[Collins] Differences Between the 51J and the R-388

[BSugarberg]

 From "Un-crossing the R-388 Cross Modulation" by Chuck Teeters,
W4MEW.  Electric Radio, Issue #81, January, 1996:

"A valid complaint however is the amount of cross modulation
in the receiver. The cross modulation is the result of the
direct connection of the antenna to the RF amplifier grid.
Despite the coax connector, the input is high impedance."

The author then tells about testing the 51J at Fort Monmouth
at the start of the Korean War:  "A shock mounted rack was
installed in the AN/GRC-26 with 2 51Js, and everything
tested out fine except sensitivity.  The low impedance of
the receiver connected by 12 feet of RG-11 to a 6 foot whip
didn't cut it.  W2VQR, the test director, said treat it like
an automobile BC receiver, and try a hi-Z input.  It worked
great with a direct connection of the short antenna to the
6AK5 grid."

"After some negotiations with Collins, we had a JANitized
51J with hi-Z input, antenna trimmer, and IF output."

"Col. John F. Rider's publication branch glossed over the
high impedance input, in their rewrite of the Collins IB
into TM-11-854."

"You need to restore the R-388 to a 50 ohm input.  The
R-388 eliminated the primary windings on the RF coils.
It is easy to put new windings on the RF grid coils, and
rewire the band switch."   The author now tells you how to
do that.

"For about 2 hours work, you have given your R-388 a
normal input that will match the coax most of us use,
and will have eliminated any strong local RF from
getting a free ride into your receiver."

73, Bruce WA8TNC



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