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

[ANTHONY CASORSO]

The RF amp is labeled as a 6DC6 (I had it out yesterday). I haven't pulled all the tubes recently but I did at one time and I didn't notice any that were wrong. I will check this. Mistakes happen.

 

I know about the graph. I will have to check it out.

 

AGC line resistance to ground on the IF deck alone is about 500K. With everything plugged in, it reads 350K-ish if I remember correctly. 

 

I did hook the signal generator up to the IF input and drive 455KHz into it. I was able to develop over 15 volts of AGC with the generator at about 1volt. I checked Z-503 and noticed that it did not seem to have a clear peak unless I drove it hard. In other words, with a modest input to the IF deck (say 150 microvolts) I couldn't locate the peak when adjusting z-503. I had to crank the generator up until I was getting -10 or so on the AGC bus before I could see a clear peak in the adjustment. I was looking for a peak by monitoring the AGC bus voltage. I don't recall it being like that the last time I looked. There seemed to be plenty of AGC voltage regardless of the adjustment of Z-503. I removed the z-503 cover and checked the coil. It reads low ohms, so it is not open.

 

I wonder if I could have shoved an incorrect tube into the IF deck at some point. I doubt it though.

 

Tony
 
> CC: david_wise at phoenix.com; r-390 at mailman.qth.net
> From: k1lky at earthlink.net
> To: canthony15 at msn.com
> Subject: Re: [R-390] R-390a AGC Issue/Observation
> Date: Tue, 14 Apr 2009 01:42:41 -0400
> 
> 
> 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
> 
> 
> 
> 
>