Question Can 3.7V at the 3.3V rail prevent an (old) mobo from POSTing?

Jun 23, 2021
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TL;DR: I'm troubleshooting an old PC that won't POST anymore and the only symptom out of the ordinary is that the PSU 3.3V rail is reading 3.7V on a cheap PSU tester. Can that be it??

I have an old PC that used to work fine (passing several stress tests and all), and now it won't post. It powers on, CPU and case fans spin ok, RBG lights turn on, but about 8 seconds later it turns off. Then after 2 seconds it turns on again and repeats the process. In the process I get no video and no POST beep (which I used to).

Here are the specs:
  • CPU: Intel i3-2100/i5-3570 (non-K)
  • Mobo: Gigabyte GA B75M D3V
  • RAM: Corsair Vengeance LP 8GB (2x4GB) 1600Mhz DDR3 C9 Black - CML8GX3M2A1600C9
  • GPU: integrated Intel graphics
  • PSU: Nameless 500W
  • SSD: Kingston A400 240GB
Here's the troubleshooting I've done:
  1. I've removed the mobo from the case and tested on my bench (table with non-conductive mat): no change;
  2. I've tested the rig with each RAM stick in each slot (there's 2 in this mobo): no change;
  3. I've removed both RAM sticks so the mobo ran without any RAM: the board beeps continuously without stopping or turning off... beep, beep, beep, beep, beep... so the beeper works. No video.
  4. I've replaced the i5-3570 with the i3-2100 I have laying around. No change, except in the way the CPU fan is driven:
  • With the i5-3570: gently turns the CPU on at low RPM;
  • With the i3-2100: full throttle, like the fan is a jet engine;
I then checked the PSU with those cheaper PSU testers and it's complaining that the 3.3V rail is at 3.7V. Is that enough for the motherboard to not POST? Is that enough for me to trash the PSU altogether?

I'm now planning on probing the PSU with a scope. I have a few tools available for that, but I'm not sure what I should look for. Just voltage level and ripples? I know I should hook up some loads to reach minimum current specs, but I don't have the right power resistors handy.

There's a bit more info here:

https://www.reddit.com/r/buildapc/comments/o6ej8i View: https://www.reddit.com/r/buildapc/comments/o6ej8i/suggestions_for_troubleshooting_an_old_i53570_rig/?utm_source=share&utm_medium=web2x&context=3


Thanks!!
 

jay32267

Glorious
I think that 3.7V on the 3.3V rail is enough to cause problems.

The specification for the high side is about 3.47V.

....and if it's a nameless PSU...I wouldn't trust it with my hardware.

In either case I would throw it out.
 

TommyTwoTone66

Prominent
Apr 24, 2021
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Those AliExpress parts are cheap for a reason dude…

The high voltage on the 3.3V along with other issues caused by using a “nameless” 500W PSU has likely overloaded the VRMs on your motherboard and popped them.

You will need a new motherboard and should throw out that old PSU and buy a brand new one, not from AliExpress.

Buy cheap, buy twice.
 
Meh, while 3.7V could cause problems you would hear or at least smell it happen, and usually the mobo caps can level small differences out.
Also the cheap tester might be off by a few % with the PSU being ok.

But what you describe is in line with a broken mobo and you should consider sourcing a mobo to see if that is the problem.

As to the cause, if you don't find anything burned on the mobo, could even be static electricity, that's the silent killer that gives no clues like smoke or popping when it happens.
 

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