[R-390] AGC problem

[Bob Camp]

Hi

Sorry to be late on this one.... I think the AGC side of this has been 
pretty well worked over. There is another side to this though.

The distortion on SSB detection is related to the way the BFO works as 
a detector. It distorts when the RF signal gets to high for the way 
it's set up. One of the things that directly affects this is the level 
of the BFO voltage injected into the detector. Since the AM detector 
diode gets used as  a mixer when the BFO is running it's 
characteristics also get into the act. Some 390's have been modified to 
increase the BFO injection and some have some detector tubes in them 
that have gotten a bit soft. In this case a soft tube *might* work 
better than one that's up to spec.

I would also agree that most R-390's do not work very well on SSB with 
the RF gain turned all the way up. If I was going to look for something 
odd I would take a look at the radio that picks up SSB fine rather than 
the one that has problems.

	Take Care!

		Bob Camp
		KB8TQ




On Dec 10, 2004, at 12:31 AM, 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
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>