Help out a noob at motherboard repair

[LeonR]

So, I've tested the board with another PSU. I found out that If I let the board "rest" for two days or so, it works OK but when I try powering it up the following day the same symptoms occur.

Any input on this would be appreciated.

On board power supply/regulator fault? If there is still some remaining charge in the capacitors it won't operate correctly, if you let it "sit" the charge drains away and it works again. That (and the BIOS battery) are the only things on board that could have any "time related" effects.

Of course, that assumes that there aren't other issues and the above is not a red herring - such as your power supply/board being warmed up/cold between the tests, the board being moved (intermittent contact because of a broken solder joint ...) and similar that could manifest themselves in a similar way.

If you rule the above out then the only meaningful thing you could do at this point would be to try to find schematic for the board (likely only on some fishy Chinese forum) and actually start measuring the regulators on the board. However, without knowing what the voltages need to be (schematics ...) and having an oscilloscope to check what the regulators are doing (ripple voltage, etc.) you won't get far.

Did you get that ESR meter and measure those capacitors in the meantime?

Not yet. I need to import one of those and it may take from 1 1/2 to 3 months. Postal service here is terribly slow and inneficient. I have about 10 packages that are over a month to be delivered.

I assume the VRMs can be tested as any other component, just need to know which values I'd expect from the testing?

[janoc]

I assume the VRMs can be tested as any other component, just need to know which values I'd expect from the testing?

"VRM" is just a fancy name for a voltage regulator/converter. It is not a "component" per se but a module ("VRM" = "voltage regulator module") containing one or several chips, perhaps some MOSFETs (for the high current ones), some inductors and a boatload of capacitors and resistors. Furthermore, it is very often a misnomer because this circuitry is commonly integrated directly onto the main PCB and not really any kind of separate/replaceable module. So don't expect that you could stick this into some cheap tester that tells you "OK"/"NOT OK" and done.

If you want to test whether it works as it should, you need to know at least what the input and output voltage(s) are supposed to be and what ripple is acceptable. And if those voltages aren't right for whatever reason, you will need to understand how the circuit works in order to have any chance to find the problem. So better start reading up on DC-DC converters ...

[LeonR]

I assume the VRMs can be tested as any other component, just need to know which values I'd expect from the testing?

"VRM" is just a fancy name for a voltage regulator/converter. It is not a "component" per se but a module ("VRM" = "voltage regulator module") containing one or several chips, perhaps some MOSFETs (for the high current ones), some inductors and a boatload of capacitors and resistors. Furthermore, it is very often a misnomer because this circuitry is commonly integrated directly onto the main PCB and not really any kind of separate/replaceable module. So don't expect that you could stick this into some cheap tester that tells you "OK"/"NOT OK" and done.

If you want to test whether it works as it should, you need to know at least what the input and output voltage(s) are supposed to be and what ripple is acceptable. And if those voltages aren't right for whatever reason, you will need to understand how the circuit works in order to have any chance to find the problem. So better start reading up on DC-DC converters ...

Yeah, I meant the components on the VRM, like the driver/MOSFETs, inductors and caps. Since the board have dual socket, I think I can use the other CPU VRM phases as reference. What should I look for? Voltage differences?

[janoc]

Yeah, I meant the components on the VRM, like the driver/MOSFETs, inductors and caps. Since the board have dual socket, I think I can use the other CPU VRM phases as reference. What should I look for? Voltage differences?

That is pretty much pointless if you are not sure whether both sockets don't have a problem - the issue could be "upstream" from them. Or whether the problem is with the CPU regulators at all - not only CPUs have their local DC-DC converters!

The first thing to check would be measure all power rails for each CPU (there are several) and see whether they are in the correct range and not sagging under load. But that will be difficult to do without an oscilloscope - multimeter will not catch any transients that may be on the power rails.

Once you have that checked you can start comparing the two sides but be aware that the the voltages will vary depending on the CPU load. So there most likely will be differences and not necessarily indicative of any fault.