[Milsurplus] BC-640

[C.Whitaker]

de WB2CPN
I installed and maintained a lot of BC-640's and BC-639's while
in AACS etc (USAF) beginning in 1946.  They were still in use
when I retired in 1966.   In the mid and late 30's the US didn't
have much interest in VHF above about 60 MHz, but the Brits
did.  The BC-640 and 639 is a Brit design.  Most were made by
Bendix under license.  The SCR-522 aircraft radio was a clone of
the BC-639.  The transmitter was Brit.  Now, they were a good
teaching machine because everything was right out where one could
see it, and everything was replaceable.   The oscillator was followed
by a tripler, followed by a second tripler, followed by a doubler, and
then the PA.  Tricky, how the doubler IPA was coupled to the PA.
They were in separate panels in the same rack.  Few people learned
how to neutralize the IPA and the PA.   The antenna had a unique
way of coupling to the PA, and that kind of thing was far too expensive
for amateur radio designers.  Yes, BC-640's were big and heavy, but
rugged.   The VARIACS (tm) let them be run off most sensible AC
voltage supplies.  You'd think that when that much RF was being
generated in a rack that had screen-covered slots it would at least
wake up the chickens.  Nowadays you won't see PA tubes running
so hot, (not neutralized properly), that the grid would show right through
the plate.  We never encountered interference when collocated with
receivers,  but along about 1954 the trend was to locate transmitters
and receivers about a mile apart.  VHF is still used at USAF and
civilian air fields, check "Popular Communications" sometime.
TRIVIA:  Harrison's Law:  "Radio communications requirements will
expand to occupy all channels on all multi-channel selector knobs."
73  Clete