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

[David Wise]

An open cap would let audio get onto the AGC bus,
attenuating the envelope.

Dave Wise


-----Original Message-----
From: Roy Morgan [mailto:k1lky at earthlink.net] 
Sent: Monday, April 13, 2009 10:43 PM
To: ANTHONY CASORSO
Cc: David Wise; r-390 at mailman.qth.net
Subject: Re: [R-390] R-390a AGC Issue/Observation


On Apr 13, 2009, at 11:35 PM, ANTHONY CASORSO wrote:...
> It seems to me that I have too much AGC. With a strong station tuned  
> in, turning on the AGC reduces the signal so much that I have to  
> crank the audio way up for normal listening. Even then, the audio  
> sounds "compressed", lacking any punch. Actually it is even like  
> this with not so strong stations.

THIS clue sounds familiar from earlier posts on the list.

Is the correct tube in the first RF stage?  And all other stages,  
too.   (A very high gain substitute, or one with a sharp  cutoff  
characteristic will do this.)  It could be that the AGC voltages are  
pretty much ok, but the response of the radio to them is wrong.

Also, is the trouble possibly in the noise limiter stage?  Pinched  
audio can come from things amiss there.

There is a chart showing expected AGC voltages with RF input levels.   
It may simply show values from a table in the manual, but I can't  
remember if that's true.  My copy is called AGCGRAPH.JPG and it plots  
four points:

RF input      AGC voltage

10uV                 -.8
1000 uV           -4.
10,000 uV        -7.5
100,000 uV      -9
(These values are approximate - its a simple graph)

There is a specification for audio output increase (in dB) with RF  
input increase of a specified amount (from X to Y).  Check this out,  
too.

I think you did report the resistance  to ground of the AGC line.   
There are expected values of that resistance, too.  You may have a   
leaky cap.

Roy


Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073