KA-4002 60hz hum

Suddenly out of the blue i get a faint, constant 60hz hum sound coming out of both speakers on KA-4002 amp. She was running without issues like this for a few months now. I also have been hearing intermittent sharp crackles as of late, not often enuf to be a big alarm or annoying, but i notice it. Not sure if related or not. All caps are new, new wires, re-flowed rails. The hum occurs even as the amp is idling. Tried a different power circuit, its still there. Do I have a output transistor going out/shorting to ground causing this hum or... im looking for ideas...

Could be a regulator transistor.

Flip the pre/main switch to separated and see if the hum goes away.

The pops are probably a bad transistor in the audio section. Not the outputs. I had to replace a few in my KA-4002.

Rob

Robisme said:

Could be a regulator transistor.

Flip the pre/main switch to separated and see if the hum goes away.

The pops are probably a bad transistor in the audio section. Not the outputs. I had to replace a few in my KA-4002.

Rob

Click to expand...

The hum does go away when switched to preamp. Could the bad transistor causing the pops also be causing the hum?

It's possible. Is the pop in both channels? It's coming from the preamp section.

Rob

Robisme said:

It's possible. Is the pop in both channels? It's coming from the preamp section.

Rob

Click to expand...

You are probably correct Rob, the problem does persist on the B channel as well. I will get it on the bench to examine the preamp circuit and post back.

We had a bad NPN transistor in the preamp circuit, replaced it and the hum was eliminated. Thanks guys.