"dw" said:
Hi All,
I've been trying to replace the ATX power supply on a PC for a friend but
without success - I've tried two different PSU's but it just won't power up.
I'm convinced it is the PSU as I've connected a microATX supply and the PC
powers up fine but obviously isn't the right size for the case.
What am I doing wrong or have I just been unlucky and bought two dead PSU's?
Tried just connecting to the mobo and also with hard drive and/or CD drive
and floppy connected with same result.
It sounds as though they are 'generating' power as they emit a high pitched
whine.
TIA
dw
The high pitched whine is telling you something. It is not a
symptom I would ignore. Since you've tried two PSUs and they
do the same thing, I think there is something abnormal about
the motherboard or plugged in hardware. The older microATX
supply could be allowing it to power up, because the older
supply doesn't have as many protection features on it, as the
newer supplies have.
You can try a third supply, from a quality brand name, but
I think there is a good chance you'll hear more whining.
Since I own a clamp-on DC ammeter, I'd be reaching for that
tool, and measuring the magnitude and direction of current
flow, on each of the DC rails of the PSU. I would use the
microATX supply, since it will remain running while making
the measurements. Based on the numbers you see, that may
suggest what is going wrong. For example, if one rail was
shorted to another rail (a partial rail to rail fault),
I bet the power supply wouldn't like that too much.
Since most PC repair is done by swapping stuff, I don't
see much choice, but at some point you are going to have
to try another motherboard. Have you done a visual inspection
of the current motherboard ? Are there any bulging capacitors
on it ? Any discoloration or signs of heat damage on anything ?
When your friend first had problems, was there a lightning
storm, or some other interesting story that your friend told
you ? Knowing a few details about what happened, might also
hint at a solution.
If there is a Radeon 9700 video card in the computer, try
pulling the video card out of the computer, then connect
one of the two new power supplies, and see if the computer will
start. It'll beep like crazy, and tell you there is
no video card, but by doing that, you can listen to
see if the whining noise is gone. The first batches of
Radeon 9700, had some problems with connecting certain
voltage rails together (a diode power sequencer perhaps),
and I believe that might cause whining.
Paul
