Testing a power supply

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J

JJ

I was just given a dead computer, nothing at all happens when I turn it on.
Have a few vom's laying around. how would I go about testing the supply ?

Thanks
 
I was just given a dead computer, nothing at all happens when I turn it on.
Have a few vom's laying around. how would I go about testing the supply ?

Thanks
Click to expand...

Nothing even means the power supply fan isn't turning?

Unplug all power supply leads except one to a hard drive, then plug
the AC cord back in. Measure 5VSB, should be near enough to 5V.
Measure PS_ON, should be 3-5V.

Next short PS_ON to ground, at which point the PSU should turn on, fan
spinning and hard drive spinning. You can then check 12V and 5V on
another power plug. If all is well so far, unplug from AC, plug back
into motherboard and disconnect all non-essential parts from
motherboard, leaving NOTHING in the board, no CPU or memory or video
card. With nothing in the board the power-on switch connected, it
should turn on the power supply. If it does turn on in this barebones
state, power off, remove AC cord, and add CPU, Memory, Video card.
Try powering on again. If no POST, try clearing CMOS. If it does
post, continue adding parts until you find the source of the problem.


If the power supply didn't show any signs of life from any above test,
unplug PSU AC cord, remove power supply, and after a few mintues have
elapsed, open it and examine for signs of problem like vented
capacitors, spots where there "used" be a be a capacitor but now only
some tin-foil floating around inside, burnt resistors or diodes, blown
fuse or MOVs, etc.

The PSU ATX connector pinout:
http://69.36.189.159/usr_1034/atx_on.gif



Dave
 
JJ said:
I was just given a dead computer, nothing at all happens when I turn it on.
Have a few vom's laying around. how would I go about testing the supply ?
Click to expand...

To test an ATX PSU:

First, disconnect everything from the power supply (including MB.) Plug
AC cord into the rear of the PS. IF the PS has its own rocker on/off
switch, turn it ON. There should be +5v on pin 9 of the 20 pin connector
(+5vsb, usually a violet wire.) There should be >0.8v on pin 14 (PS-ON,
usually a green wire.) This is from an internal pull-up to the +5vsb.

Connect PS power leads to 1 or 2 IDE HDs (for a load). Jumper pin 14
(PS-ON) to pin 13 (ground, black wire.) At this point, fan in PS should
start spinning, drives should spin, and + 5/12v, -5/12v, +3.3v, and
+5vsb should be present at 20 pin connector.

Disconnecting pin 14-13 jumper should turn supply back off.

As far as your dead computer:

When your system fails to start, what is the voltage on pin 9? If it is
less than 4.75v, either a) the logic on your motherboard is shorting,
b) your +5vsb is under powered, or c) your PS is bad. It is
most likely a combination of b) and c).

If your +5vsb is OK, measure pin 14 of the PS/MB connector. If it is <0.8v
(and the +5vsb is OK), the power supply should be on. If pin 14 is <0.8v,
the +5vsb is OK, and the PS is OFF, the PS is bad. If pin 14 is >2.0v, the
power supply is being told to stay OFF by the MB. If depressing the front
panel ON/OFF switch does NOT lower pin 14 to <0.8v (and the +5vsb is OK),
then the problem is NOT the power supply, but rather MB related.
 
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