dual diode becomes leaky between coin cell battery and 3VSB, effectively discharging the coin cell battery into 3VSB.
Cheers, that makes sense.

If it was the case that the diode was leaky, would you expect the battery to stop discharging into 3VSB if the ATX PSU was connected but still in standby?

As I have removed south bridge it can not be discharging into that, though it did reduce the current from the battery from 40mA to 27mA so some was going that way.

I changed the SIO which must have been faulty as the motherboard does now power up which it did not before. I doubt it is discharging into the SIO but I will lift that one leg just to eliminate the possibility.

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Old 03-21-2020, 02:00 PM   #8
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Default Re: Anyone have schematic/boardview MS-6178?

Ahaa - as there is a diode like you say and I read a foreward biased diode junction from CMOS battery +ve to ground then it would make perfect sense from my little circuit diagram that I have a short circuit from RTC power rail to ground even with the south bridge removed.
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Old 03-21-2020, 08:40 PM   #9
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Default Re: Anyone have schematic/boardview MS-6178?

Ah if you removed southbridge you should be able to check RTC power rail resistance/diode mode to ground easily by identifying corresponding ICH pad from ICH datasheet ballout.
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Old 04-06-2020, 03:45 AM   #10
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Default Re: Anyone have schematic/boardview MS-6178?

I found where the 27mA was going

Basically it was my own fault, I found a small solder bridge on the SIO on the BAT+ pin to the adjacent one which would have been from when I replaced the SIO. I also found another solder bridge between two other pins so this is something I need to check more thoroughly in future

As the current from the CMOS battery went down from 40mA to 27mA when i removed the south bridge, and I have already ordered a replacement, I decided to put this on one side again until the parts arrive. I have some more SIO here (I ordered a pack of 5 as they were so cheap) so if i need to change that again it isn't a problem

Now the current from the battery is zero so the SIO is not drawing any.

I found the circuit diagram and that helped me. Here it is for anyone else who wants it.
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Last edited by dicky96; 04-06-2020 at 03:47 AM..
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