RACAL RA-237B Gerry O’Hara
4
balanced mixer, though a
hole could be drilled for
this purpose also
(however, its adjustment
is very broad and makes
little difference).
Restoration Work
As noted above, the unit
was in reasonable shape
on receipt from the
seller, and testing some
of the passive
components revealed that
they were in reasonable
shape – much better than
those that were in my
RA-117! Most resistors
tested within tolerance
and the capacitors were
mostly serviceable (the
electrolytics were not
that great). However, as
I had completely
replaced all paper and
electrolytic (and some
silver mica) capacitors in
my RA-117, I decided to
do the same for the RA-
237B to promote long-
term reliability (photos,
left and at top of
following page). All
tubular ceramic caps
tested ok and were left in
place. While doing so, I
noticed some discrepancies between the schematic in the manual and the unit on my bench: these
changes were apparently modifications made to improve performance and reliability, including
inserting a blocking capacitor between the RF stage anode and the 1MHz low-pass filter (this I
suspect to avoid disaster if one of the capacitors in the filter shorted, as this would burn out the
preceding inductors), some changes in the harmonic generator (damping the anode tuned circuit),
and including a 1pF coupling capacitor in the bandpass filter.
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