[R-390] R-390a AGC Issue/Observation

[Tony Casorso]

I have been annoyed for quite a while with the fact that I get a big drop in 
gain when I switch from MGC to AGC, even when tuned to just background 
noise.I checked the AGC bus voltage and found that it was about -.9 volts in 
AGC and 0 volts in MGC. I wasn't sure if -.9 was reasonable for the AGC with 
no signal but I suspected that it was not becasue of the big drop in gain 
and the fact that this radio is supposed to have delayed AGC.

I first checked the resistors on the IF deck (R544, R545, and R546 if I 
remember correctly). They were fine. I also disconnected my 2 diode AGC mode 
(the Lankford mod). That made no difference. I could see a postive voltage 
on the plate of the AGC rectifier but negative on the AGC bus. When I pulled 
the AGC time constant tube (V506 which also doubles as the detector), the 
AGC bus went to +1.2. That surprised me since it is only the grid of V506a 
that is connected to the AGC bus. But we are trying to pull the grid 
positive so maybe it makes some sense. Anyway, I swapped the tube with 
another 5814a. It was a little better but not much. Still about -.7 volts. I 
stuck in a 12au7a and got about -.3 volts. Finally, I put in a 12ax7a and 
the AGC bus stayed near +1.2 volts with no signal. With the 12ax7a in there, 
there radio acts like I expect. On weak signals, switching from AGC to MGC 
makes little difference. The radio seems livelier too. I'll have to try it 
this way for a while, but I think I like it.

If anybody else is so inclined, I would like to hear what your AGC bus goes 
to with no signal. You can measure it on the back panel so you don't even 
need to open it up. Also, I would like to know if you hear a substantial 
drop in the background noise when you switch from AGC to MGC with no signal 
present.

My radio is a 1967 EAC with all EAC modules.

Thanks,

Tony