PMH power supply

[PeterB]

Hello
I have a problem with the power board (SPS600) of a Behringer PMH 3000.
Everything if fine for about 4 minutes, then the power fails. It will not work again until being left for a few hours. Then the same thing happens even with the mixer section and the power amps supplies disconnected.
All the caps look ok and I've found no obvious problems when prodding around with a multimeter.
Any help would be appreciated.
Peter

[pmaillot]

something gets hot... a short or close to it somewhere?

and works again when cooled down.

[PeterB]

Thanks for the reply.
Nothing seems to get hot or smelly.

[pmaillot]

You can also look for any suspicious electrolytic capacitor. These have a tendency to wear quickly in power supplies. If suspicious... replace it.

[MJKlein]

You cannot always tell bad capacitors by looking at them. You have to de-solder the negative end of the electrolytic and use a Z meter to check them. In the old days, Grady and I would just replace every single electrolytic capacitor on a console once it got past a certain age, regardless of anything else. That always worked out very well.

[PeterB]

Thanks for the reply.
Agreed, a faulty cap is probably the best suspect .
I have checked the larger caps in circuit with an ESR meter, perhaps I need to go around again a bit more carefully.

[MJKlein]

It takes time to lift one end of each cap, but that Z meter instantly tells the story. It's so fast that very often it's not even worth trying to figure out what part of the circuit has the problem. Grady showed me that workflow once when we were working on a cart machine at a local AM. He pointed out that depending upon whether the capacitor was in series or shunt configuration, we should be able to look at the circuit and deduce where the problem was, based on whether the audio issue was loss of high end, or loss of bass frequencies. We spend a few minutes looking at the drawing when he said "It's not necessary that we engineer a fix, we just have to get it fixed as quickly as possible." The soldering iron had been already heating up while we looked at the drawing. He pulled the card and had the negative end of every cap lifted on the card in just a few mins. The Z meter found the bad one right away (he did check all of them, btw). All that remained was to replace the bad one and re-solder the leads on the rest of them. It took between 10 and 15 mins total.