Behringer PMH 660m

[Jazz P Bass]

That 20 ohm resistor is a soft start.
The relay is 'supposed' to take over.
Not sure if these schematics are valid for the 660.
Power amp & Power supply attached.

[Jazz P Bass]

Another thing, each power amp (left & right) has there own power connector.
It may very well be that one is good.

I have seen where a faulty amp will kill that 20 ohm resistor.
(Some versions used two in series)
It usually happens when the owner 'tries it again' after the initial failure.

The high voltage power supply itself was not tested with a load.
So maybe it was on the brink & hooking it up killed it.

[Enzo]

My files say the PMH660M used the SPS400 power supply.

[Jazz P Bass]

Behringer SPS400 Schematic

O/k then.
Here she is.

[kalvinb]

Thanks for the help and schematics gentlemen. The power supply is an SPS 1000/02. I appreciate the schematic--I've just been trying to follow the circuits with no real direction. It does remind me of working on my old Dynaco ST-400--it used a similar soft start relay with resistors. I replaced the burned 22 ohm/10W resistor (2 in series and it's mate is beginning to crack too). I'm not in a great rush to get it fixed so I'm going to haul the whole thing into the TV station shop next week and go through it with a Huntron in-circuit tester to see if I can find the fault(s). If it's a quiet day I'll probably have one or two maintenance engineers looking over my shoulder helping too. Thanks again for the schematics and information--I'll study them this weekend.

[Jazz P Bass]

The switch mode supply has to have everything right or it will not turn on.
1: the main switchers T3 & 4 must not be shorted, as well as D1.
2: the drivers can be checked with a DVM as you would any NPN/ PNP transistor.
3: IC1 has to function as that is what drives the main switcher circuit.
4: the TOP221 has to function. It's like a 'housekeeping' circuit.

A lot of times a simple resistance check on the TOP221 & IC1 will find that it is bad.
Simply look up each datasheet & consult the block diagram and make a few intelligent readings.

That is the primary side.
As to the secondaries, about all you can check are the diodes

[Enzo]

Guess my files lied, though such upgrades often happen during a production run.

SHorted rectifier diodes on the secondary side is a pretty common SMPS failure and will shut one down.

[kalvinb]

My thanks to both of you for all your help. I'm leaving it alone for the rest of the weekend--weather is nice here and I have yard work and painting to do. The schematics are a great help and I'll check all the items you've suggested early next week and let you know what I find.