ReForming Capacitors

[bigbeark]

I oftern find motherboards that have been decommissioned for some time will
"almost" boot. Sometimes they will boot if the machine is shut down and powered up again.

Since I generally buld using SCSI, powered off the PCI slots, I have come to the conclusion that the small 8mm diameter caps used near the PCI slots tend to "deform". These are frequently really good caps like Rubycons or Sanyos.

I have often found that replacing these caps is enough to get the board working again.

Has anyone been successful at "reforming" caps without resorting to recapping? if so how?

I have recapped 15 or 20 boards so far, success rate about 80%, but I hate to throw out perfectly good Rubys. When I remove them from the board, they always test fine.

[Per Hansson]

Re: ReForming Capacitors

Have you tried just letting the system run for a couple of hours to see if that does the trick?

[Krankshaft]

Re: ReForming Capacitors

I've only heard of reforming on old lytics in tube radios not modern ones. I always replace old tube radio lytics it's not worth the potential house fire.

The process involves applying the caps working voltage through a resistor for a certain period of time.

First off applying the caps rated voltage while in circuit will almost definitely fry something. The circuit the cap is in isn't rated for the caps max WV the max WV is the absolute limit the cap can take. Any engineer worth his salt isn't going to run at cap at it's max WV. The voltage of the circuit will be lower.

So all that aside the suspect cap would need to be removed from the circuit. If you're removing the cap for this why not replace it?

[bigbeark]

Re: ReForming Capacitors

No, although I have let the "second time" booters run for extended periods of time, but they still seem to insist on the "boot me again".

Have you had success with thie "extended run time" approach?

[Krankshaft]

Re: ReForming Capacitors

If by extended runtime you mean run the PC until the caps work you are not getting any permanent fix.

What you are doing is heating the caps up by using the mobo when caps heat up their ESR goes down. Which will give you the illusion that the mobo is fixed.

But if you shut the PC down again and the caps cool you're back to square one.

This is why when people leave PSUs with bad caps on all the time they will keep running but when they shut them down once they won't boot anymore.