[R-390] R-390 AProduct Detector

[Kenneth G. Gordon]

The very BIGGEST problem with installing any sort of internal product 
detector in the R-390(*) is the completely crappy original AGC system.

Although it was probably adequate for the service for which it was 
intended, it is no longer adequate at all.

As I mentioned here a week or so ago, there is an article in a recent 
issue of ER magazine (www.ermag.com) detailing an absolutely 
magnificent, simple, AGC system that completely rectifies all the known 
deficiencies of the stock system.

Attack is almost instantaneous (611 microseconds), overshoot is 
completely non-existent, and audio distortion is very much reduced.

The matter of low BFO injection can be taken care of very easily by the 
simple addition of more capacitance in the right place.

At one time about 30 years ago, I installed a simple triode product 
detector circuit (stolen almost verbatim from Heathkit as used in their 
SB/HW transceivers) in an R-390. I used that 1/2 of the dual triode 
(12AX7?) that was normally used to feed the IF signal out to the BNC 
on the back panel.

Despite the crappy AGC system, my mod worked quite well for SSB, 
and didn't seem to effect AM.

Ken Gordon W7EKB