[Solved] P2/P3 Motherboard standby power issue

[spilihps]

Hello,

Pulled out a P3 rig from storage to play around with. Noticed that as soon as PSU is connected power "leaks" in standby, causing CPU and GPU fan to run for example.
Power LED in front is also lit, but very dim. Likewise using a postcard the power indication LEDs on that is also lit, but very dim

Checked the PSU, and it is not providing anything more than the +5VSB as it should.

System power on and works as normal.

To me it seems like the VSB on the motherboard is leaking to other rails for some reason. I did not want to keep it plugged in more than necessary with this issue.

Any tips or hints what might be the cause, or what to check?

The motherboard model is unknown, it only has "INTEL BX-B" written on it. Its one of these built to order boards from late 90s.

I actually have two of these boards, one that failed in a worse way, but it was saved in this thread: https://www.eevblog.com/forum/repair/broken-ic-on-old-pentium-2-motherboard/ 

I attach a image of the board for identification.

[m k]

What voltages those other pins have?

VSB is very small, it can't really do anything.
Maybe one low current voltage is on, so regulation is not really starting around other higher current voltages.

[amyk]

What does the BIOS POST screen show? That should help identify the model.

As for troubleshooting the actual issue, "BX-B" suggests this is one of the many mobos based on the famous 440BX chipset, for which the reference schematics are easily found: https://elektrotanya.com/intel_440bx_rev_1.0_sch.pdf/download.html

[coromonadalix]

1st:  check all caps on the mobo  if they are inflated, 

2:  the psu in theses must be very old ... it would be normal to have caps failing in it ...

for tests, you remove the psu form the mobo

the only power you should have is the 5vdc  standby pin / power pin active,  you ground this pin and the psu should come alive,  should be the green wire

you have  many 20 pins   atx  psu pinouts on the web to cross check the voltages,  but at some point you need to check if you have ac  noise / voltages  not filtered correctly coming from the psu, a scope would help

the psu would be the 1st part   i would change  if the mobo caps are not buldged ( they seems fine from the photo ? )

[spilihps]

Hey everyone!

Thanks for your help so far!

However, It turned out to not be the motherboard at all, but a pretty weird issue with the PSU.

Opened up a new thread on the forum for this, as it is a completely different topic all on its own.
https://www.eevblog.com/forum/repair/atx-psu-ps_on-issue/