Question PSU loses power when the 4-pin 12V MB is connected

Aug 9, 2020
9
0
10
0
Skip this paragraph to jump straight to the problem. Trying to get an old computer running again in my restaurant. We just use it to stream Pandora in the dining room (I take the audio out and feed to and amplifier) and chefs use it to look up recipes and do a few things with Word and Excel. It has an old Windows OS on it, I believe it's Windows 7. Restaurant is closed so I'm trying to be frugal and have time to troubleshoot and get things ready for when we can open again here in Santa Monica CA.

PSU is Thermaltake TR2 430W, HD is WD2500AAJS, Motherboard says Asrockk H61M-DGS and one Kingston RAM 8 GB installed. No other peripherals. Monitor, keyboard and mouse disconnected. One day the computer failed to work. Abrupt failure.

Front panel power light comes on but does not boot up and PSU and CPU fan do not come on. When I disconnect the 12V 4 pin connector to the MB and power up the computer the PSU and CPU fans come on and stay on. Back probed all the colored pins on the 24 pin connector: 5V on reds, 3.2V on oranges, 12V on yellows, -11.3V on blue, 5V on purple and 5V on gray. I believe these are within spec. If the CPU 4 pin is connected then I loose all voltages (OV back probe). Can a bad MB or CPU cause the PSU to loose all its power? Or is the PSU failing because it has gotten so weak that it cant power the MB with 12V? PSU has no trouble powering the 12V CPU fan and the PSU fan when MB is disconnected from 4-pin 12V connector. If I had spare PSU I'd try it out but I don't want to spend $ throwing parts at this computer before having some expert recommendation. I prefer troubleshooting rather than throwing parts at the problem if possible.

Other things:
All the MB capacitors look fine, no swelling or leaking.
Don't see or smell any obvious bad components or traces on either side of the MB.
Cleaned MB of all dust and cleaned again with electronic contact cleaner.
Disconnected front panel USB connectors. (the rear USBs are integrated into the MB so cant disconnect them).
Removed RAM as a test (made no difference).
Removed CMOS battery (measures 3V) and held down power button for 1 minute with power cord disconnected.

Please help.
 
Last edited by a moderator:

ex_bubblehead

Champion
Moderator
It'e entirely possible for a failed power supply to be able to power such low power loads as fans while not being able to handle a much larger load such as the motherboard and CPU. At this point, unless you have the necessary equipment for load testing the power supply, you're going to have to start swapping parts. And, swapping parts does not indicate "lack of troubleshooting skills", there are times that that is the ONLY thing that one can do. However, it's your system, do as you will. I'm out.
 

Deicidium369

Permanantly banned.
BANNED
Mar 4, 2020
390
59
290
6
If the PSU is shorted why is it able to produce 12V and power the 12V fan when the 4-pin MB disconnected? You used the term "short". Is that accurate? I already know it's either the MB or the PSU. Looking for help on troubleshooting. Is replacing parts the only way to troubleshoot at this point? That's what mechanics do to cars when they lack troubleshooting skills. I would replace the PSU as a test and get on with life but I don't want to buy one to use as a test and then find out it was the MB.
There are PC PS testers - they are inexpensive and can tell you whether the PSU works at a basic level or not (12V 5V 3.3v)

What I see is that the PS is putting out on at least the 12V rail - but when connected to the 4 pin, it can't produce sufficient amperage - and so the motherboard can't boot. Can either be the power supply not producing enough power (partial failure) or the motherboard pulling too much power due to an internal fault. That's pretty obvious. Could be a continuity issue as well.

So it's not an easy problem to test - especially remotely - and the only real way to determine the issue is to start replacing - I agree that some people immediately jump to the replace step - but in this case, that may be the correct way to go.

Do you not have another PSU from another machine? If it were me, I would start with replacing the PSU - that seems the logical place to start.

If you wanted to get all multi meter testing - test the continuity of the 4 pin terminals in both the motherboard and PSU - lack of continuity could be caused by a cold solder joint or a pulled pin.

Other options are to look into a used mini PC - a quick search for "mini pc" on Ebay shows several in the $130 range complete with Win 10 OS. Solves 2 problems - 1 that it actually works, and 2 that is has Win 10 and can continue to get updates. A power supply would be half of that - but if it isn't the PS - a motherboard would cost at least the same as the PS.
If you change your motherboard - Win 7 may or may not freak out and invalidate the license.
 
Aug 9, 2020
9
0
10
0
Something else I can test/troubleshoot? I'd rather not buy a PSU to "see if the system is revived". Can a problem on the MB or CPU cause the disappearance of all the voltages? When I leave the 4-pin 12V CPU connection off I have all the voltages and the 12V CPU and PSU fan run fine. I even disconnected the 12V CPU fan and still the voltages disappear when the 4-pin is connected. I would think if the PSU can power the 12V CPU fan then it could provide 12V to the MB if the fan was disconnected. What on the MB is demanding so much 12V that makes the PSU unable to supply any voltage?
 

ex_bubblehead

Champion
Moderator
It's one of two things here.

  1. Power supply
  2. Motherboard
Voltage dropping to 0 when connected to a load indicates a short somewhere. You're going to have to swap one or both of those. My money is on that old power supply.
 
Aug 9, 2020
9
0
10
0
If the PSU is shorted why is it able to produce 12V and power the 12V fan when the 4-pin MB disconnected? You used the term "short". Is that accurate? I already know it's either the MB or the PSU. Looking for help on troubleshooting. Is replacing parts the only way to troubleshoot at this point? That's what mechanics do to cars when they lack troubleshooting skills. I would replace the PSU as a test and get on with life but I don't want to buy one to use as a test and then find out it was the MB.
 
Aug 9, 2020
9
0
10
0
Thank you for such a thoughtful response Deicidium369. I pulled out the PSU from a good PC and swapped it in, I hope that will not damage the donner PSU. As I suspected the same problem with the replacement PSU so I can rule out the PSU as the problem. So now it's down to the MB or CPU. I might try taking off the CPU and checking my voltages with everything connected. But...I'm moving in the direction of buying a used mini PC for cheap ss you suggested. Thank you so much for your help!
 

ASK THE COMMUNITY

TRENDING THREADS