I repaired an ST70 for a customer. It had a loud audible hum on both channels with nothing plugged in.
I ended up replacing the cap can which helped but didn't solve the problem. I went further and found that the 470k resistor was missing on one channel, and on the other the connection from the input tip to the mono/stereo middle switch was also missing.
Fixing all three of those problems made the ST70 dead silent when nothing was plugged in.
Next I connected up a PAM-1 and the darn thing hummed like a son of a bitch again.
Here's the thing, it hums badly even in the PAM1 output is not connected to the ST70 input. All I need to connect is the take off socket octal plug to the PAM1 and the hum is there. This happens on either channel.
The PAM1 has the original CAP-can in it. It tests OK for capacitance and ESR, but I didn't check for leakage yet at full voltage.
Anyone have any ideas? Could a shot cap can in the PAM1 cause this behavior with the ST70 even without the output connected up to the power amp input?
Thanks everyone. I don't want to start replacing parts blindly and I really appreciate your insight.