[hagtech]

Ok, so it looks perfect.  Is the hum coming through the headphones?  Or a mechanical vibration of the chassis?  Hum or buzz?  Both channels?  Does volume setting matter?  Input connected or not?

jh

[Braden]

There is some low level hum that is coming through both channels of the headphones, remaining constant regardless of volume level or connected input. The hum coming from the unit is more of a buzz, a mechanical buzz, and is either coming from the power transformer or the 156 next to it. I can't quite tell. The buzzing doesn't start until after at least a half hour of warm-up. The low level hum in the headphones starts on power-up.

[hagtech]

Yes, there is a slight residual 120Hz hum caused by magnetic coupling between the first 9H and and the output 30H chokes.  It is about 98dBA down, but still audible in a quiet room.

The buzz is a bad tranny or loose mounting.  As it happens after warmup, check to see if tranny and choke mounts are tight.  Maybe you can track it down by holding various parts of the machine (fingers should mechanically damp it)?

jh

[Braden]

I can stop the buzzing by putting some sideways pressure on the tranny. It's tightened down pretty well, but I'll see if I can get it a little more. Could a bad tranny vibrate after warm-up as I described?

[hagtech]

Sounds like the varnish is broken.  After the tranny is built the vacuum impregnate them to hold the coils in place (otherwise it is a motor).  Maybe it is time to try out a homebrew dip?  I've been thinking about this for production, perhaps a dipping in paint, or something.  Lacquer?  Anybody have ideas? 

jh