[R-390] AGC problem

[Bill Abate]

OK guys, thanks for the info.  First off I have the gain pot a little on 
the low side for a better s/n ratio.  If anything that should help.  My 
sensitivity is between 0.6 and 1.1 microvolts for 10 db s+n/N.  All the 
tubes are correct and have been tested in a Hickok before aligning the 
rig.  It really is perking quite well.  I have that darn linearity 
adjustment to do for the Cosmos PTO, but I got sidetracked on this SSB 
thing.

I pulled the IF deck and played around with it for most of the day.  I 
replaced a few resistors and caps but not because they were that bad. 
Same results.

I got one more piece of data that makes no sense.  The suppressor grid 
of V504 and V508 (pin 2) has +6 V on it instead of the -2.5 that should 
be there according to my manual.  The voltage does not change with AGC 
or MGC selected with the function switch so I conclude that it is coming 
from V509 and not the AGC bus itself.  Now the no signal voltage on pin 
1 or 2 of V509 is -.5 V and should be -1.9 V.  Maybe that is why I am 
not getting the AGC voltages that Ed, WB2LHI, said his manual indicates.

One thing that I think has helped is to lower the B+.  The military put 
diodes in the place of the 26Z5's and the voltage was 255 which is a 
little high.  I inserted a 10W 40 V zener and got the voltage down to 
215.  The distortion is not that bad now.  I can copy some stations with 
full RF gain but it still does not have good fidelity.  Maybe I moved 
the overall gain of the rig to where the AGE has more of an effect?  I 
am still blaming the AGC voltage.  Z503 has been peaked but maybe there 
is something wrong with it?

Not ready to give up yet.

Thanks again,

Bill, K3PGB



Bill Abate wrote:
> My 390A is almost finished its restoration.  Looks pretty good and works 
> well.  Well, except for SSB reception.  If I try to tune in a ssb signal 
> I get a lot of distortion in the audio and the signal can't be tuned in 
> properly.  If I try the old method of max audio gain and reduced RF 
> gain, all is well.  Of course this is not the way other R-390A's work. 
> BTW, AM reception is pretty good but has some distortion in the audio.
> 
> I figured that the AGC was not working properly so I ran the AGC test in 
> the manual.  I adjusted the ,local audio for 1.7V of audio with 5 
> microvolts of RF.  I then increased the RF sig gen to 1000 microvolts 
> and the audio output went to 20 volts.  Its supposed to increase to 3 
> volts, so I found the problem.
> 
> The problem is I can't seem to find the cause.  I have AGC voltage on 
> pin 2 of v506 (AGC line).  I got these AGC voltages with my sig gen:
> 
> 5 mv    ->     .04VDC
> 50mv    ->     -.2 VDC
> 500mv   ->    -3.8 VDC
> 5000mv  ->    -7.1 VDC
> 50000mv ->    -9.6 VDC
> 
> I also found the AGC voltage on all the tube grids it was supposed to 
> control.  The only thing I can conclude is that the AGC voltage is 
> insufficient but the manual does not say what the voltage should be with 
> varying inputs.  At least I could not find it.  I also tried slow, med 
> and fast AGC and the results were the same.
> 
> Anybody have data on the AGC line?  Any ideas as to the cause??
> 
> Thanks,
> 
> Bill, K3PGB
> 
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