[R-390] R-390 product detector

[DJED1 at aol.com]

I've played around with a variety of  product detectors- so here's my .02 for 
what it's worth.  I started using  the R-390A alone, and found I could copy 
SSB pretty well just by using the  medium AVC speed.  I then put in the pair of 
diodes to speed up the attack  time, and that helped.  Probably OK for casual 
listening.  I picked up  a bunch of CV-591As for $35 each (ah- those were the 
days!) and used one for a  while.  The passband tuning was nice, as was the 
ability to switch  sidebands.  As I recall it didn't feed AVC back to the 
receiver, so the  radio's AVC was still operating. But I didn't really have room on 
the desktop  for the converter, and I didn't like the lack of a speaker 
output, so I sold it  off in the 1970s.
A few years ago when I rekindled my interest in the R-390A  I decided to 
design my ultimate SSB adapter, and set the following  criteria:
-no modifications to the radio- connect only to available rear  connections
-provide fast attack AVC to the radio
-small and  unobtrusive

I built an adapter using a 1496 chip, an amplified AVC  circuit, and a 
tunable BFO.  The unit takes the IF from the back panel of  the radio, generates AVC 
voltage, and demodulates the SSB, providing  audio.  The audio is fed back to 
the radio via the diode load jumper, which  is removed.  A switch in the 
adapter allows the radio to operate normally  for AM and CW.  I was going to open 
up the AVC jumper and use only the  adapter's AVC, but found the signal 
strength meter didn't work properly, so I  just feed the adapter voltage to the AVC 
buss, leaving the jumper intact.   The adapter provides the fast attack, and I 
set the radio to "slow" AVC.
This  is the best solution, in my opinion.  The radio operates using all the  
normal controls, and the meter works normally, except you can see the fast  
attack and slow decay on the signals.  Audio is controlled by the local  gain 
knob.  As with the normal radio operation, you need to offset the BFO  
depending on which filter bandwidth you are using, and you need to recalibrate  when 
you shift the BFO. Audio is clean and clear for all signal levels, with a  nice 
low frequency response because the adapter's audio is not band  limited.
I've added Alex's crystal controlled BFO, but since the frequencies  are not 
set exactly for each filter bandwidth, it's a more stable solution but  not as 
convenient for different bandwidths.  
Next, I undertook to do  what I thought would be the ultimate adapter, 
somewhat similar in function to  the CV-591.  I had a surplus 100Kc crystal filter, 
so I downconverted the  455 Kc IF to 100 Kc, built a 100 KC crystal BFO, and 
added the same AVC  circuit.  The downconverter had two oscillators to switch 
sidebands. No  more recalibrating when I switch sidebands.
However, it was a  disappointment.  The receiver IF filter had to be set at 
twice the passband  of the adapter, in order to pass both sidebands.  However, 
the radio's AVC  circuit operated at the receiver bandpass, so strong signals 
in the unselected  sideband drove the AVC.  I tried running the AVC through 
the sideband  filter, but then the meter didn't work right.  That was when I 
added the  crystal oscillators to my original design, and have been using that 
now for a  couple of years.  
Alex, AI2Q, has a clean schematic of this adapter for  those who want to roll 
their own.  Jan Skirrow sells a similar product  detector, with a synthesized 
BFO, but, regrettably, without the AVC circuit,  which is essential to good 
operation.
I've lusted after a HC-10, but never  have had the opportunity to try one.  
Having notch tuning would be nice,  but I don't know what they do with the AVC.
Ed  




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