[R-390] REALLY SNAZZY AGC mod for R-390A

[Kenneth G. Gordon]

On 9 Mar 2007 at 13:32, Gary Gitzen wrote:

> Hi Ken,
> 
> > BTW, ER also recently printed an article on a mod to the AGC system
> > of the R-390A. The result is an almost perfect AGC system with no
> > overshoot, and almost instantaneous rise time. It requires no added
> > holes and only a few added components. It makes the R-390A an
> > extremely good SSB and CW receiver, yet doesn't change its AM
> > characteristics.
> 
> If you happen to scan any schemtatic of that, please share it
> with the 390 list. Thanks!
> 
> Gary

Gary:

The entire article is important. The schematic alone won't be of much 
help, as there are things you must do that can't really be shown on a 
schematic.

The ER in question is Number 208, September 2006, page 30, entitled, 
"A High Performance AGC for the R-390A" and written by Clark Hatch 
W0BT of Topeka, KS.

Ray N0DMS the editor writes: "I have installed Clark's AGC circuit in my 
R-390A and it is a "keeper"!. My receiver has been used almost daily 
for nearly 30 years and I highly recommend this modification because it 
is the best performing AGC circuit I have ever used."

In ER a few issues before this one, Ray published a two part article on 
some upgrades and mods he had made to his own R-390A, including 
one on a better AGC system. But Clark's works so much better than his 
own, that he removed his, and installed Clark's.

Ray shows an accompanying test and a photo of an oscilloscope 
screen. When he switched his sig gen from Off to On with a 0DB signal 
(224 mV) the rise time of the AGC system was measured at 611 
microseconds, and there is NO overshoot or ringing.

For a second test, he used a +20Dbm (2.2 Volts!) input signal. Rise 
time was unchanged and total harmonic distortion was 4%.

Further, the annoying receiver blocking that occurs when switching 
from SLOW to MEDIUM is gone because C-551 can no longer 
discharge into the AGC line.

The earlier articles also show o'scope photos of the ORIGINAL AGC 
system, and there is really terrible overshoot and ringing.

Ken Gordon W7EKB