PSU or motherboard failure. What to try?

Our company historically purchased all it's PCs from a rather inept IT contractor.  The only things the PCs have in common is that they use cheap hardware.  This morning a user reported that his PC had stopped working and could not be powered up.  I took it back to my office and I could not get it to boot.  The PSU makes a whistling noise when connected to the mains supply and so I assumed it had failed.  Swapped out for a spare, but this did not work.  I took a third one from a PC I knew was good and this one behaves strangely.  The fan runs as soon as the PSU is turned on.  The tiny green LED on the MB is illuminated, but I cannot boot the PC using the switch on the front.

I've tried disconnecting all the internal hardware (SATA HDD, RAM, IDE CDROM), and taking out the BIOS battery and leaving it for a while.  Doesn't make any difference and I'm out of things to try.  Can anyone make any suggestions about what else to try before I declare the motherboard DOA and bin the PC?

I'm a little concerned in that we have a lot of these type of failures (three this week out of ~100 PCs) and I'm not a hardware engineer, so worry that I'm missing something simple and trashing good hardware......  Having said that for the last year we have been buying HP workstations instead and not had a single failure of any kind on those, so I wonder if it is the cheap hardware......

Any advice gratefully received.
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Is the cheap hardware in the pc's identical or similiar.
Then there could be a some parts, from the same series, that are bad.
According to the description, it sounds like, it would most likely be the motherboard,
that is faulty.
Also you should check if your power at the office, is stable,and regulated, so that
the voltage should not be changing too much.
PC Hardware are very sensible to that.
Richie4236Author Commented:
We have ~100 PCs built by the same supplier over a 10 year period.  No two PCs are the same or even close.  They just used whatever the suppliers had on special offer at the time of construction.

Power supply is extremely dirty.  The UPS in my office is constantly jumping back and forth to compensate for high or low voltage.  Didn't realise PC hardware was more affected by it than other office hardware.
In order to help you out to identify whether is the motherboard faulty, have a check for the bad capasitor of the motherboard :

>> I've tried disconnecting all the internal hardware (SATA HDD, RAM, IDE CDROM), and taking out the BIOS battery and leaving it for a while.

But normally if you unplug all the RAM out and on the PC, the PC will keep beeping with the sound ....
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Start inspecting your motherboard, PSU, and video card capacitors.
Cheap parts get cheap caps and bad caps can cause any PC problem you can even think of.

Bad caps = bad (noisy - like EMI effects) power in places nice clean DC power is needed.

If you have 100 machines that were built like that to keep going you might want to consider joining Many of the forum members there [me included] rebuild motherboards, power supplies, and LCD screens at the component level on a daily basis. In depth knowledge on that sort of thing is a little out of EE's league. I try to bridge that gap but I'm an army of one - and I can't always be at either place, let alone both. If you are more of the mindset to just replace [vice repair] bad motherboards and so forth then EE is fine.

Gary CaseRetiredCommented:
"... The UPS in my office is constantly jumping back and forth to compensate for high or low voltage.  " ==> NEVER buy a UPS that doesn't have AVR (automatic voltage regulation).    With an AVR circuit the UPS can regulate voltage fluctuations within a reasonable range (typically +/- 15%) without switching to the battery/inverter.   This is MUCH better for both the UPS unit and the connected equipment.    It sounds like you not only have inexpensive PC's, but also cheap UPS units.

... if all of your PC's aren't connected through UPS units, this constant voltage fluctuation, along with inexpensive power supplies, could easily explain your relatively high failure rate.

It does sound, from your description, that the most likely issue here is a failed motherboard => very likely (as PCBonez noted) due to failed capacitors (and the power fluctuations were likely a catalyst that accelerated these failures).

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Good power supply is vital for computers

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Here a link to HDD makers Hard Drive Diagnostics Tools and Utilities
Download FREE and burn DOS bootable CD on computer with CD burner.
Put in your drive
Hit "c" (+"Alt")  during power on to boot from CD, then run HDD diagnostic Tools.
These DOS boot CD's have HDD tests and many other must have DOS apps.

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