I have a small form factor HP computer. After the computer has been
unplugged for a while it is hard to get it to turn on again. The
momentary switch that turns on the computer had soda spilled on it and
since it was dirty inside I just replaced it with a new one. Still the
problem persists. Pressing the button usually gets no immediate
response from the LED that shows the computer is on. Then after a
variable length of time, from a few seconds to as long as 3 or 4
minutes, the LED glows and the computer starts. Or there is no action
at all and the computer does not start. If I press the button again it
may or may not start, again with a variable delay. I removed the cover
of the computer and wired in a switch across the green wire and a
ground wire on the power supply. Closing this switch turns on the
power supply and the computer but when the switch is opened the power
supply shuts off. So now I'm thinking that maybe there is something
wrong with the motherboard circuit that keeps the power supply on. I
did check the motherboard to make sure it had not also been bathed
with soda and it had not. Nor had the power supply. Only the power
switch and a card reader module had been contaminated. I do not have
another power supply to swap or I would have done that already. Any
thoughts?
Thanks,
Eric

wrote:
I removed the cover of the computer and wired in a switch across the
green wire and a ground wire on the power supply. Closing this switch
turns on the power supply and the computer but when the switch is
opened the power supply shuts off.

This is how that green wire is supposed to behave. The motherboard
keeps it grounded for as long as it wants the power supply to be fully
on.

So now I'm thinking that maybe there is something wrong with the
motherboard circuit that keeps the power supply on.

Possibly. The "standby" output of the power supply may also have
trouble. This is a +5 V output at 1 or 2 A or so that is always on when
the power supply is plugged into the wall. It may run the "turn on"
circuit that is connected to the front panel power switch. It also
usually runs the Ethernet interface, to enable tricks like sending a
magic network packet to the PC that makes the PC power up completely.

You might measure the output of the +5 V standby (+5VSB) when the
computer is plugged into the wall and not powered on. It *might* be
a purple wire; it's on the other row from the green turn-on wire and
towards the other end. Measure between +5VSB and any of the black
(ground) wires.
http://image.pinout.net/pinout_24_pi...atx_pinout.GIF
Do this with the power supply still plugged in to the motherboard.

If you get it powered up completely, you can also measure the voltages
on all the other power supply pins (power supply still plugged in to
the motherboard). The +3.3 V and +5 V should be pretty close to right,
like within +/- 0.1 V. The +12 V may be a little further off, but still
within about +/- 0.5 V. If any of these are way off, suspect the power
supply.

If you get it powered up completely, make sure that the fan inside the
power supply is spinning. You can usually see it, or you can feel for
the air movement. If the fan has failed, the supply may be permanently
cooked.

You can also try things like unloading the power supply. Unplug the
power connectors for the hard drive and CD/DVD drive (if equipped), then
try the front-panel power button again. If it always starts right up
like that, then the power supply may be getting weak.

People have claimed that the clock/CMOS battery going bad can cause
this, but I've never seen it myself and am skeptical. This is usually
a coin-cell battery (CR2032 or similar) on the motherboard. It's easy
enough to pop it out of the holder and measure it; they are usually
3 V nominal.

I do not have another power supply to swap or I would have done that
already.

If you have an ATX power supply from a full-size desktop, you can test
with that, even if the power supply won't fit mechanically. If the
full-size supply has the 24 pin connector and your PC only has 20 pins,
you can just let the extra 4 pins hang off the side, if there is room
on the motherboard.

It is also probably worth inspecting the motherboard for failed
electrolytic capacitors. Look for bulged tops, and brownish goo leaking
out at either the top or the bottom. Some examples...
http://www.badcaps.net/images/caps/kt7/image001_sm.jpg
http://www.badcaps.net/images/caps/kt7/image004_sm.jpg

If you have bad caps, this is fixable, but you'll need to take the
motherboard out of the case to unsolder the old caps and solder some
new ones in.

Matt Roberds

I owuld also Google for the ATX pinout. One of the pins is called "power good" and it is supposed to tell the motherboard that the PS is in regulation and ready to go. If it does not do that, there will be no boot.

I would measure all the supplies because in a switched mode power supply like that, all the sources' levels are chiefly determined by the secondaries of the transfomrer, but only one is actually in the feedback loop. There may be subregulators on some supplies, there usually are, but there is a good possibility that when one supply drops because of poor filtering, that other sources will rise because it has affected the feedback loop. That can fry **** out.

It's about lke the PSes in the older VCRs where the 5 volt line would drop slightly, and the regulation would make it crank out voltage until the 12 volt line hit like 17 volts. at 16.9 volts, everytihng that dissipates pwoer on a 12 volt line dissipates twice as much. That can fry **** out.

Of course Zeners are supposed to protect, but.....