[R-390] 2 diode agc ssb mod c/l meter linearity

[David Wise]

Back when I was testing the Gary Gitzen/Dave Wise Universal Carrier Meter Retrofit*, I investigated the suppressor grids.  In truth, they are just about worthless as diodes.  The real source of AGC-delay clamping in the R-390 and R-390A is the control grid of AGC Time Constant triode V506A, via R547.  The main effect of the suppressor grid wiring is that the AGC line exerts a bit more control on the tubes, because it's talking to two grids.  That effect is so slight as to be meaningless.  Somebody had good intentions without deep knowledge, that's all.

Part number aside (Larry got it right), the diode that Larry recommends adding (the 1N1693 from V506 pin 2 to ground) is the same diode I added in the Universal Carrier Meter Retrofit.  (It's needed in that mod because V506A cathode is higher off ground, making the control grid useless as a clamp.)

73,
Dave Wise
SWL in Hillsboro Oregon (inactive)

* An alternative to Jan Skirrow's op-amp mod, this minor IF Deck design change lets you drop in virtually any meter movement without modern technology.

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Larry H
Sent: Thursday, August 26, 2021 8:43 PM
To: R-390 Forum
Subject: [R-390] 2 diode agc ssb mod c/l meter linearity

Hi all,  The c/l meter linearity may be skewed if you have the Lankford 2 diode agc mod on for ssb reception.  The reason is that the original 'suppressor grid' clamping at the junction of R544, R546 and R547 is not effective enough to overcome the increased voltage from internal noise, an RF signal or the antenna noise.

The clamping circuit provided by this fix (Mod 3), improves the situation considerably, but does not completely solve it for a high noise antenna.
This problem has been there all along, but with the original design, it was not very noticable. With Mod 2 and Mod 3 on, it looks like the c/l meter noise tolerance operation is a little better than with none of these AGC mods on. Mod 3 also improves the c/l meter linearity close to what it was without these mods on. How does this affect the c/l meter linearity, you say? Well, with mod 3 on, the meter 0 set point is at a better place. This is what improves the linearity. The diode needs to be 200 volts at 400 ma with a .6 volt forward drop or less because of the high voltage pulses created when switching the AGC speed.  There were a number of diodes made that meet the requirements in the 60's and 70's, but very few available now. Ones I found are: 1N1693, 1N1694, 1N1695, or 1N2092.

However, if you have the 'moment of silence' fix on for the AGC speed, then a lower voltage one (such as a 1N63) can be used.

I have attached the updated schematic for this fix.  If you don't receive it from the list server and would like a copy, please let me know.

Regards, Larry