[Milsurplus] BC-645 Conversion

[John Vendely]

These turn up fairly often at hamfests even today, typically in like-new 
condition and attached to their plywood packing board, just as G&G and 
others sold them years ago.  Asking prices are usually silly, the 
sellers often believing these are rare and wonderful items.  I knew a 
coupla guys who years ago did the conversion on two of these sets, but 
had very poor results.

The story I heard many years ago was that this early IFF system was 
somehow compromised almost immediately, and the remaining stocks were 
therefore never issued.  The story seems plausible, though I have no 
idea if it's actually true...

73,

John K9WT

On 11/3/2013 3:32 PM, Hue Miller wrote:
> 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
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