KiT 6550 ECF 80 Pin 6 Voltage

[Richard]

Hi Tim,

The power supply is common to both channels so I wouldn’t expect it to affect hum differently between channels when the volume is changed. Have you checked different input selector positions (if you have them) and using different sources, and any earths?

Re. the psu, parts I’d check are;
Choke L1
R5 5k6
C5 120uF 500V

C5 is close to its voltage limit so change it out if it’s not new.
R5 should be checked cold and hot after running a while for any change in correct resistance.
Choke L1 should be checked for inductance 2.5H and resistance approx 28R cold then hot after running a while.

I had a similar choke in a different amp which shorted its turns after running for about a minute when it warmed up and a hum kicked in on both channels (it was still conducting but lost its inductance so didn’t smooth the DC). I found the problem with the cold/hot resistance check when the resistance measured much lower hot. Be very careful metering there as voltage is very high. The 10V drop across the choke can be checked running carefully with a probe clipped on each choke terminal before powering up. The choke DC resistance is best checked powered off and cold, then soon after running with it powered off again when the voltage has dropped but the choke is still warm.