Assistance with diagnosing my mystery fault

Hi Jayeff, thanks for your very in depth reply. I’ve already tried the RAM and Clear CMOS steps. Tried them again just now with no luck. I also don’t have any PC beepers right now, so I’ve ordered one off eBay which should be here in a week or so.

Tested the voltages just now. All of the voltages were slightly higher than expected but this may just be within margin of error or because of my pretty cheap DMM. All of the 3V pins read around 3.52v, all of the 5v pins read 5.33v except pin 8 which read 5.25v. All of the 12v pins read 12.88v. Tested the ground pins by making sure there wasn’t any open loops, though I always tested it against pin 3, should I check all the ground pins against each other or have I done it right?

Additionally, I was just thinking back to where you said the BIOS may have been corrupted. Do you think it’s worth trying a BIOS reflash if that’s the case? Maybe something went wrong after I shut it down so early after the brief test.

Any more advice would be greatly appreciated.

Hi @raptorrepairs,

Did you try the to reset the CMOS by removing the coin cell battery on the motherboard as well? Might be worth a shot if you didn't.

Make sure that there is no power connected to the PSU when you do this.

When you have removed the battery, assuming that the case Power On button now works (not sure from your post if it is now working or not) press and hold it operated for a full 30 seconds and then release it. The rationale behind this is that doing this will drain every last vestige of residual power in the motherboard as it attempts to turn on but obviously can't, thereby resetting the BIOS back to default values. The Clear CMOS header pin method just sends a reset signal to the CMOS chip and usually this is good enough but sometimes not. It is the coin cell battery that maintains the CMOS settings when there is no power applied to the motherboard.

Reinsert the battery, reconnect power to the PSU and try turning it on and check

I think that the PC needs to pass POST before you can reflash the BIOS as you need to be able to access the BIOS and also have a working display.

You may just have to be patient and wait for the system speaker to arrive and hopefully connecting it will enable you to know what is happening.

Regarding the PSU voltages, I agree that they're most probably within tolerance values, (all were within +/-10%). As long as each particular voltage supply wire from the PSU had its specified voltage on it and not that it wasn't there at all or totally out of spec they should be OK.

Your testing sounds OK. Usually when the harness cable is plugged into the motherboard, the ground plane is multipled on the motherboard anyway so that there will be a return path using the other ground wires if one were disconnected in the PSU for example.

@jayeff Hi again. Yeah I did try the CMOS battery removal with no luck. My power button on the case still doesn’t work.

Good news is that the PC speaker arrived today. Bad news is that it doesn’t actually produce any beeps. No beeps at all. Tested it on a different board and it worked fine on that PC, so this is pretty strange. Any suggestions on where to go from here?

@raptorrepairs

Have you tested the PC power button with an Ohmmeter to see if it short circuit when the button is operated? Disconnect it from the motherboard when testing

if the button doesn't work, how are you starting the PC, are you applying a momentary short across the motherboard's power button header pins then?

no beeps at all means that it is not even getting as far a POST, just the initial powering up I guess.

By base components I assume just a PSU, motherboard, ram, monitor and keyboard (and mouse perhaps) and now system speaker, is this about right.

Have you checked that the CPU is seated properly?Remove it and check that none of the pins are bent out of alignment with all the others etc.

It seems like the CPU is not even starting

The motherboard isn’t currently in the case, I took it out a while ago to make access easier and wanted to rule out any electrical shorting. To start it up now I just short the two power on pins with my flat head bit.

You’re correct with the base components list. I’ve tried two different screens too, just to rule that out. One with HDMI and VGA and the other with just VGA. Tried two different cables with each, no display still obviously.

Reseated the CPU. No bent, broken or missing pins, all looks pristine under there. Still no beeps or display.

The cheapest AM4 CPU in the UK is £38 for a used Athlon 200GE, but as you can probably imagine, I’d rather not buy more parts and then they turn out to be useless. I could maybe borrow a PSU from someone to see if that’s causing the issue with the CPU not getting power, but I’m not 100% sure I’d be able to find someone who’s willing to risk their supply.

Once again I really appreciate your time and effort, any more assistance would be appreciated