[R-390] Product detector for 390

[Dan Merz]

Hi,  a short report on my success constructing the product detector designed
by Ed Newman/ modified with xtal control and posted by AI2Q,  with reference
noted earlier on this list.  I deviated somewhat from the posted diagram and
Ed's design by using a Hartley oscillator with a National NC-100 bfo coil
but more or less the same configuration otherwise.  The detector requires no
mod's to the 390 (or 390a) and is an external box connecting to the diode
load terminals,  the i.f. output and the agc terminals on the back of the
390.  With the oscillator I used,  one of the buffer stages following the
oscillator was unnecessary and a 300 mv signal to the 1496 IC pd was
accomplished with an MPF-102 oscillator followed by the MPF-102 stage to the
product detector. 

The product detector improved the clarity of ssb signals and more
importantly the design provides agc that works nicely with the rf gain
control turned all the way up,  or slightly reduced.  The agc is a big
improvement over what I accomplished with the simple two diode addition.
The 390 noise limiter does not operate in the product detector mode.
However, the unit has a single switch to revert to normal 390 operation with
the 390 diode detector/noise limiter if desired.  I hardly use the noise
limiter so I haven't missed it yet.   

I'll probably build a second one and use an R-392 bfo coil for the
oscillator and make it compact enough to fit below the 390a in its cabinet.
A nice unit,  thanks to Ed and Alex for revealing their handiwork to the
list,  best regards,  Dan.