[Milsurplus] BC-645 Conversion

[Hue Miller]

Was reading the same Hints and Kinks, vol. 4, 1949, the article about the
BC-645 IFF unit conversion to 420 MHz transceiver. Basically the article
has some receiver mods, and then uses the WE-316 doorknob tube in
the transmitter as Heising modulated.  What I was surprised to see was
that the unmodified receiver schematic uses antenna input to a 955
mixer; but all three elements of this tube, cathode, grid, and plate, are
at DC ground. There is NO B+; the plate goes thru the first IF transformer
to ground. Apparently the grid-cathode acts as diode mixer; the osc drive
is fed right to the grid. The plate must be acting as a capacitor plate, I
assume, to pick off the conversion frequency. I do not know why the plate
has to be dc grounded for this effect, tho.  The conversion article has you
lift the mixer plate off ground and connect to B+.
Apparently, I "guess", the designers had need for only so much 
sensitivity???

This is the "Identify-Friend or Foe" transponder flogged for years by such
firms as G&G Radio in New York, for years after the more desirable stuff
had thinned out. Apparently the sets were produced new, packed, then
never used as better equipment was brought out. I have never actually
seen a successfully "converted" BC-645, only some really hacked chassis
that may have been abandoned starts.
-Hue