Denon Power amp woes

[amplexus]

I really hate servicing solid state power amps. EVERY time SOMETHING has to get funky and my brain just doesn't wanna wrap its way around them. This time is no different.

Denon POA-S10. Cascode output stage, I beleive the output lead to the relay was shorted to gnd.

Seems to start up ok being ramped up on the variac, then suddenly as it approaches full voltage it takes off. All output devices test good, driver MOSFETs test good (TR211/212), but TR209/210 are shot.

Replaced with IRF540/9540, no more runaway but bias is barely adjustable and there's like 7V of DC offset and R274 is smoking (which is odd cus C226 should be blocking DC there) and of course the relay won't close to the spkr terminals but I assume that the TR220/223/224/225 network there is meant to tell the control circuit to stay open if there's signficant offset present.

Are these IRF replacement fets unsuitable subs? Almost every other component on the board tests correctly compared against the working unit next to it. The bias TR unit TR221 also tests fine.

I swear every time I work on a SS power amp I feel way dumber than I'm sure I am

 

[JohnRoberts]

Have you measured and compared node voltages on the schematic, to the unit being troubleshot? This might reveal where to focus attention.

JR

 

[amplexus]

Have you measured and compared node voltages on the schematic, to the unit being troubleshot? This might reveal where to focus attention.

JR

If i have it on long enough to probe I'll definitely smoke that resistor.

Removing it from the circuit temporarily probably won't alter things too much I suppose tho

 

[JohnRoberts]

Back in the day when working on power amps we used Variacs to bring the circuits up at lower mains voltages, and also wired something like a 100W incandescent light bulb on series to act as a roughly 1A current limiter. After the amp was up to voltage and working, we would short across the light bulb for power testing.

JR

PS: Do they still sell incandescent light bulbs?

 

[amplexus]

I have it on a variac, bringing it up slow was where i found the initial issue. Now it'll come up and idle more or less where it should ~800-900mA, but the offset is massive and it's making R274 real hot and bothered.

I think you can still buy them in limited varieties- I have a box of various ones stashed away for those places where I like em better. Almost like the old stash of TV and radio tubes some guys keep in the attic