OK, here is the whole board. Audio amp. where should I be looking?

Remove the dark glue and you may fix your problem.

Read the third bullet dot down at:

Capacitor plague - Wikipedia, the free encyclopedia


"The glue is itself sometimes harmful and can corrode leads and tracks covered by it, leading to leakage current or open-circuit"

Hi,

Take out the two fat diodes (first photo, just above main xfmr), you will be isolating the main output to the amplifier circuit. One of these diodes could be shorted...just as a tip...
Also there are three smaller diodes, providing auxilliary supply to amplifier circuit.
The small daughter board is actually the controller for the main mosfets.
There are two "reddish" capacitors between the daughterboard and the big PS capacitor. Those two are part of the half bridge topology of the input converter. The other half being the two mosfets. As I said in a previous post, its a self oscillating half bridge dc-dc converter. The small daughterboard should have also a DIAC on it, which provides for a turn-on pulse and afterwards it is kept off by the oscillations of the converter, of course if it starts. This kind of converter also needs a minimum load, to continue operating. This kind of PS were used in early days as "lightweight transformers", to power the halogen lamps.
At the moment I don't have a generic schematic handy but I believe you can find one on the net, just to get an idea, how the circuit operates.
It is very possible, that you have a short in the amplifier circuitry, preventing the main PS to start and remain locked, blowing the fuse. The advice to use a 100W bulb instead of a fuse is fully correct and enables you to actually power the circuit without blowing things.
What was the rating of the original fuse?
Tonight I traced down the schematic of the other daughter board, the one which works at the secondary side, doing the main job of tracking the output power needed by the amps, by varying the voltage supplied to the output transistors.
Probably if I get too pissed off by this lousy circuit I'll do the same for the input doughterboard, even maybe for the entire circuit... Then I will post it. I hate when people try to hide "hot water" inventions...

leal said:

This is a class D Amp and there are some tricks to know about trouble shooting this type of Amp. this is not like the old class AB amp where use too working on. So pay special attention to wave form. It works like a PWM. Just a hint.

Click to expand...

that's a Klipsch amp board. from a SUB10, SW350, or SW450.Klipsch sells the whole backplate assembly.

these boards fail a lot, and trying to repair them is a real pain. Klipsch doesn't provide schematics on this. the subwoofer also has a 5 year warranty IIRC

OK guys,

My amp repaired!!!
As I told you before, it is a bridged AB class amplifier, with variable tracking power supply. In no signal condition, the main supply voltage to the amplifiers' output transistors is +/-3.6V!(referenced to the virtual ground). Yess you read correctly. This voltage appears at the two fat resistors of 150ohm/4W each, which are there, to provide for a virtual ground, which all of the low-signal circuits are referenced to. This main supply then tracks the input signal. If the input signal rises, the main supply rises as well(it can get up to +/-40V). I loaded the amplifier with 2.66ohm dummy load(4ohm in parallel with 8ohm) and measured a whooping 27.5Vrms across the output terminals(make the math but I think the power is around 278Wrms...).
The problem that was driving me mad, was actually some tech who put hands on the circuit and messed around for a while, before I got it for repair. Basically I found the main MOSFET(the first in the row of the amplifiers' output transistors), which is part of the tracking PS(yes it's a switching BUCK topology and it is inserted in the negative branch - this is the reason for the big inductor), to be FUBARd(it was shorted between S&D). Due to this shorted transistor, the full tracking voltage appeared immediately after switch on at the two fat resistors, causing them to glow reddish...and of course stopped the tracking regulator to operate.
Basically the amp worked correctly all the time, just the time was too short, if I wanted not to roast up the two 4W resistors...
Possibly the original MOSFET was the one I found in the circuit(IRF530). I didn't have it handy so I replaced it with an IRF640.

So this is the story, about a mysterious circuit, repaired without a schematic. Of course I prefer not to mention all the time I spent into it...

Cheers