My old KEF PSW2000 sub (which I got broken many years ago for a Pizza and could fix by replacing an opamp) failed again. Power came up but no output and no switch on/off noise.So I took out the electronics. See front and back side:
View attachment 363578
View attachment 363579
A visual inspection shows something I haven't seen outside a battery chamber or in a device flooded by salt water:
View attachment 363581
Everything else just looked fine:
View attachment 363582
So I replaced the opamp (BF4560) in the preamp section by a NE5532. Checking the output pin revealed that it was oscillating at 360kHz, so I removed the 220 nF cap between output and ground (the dark monster in the pic 2nd above which was soldered across the board) to check it. It was fine but then I saw that the oscillation was gone without it. So I left it as is:
View attachment 363583
The sub still didn't work although the new opamp now worked fine - the scope showed the expected signal I fed into the input of the sub.
So I checked the power amp and its output (before the protection relais) sat at 50V DC. OMG. Checked voltages at the output transistors, the drivers, the VAS stage - everywhere clinging to one of the voltage rails. And then - oh wonder - I measured a good voltage (close to 0V), checked again and now the sub was working. How could this happen? Had I touched somewhere stupid and the opamp in the input stage had latched up? Anyway, several power cycles showed the amp in working order so I put it back into the box and hope it lasts. Phew ...