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

[ANTHONY CASORSO]

Looks like it is fixed. The problem was a way too hot v508, AGC amplifier. The tube had a Gm of 4000 when the roll chart showed 2050.
 
Here are the AGC measurements (generator terminated in 50 ohms applied to antenna input so the signal is actually a little weaker than indicated}
Signal              AGC          TM11-856
10uv                -3.3v           -2v
100uv              -5.6v            -4.5v
1000uv             -8v              -7v
10000uv           -11.1v          -9.5v
100000uv          -13.9v          not shown
 
After replacing V508 with a 6ba6 (old tube was 5749) I remeasured the 1000uv point and got -6v.
On this radio, lower AGC sounds much better.
 
The S meter was also too hot, reading 80db when it should have read 60. With the new tube, it reads 65db.
 
Now to use it for a while.
 
Thanks for the comments. It got me thinking about it the right way. I went for the AGC amp because it is the only gain in the AGC loop that is not in the signal path. If it is too hot, the audio will get weak. For all other stages, a hot tube will increase the signal along with the AGC.
 
Tony
 

 
> From: David_Wise at Phoenix.com
> To: k1lky at earthlink.net; canthony15 at msn.com
> CC: r-390 at mailman.qth.net
> Date: Tue, 14 Apr 2009 10:37:33 -0700
> Subject: RE: [R-390] R-390a AGC Issue/Observation
> 
> 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
> 
> 
> 
> 
>