[R-390] Gain drift

[dog]

Not sure how to explain it except 'gain drift'. I thought it was dirty 
2nd Osc. bandswitch, but don't think so now. When I turn on the 390A 
with no antenna on it, let it warm up so I can get some audio noise out 
of the diode load to a small SS amp, then let it sit for a while 
listening to the white noise, after about 5 minutes or less, the audio 
drops rather rapidly way down and I mean to the point where I have to 
turn up the SS amp volume a whole lot. Then if I do just about anything 
to interrupt the receiver, like turn to STBY, or even sometimes MGC, or 
rock the Bandswitch, the volume pops right back up. But none of these 
things are definitive to correct the attenuation, it can be any of them. 
It's almost like the gain through the RX had been attenuated somewhere 
and any interruption or change in the signal makes the attenuation go 
away. Tried switching tubes, RF and IF, haven't tried the PTO. Even just 
hooking up the antenna will do the same thing. Now if I have the antenna 
hooked up, it never seems to do this. I've tried poking around the 
different modules, tubes, etc when the volume drops and I just don't 
seem to be able to figure out where this attenuation is occurring or 
why. Any help here? I'm kinda shooting in the dark.

Dave N3DT