A somewhat interesting case, troubleshooting and 'repair' of an audio amp.
I was presented with a "dead" 'classic' transistorized audio amp.
The owner had checked one of the fuses that was located in the back panel power socket. There were also two fuses on the main board, also good.
The owner had forgotten to run off the amp and a thunderstorm had come through the area. He found the amp totally unresponsive when he came home.
Hm, fuse or diode or switch? No visible signs of damage. No smell of smoke.
The amp weighs close to 30 pounds due to the HUGE toroidal shaped power transformer.
My first step was to try to verify the power circuitry. I didn't find any AC voltage at the screws of the terminal strip. Aha, I though, as I started tracing the AC input that came from the back panel to the power switch and then through a thermal cutout device on the large heat sink, to the terminal strip. It must be the switch or the thermal cutout.
Switching the meter to ohms and removing the power cord, I confirmed that the safety ground was contiguous and that the neutral was also. But I could not find continuity through the hot side to anything. Must be the thermal cutout mounted on the transistor heat sink!
I finally cut a small hole in the insulation on the wires leading to the thermal device. (no needle pointed test leads available) Hm. Thermal cutout was ok.
Maybe the switch? Pulling the back panel fuse, I confirmed that I had continuity to the amp end of the fuse holder when the switch was on, but the hot end of the fuse holder showed NO connection to the adjacent power plug pin!
Pulling back the 'boot' that covered the back of the panel power plug, everything looked fine but there was no continuity between the fuse holder and the strap that tied it to the power plug pin. A few seconds with the soldering iron and everthing now checks ok!
The solder joint ON the back panel power plug had failed without any visible sign!