Any tips to find / revive caps?
 

I have a HP UNIX workstation which has sat in a garage for a couple of
years. I've bought it out of storage, powered it up and it worked for
about 30 hours or so. Now it has died. I have reason to believe the
switch-mode power supply might not be fully functional, although it is
hard to tell, as it requires a signal from the motherboard to power it
up. (The PSU is not completely dead and is providing power to the
motherboard. But it is not energizing the high current lines).



One possibility is a dud cap that has dried out. Replacing all the
electrolytic in the power supply will be a bit expensive, on the
off-chance it is one of them. I might as well spend a bit more and buy a
new computer.

Has anyone got any tips on how one might find a doggy cap? Freezer
spray? Heat? Tapping them?

Debugging SMPSs is hard enough with a circuit, but I have no circuit and
no idea what lead is supposed to bring this thing into action.

--
Dave K MCSE (Minefield Consultant and Solitaire Expert)

Please note my email address changes periodically to avoid spam.
It is always of the form: month-year@domain. Hitting reply will work
for a couple of months only. Later set it manually.

Any tips to find / revive caps?
 

Dave (from the UK) wrote:
I have a HP UNIX workstation which has sat in a garage for a couple of
years. I've bought it out of storage, powered it up and it worked for
about 30 hours or so. Now it has died. I have reason to believe the
switch-mode power supply might not be fully functional, although it is
hard to tell, as it requires a signal from the motherboard to power it
up. (The PSU is not completely dead and is providing power to the
motherboard. But it is not energizing the high current lines).



One possibility is a dud cap that has dried out. Replacing all the
electrolytic in the power supply will be a bit expensive, on the
off-chance it is one of them. I might as well spend a bit more and buy a
new computer.

Has anyone got any tips on how one might find a doggy cap? Freezer
spray? Heat? Tapping them?

Debugging SMPSs is hard enough with a circuit, but I have no circuit and
no idea what lead is supposed to bring this thing into action.


Short of having a cap checking ESR meter, there is a little you can do.
YOu can take the PS apart and check all the solder joints. A bad solder
joint on the PC board can cause a problem. Look at the top of the caps.
If any are "domed" looking or look split open at a seam, they could be
leaking and bad. Also look at the caps bases for any white leakage gunk.
Any suspicious cap should be replaced.

Check the mother board caps the same way. Re seat the cpu and ram in
there sockets. Make sure the fans are not seized.

Bob


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Any tips to find / revive caps?
 

"Dave (from the UK)"
wrote in message ...
I have a HP UNIX workstation which has sat in a garage for a couple of
years. I've bought it out of storage, powered it up and it worked for about
30 hours or so. Now it has died. I have reason to believe the switch-mode
power supply might not be fully functional, although it is hard to tell, as
it requires a signal from the motherboard to power it up. (The PSU is not
completely dead and is providing power to the motherboard. But it is not
energizing the high current lines).



One possibility is a dud cap that has dried out. Replacing all the
electrolytic in the power supply will be a bit expensive, on the
off-chance it is one of them. I might as well spend a bit more and buy a
new computer.


Electrolytic capacitors are pennies to a couple of quid or so. That PSU must
have hundreds of them to make buying a new computer more economical!

Seriously, it'd be a real shame to junk that machine for a few bad caps. Try
heating each cap (on the metallic top bit) with the tip of a soldering iron,
taking safety precautions of course, like disconnecting from the mains
during the heating process. Get the caps hot to the touch but not so hot
they explode. After heating each cap try powering on until you get a result.
It's usually the little 'uns which cause power-up problems, like 10uF or
thereabouts.

You can use a heat gun or hairdryer to heat the caps, but the snag is that
will only tell you there are bad caps, not which ones as it heats everything
in sight!

If that doesn't work then it gets more complicated. An ESR meter would help-
that's assuming it is indeed a cap problem.

Incidentally- it could be a dead motherboard, which would be a real shame.

Dave