by jwhitmor » Sun Feb 22, 2015 3:57 pm
Thanks Shannon,
I did exactly what you recommended. I already had the (green) earth gound connected to the metal chassis, and I conntected the transformer C.T. to the PCB "J6" terminal as the schematic indicated. I was not sure about also tying the transformer C.T. to the earth ground. I tested first with the C.T. only connected to the PCB, and there was a little residual hum. Then I checked to see if there was any significant voltage difference between the C.T. connection on "J6" and the chassis. I also checked to see if any current was going to flow between them if I connected them.* Both checks indicated no ground loop, so I ran a wire from the chassis earth ground connection to the C.T. common connection on PCB "J6" and the hum went away. I also had a nasty "crunch" sound when I switched the amplifier AC off, so I put an X2 rated safety cap (.047uF-250VAC) across the switch and it fixed the shutdown noise.
* (Someone may wonder how this is done. I bought a few 0.01 amp glass cartridge fuses years ago, and I put one in series with my milliamp meter test lead and then check for a current loop.)
If it is not making X-rays, your B+ is too low.