Altec 1567A mixer hum, help needed
I got an Altec 1567A mixer and hooked up a line level signal (like a CD player) to input 1 octal socket pin 7 hot and 8 ground. Pins 5 & 7 are linked. The output is connected to output octal socket pin 7 hot, pin 8 ground. It has a bad hum/low frequency buzz, I think its the way its hooked up. I don't think its the unit itself and I tried lifting grounds on everything except the mixer. The source is battery powered and the amp is a quiet soundcraftsmen amp. Anyone who can help I'd be happy to trade studio parts, tubes, caps, octal plugs. Incidentally, I shorted pins 7 & 8 of input and the hum/buzz is still there.
The schematic and manual are at http://www.triodeel.com/al1567a.gif
Also, the hum gets louder when I raise the master volume or ch. volume. The hum is the same on all ch.
Yes, those units suffered badly from ground-induced hum. DC heating of the input valves (tubes) was a nice idea, but it causes heavy pulse currents through the rectifiers that permeate the rest of the circuit. The optional input and output transformers were not there for fun - they were needed to isolate the mixer from what was driving it and what it was driving.
Is the ground clean?
When was it last swept?