[R-390] AGC Slow - Moment of silence

[David Wise]

I would not notice a 20% reduction in the SLOW time constant. 
The symptom appears when you switch to MEDium, but the situation got set up during SLOW.  Your solution eliminates the voltage jump at the positive end of C551.

On paper, none of the commonly available 22uF/450V aluminum electrolytic caps has acceptable leakage, but I expect that actual parts beat spec by a wide margin.  You can also use a film cap (a motor-run type can be less than $10) but it can't go behind the front panel.
While the low end of C590 averages 0V, you'll get a bump if you go to SLOW during an excursion or during the first minute or so after power-on.

I attacked the problem differently, running the Miller integrator on MED as well as SLOW.  See "Ending The Moment Of Silence" at www.r-390a.net .

Larry's mod is easier and still pretty good, and I encourage the rest of you to try it first.

Regards,
Dave Wise

-----Original Message-----
From: R-390 [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Larry H
Sent: Sunday, May 08, 2016 6:16 PM
To: R-390 Forum
Subject: [R-390] AGC Slow - Moment of silence

It looks like the problem with the circuit is the charge time of C551 when switching the AGC into the slow position.  This causes the agc line to be pulled + for a second while C551 charges.  Even if it had zero leakage, one end is switched from ground to B+ potential, causing the problem.
I came up with a fairy simple solution to this problem.  Add a 10 meg to 20 meg resistor to the AGC switch contact 8 to ground.  Remove the wire that goes to that same contact and add a 20 mfd cap (call it C590) in series with that wire to contact 8.   This will maintain a charge on C590 all the time and will have very minimal affect on the operation of the AGC circuit.  I know that this will reduce the effectiveness of the slow position a little, but I don't think you will notice it.  Use the largest res that will overcome the leakage in C590 (should be very low), but I don't think it should be less than 10 meg.  See this schematic.
     http://s32.postimg.org/6310y6av9/IMG_7198s.jpg
I've been using this for a while and have not noticed any issue with it.
Regards, Larry