[Hallicrafters] SR-2000 Puzzle

[Critter]

Good day list,
The SR-2000 is moving along. The ps is working and the the screen choke and
shorts have been put right. It gets about 300+W on low and 600+W on high.
Thats not a concern at the time. I'm happy that its working at all.
My question/puzzle is this; USB puts the rig into full output when keying
the mic in MOX or speaking into the mic in VOX. This occurs with the RF and
AF gain set at 0.
LSB, Tune and CW work as they should. LSB only draws the 200ma bias until
you turn up the AF and speak. I see no obvious shorts or burnt parts. Has
anyone come across anything like this before?
Any suggestions on where to start looking would be welcome and appreciated.
Thanks, George