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

[ANTHONY CASORSO]

I made a little mistake last night. I forgot to mention the fact that the AGC rectifier was removed (V509) when I made the measurements. With the AGC rectifier installed, the AGC voltage remained negative, even without V506 installed. However, the rest of the story was fairly accurate. 

 

I remeasured everything with a little more care, this time with V509 installed. The original 5814a in V506 shows -.82 at the grid with no signal (V506a). I swapped tubes around. I found one from another spot in the radio that reads -.35 on the grid in the v506a position. I checked both tubes on my old Hickok 546 and both read strong (Gm of 2200 vs 1400 on the roll chart). Neither tube is gassy.

 

I then tried swapping around the AGC rectifier (V509). A different tube brought the AGC to about -.25 with no signal.

 

The V506 plate resistor reads 78k (R549). R547 reads 212k. R548 reads 30 ohms. Plate voltage with the original tube was 35 volts. With the alternate tube, plate voltage was 37 volts. B+ is about 210.

 

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. The radio sounds great in MGC with the RF gain turned down a little bit. As measured on the Line level meter, a modrate strength station drops the audio by 20db when I turn on the AGC.

 

 

 


 
> From: David_Wise at Phoenix.com
> To: R-390 at mailman.qth.net
> Date: Mon, 13 Apr 2009 17:27:11 -0700
> Subject: Re: [R-390] R-390a AGC Issue/Observation
> 
> I believe in this case that the positive voltage is
> correct AGC Delay operation, coming from B+ via R544.
> 
> Dave Wise
> 
> -----Original Message-----
> From: Jim M. [mailto:jmiller1706 at cfl.rr.com] 
> Sent: Monday, April 13, 2009 5:20 PM
> To: David Wise; 'Tony Casorso'; R-390 at mailman.qth.net
> Subject: RE: [R-390] R-390a AGC Issue/Observation
> 
> I don't know if this was mentioned yet, but an AGC controlled tube with a
> "leaky" gris can cause the grid voltage to tend toward positive as I recall.
> This can mess up the AGC line. Check all the AGC controlled tubes for grid
> leaks if your tube tester does that. Or just trial-and-error replace tubes.
> Another thing that can happen is if you get oil on the antenna trip shaft,
> on the fiber insulating clamp. The antenna trim capacitor is on the AGC
> line. If you get oil or lubricants on that fiber clamp, it will show
> resistance to ground and affect AGC performance. 
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