I have a 410hd that was having problems in the bias circuit, the pot ended up being intermittent. However, I have one lingering issue that may or may not be a problem. If power the amp off and quickly re-power it, the amp makes a terrible humming noise. If I use the Standby or let the amp discharge, it doesn't occur.
I am assuming it is more an issue of "don't do that" but wanted to see if anyone can verify if that is normal or if I need to continue to look for a problem.
The amp seems very stable now except for that issue. FYI, the amp has had the power supply recapped not to long ago and all the 105uf/50v caps were just replaced. Bias is set at .48 volts across the 100 ohm cathode resistor,