[R-390] Hartley bfo FET Prod.Det.

[DJED1 at aol.com]

"Hi,  I'm mostly thru the mechanical building part of the product  detector
for 390a similar to the one shown by AI2Q on this list,  which  has an agc
circuit in addition to the detector/bfo,  and just connects  to the i.f. out,
diode load,  and agc connections on the back of the  390a.  But instead of
using the crystal-controlled bfo oscillator   I opted to use a tapped coil
bfo unit in a can salvaged from a  National  NC 100.  In tapping the MPF102
FET bfo signal  into the base of the  2N2222 buffer stage."
 
Can't help you on the rational for where to tap off the RF, but the  original 
 oscillator circuit I used had the oscillator output from the  source, and 
the drain was bypassed to ground at RF.  So there was no choice  as to where to 
tap off for the RF.
I'd be interested to know how you make out with your project- I provided  
Alex with the original circuit which he  published with some  modifications.  I 
started with a variable BFO, and then last winter added  the crystal oscillator 
per Alex's circuit.  I still think there has been  nothing better for SSB on 
the R-390A than this little SSB adapter, partly  because it includes an 
effective AVC, and partly because it requires no mods to  the radio.  Of course, you 
have to be tolerant of mixing solid-state with  our beloved tube radio.  
Otherwise, a CV-157 is the right choice.   I've taken some measurements of input 
and output voltages which may be helpful  if you have any problems.  I've also 
started working on adding a phase  locked loop BFO so that the detector can be 
used for synchronous AM.  Not  done yet though, waiting for winter.
Ed  WB2LHI