amplexus
Well-known member
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
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
