Aahh...R4C...brings back memories! Back in the day, a Sherwood-modified R4C
ruled! I was jut a kid with no money but lotsa nerve, so I was determined to
improve my R4C despite not being able to afford the Sherwood mods. To make a
long story short, the main problem with the R4C was layout! I used a cheap
and dirty method to test, a borrowed sig gen cranked up 1 kc from rx freq
and "cut and pasted" piece of double sided copper circuit board inside the
rx to lower the leakage. When I was done, the rx weighed a few lbs heavier,
but was much improved. Leakage across the filters and even inter-circuit was
the problem...I remember one of the inductors radiated like a mother and
needed a shield can fabricated! That started me on filter leakage mania, a
Collins 75S3 benefited greatly by use of cascaded filters and increased
separation of the filter selection switch wafers (even though they had a
shield between them!), a Signal One CX7 improved when one corner of the
circuit board was insulated from the chassis (ground loop, you know) and
just to show that new rigs are not immune, a Kenwood TS 450 leaked sigs
around it's cw filter like crazy! I fixed that by putting a second switching
diode in series with the ones already there to improve the isolation! (A pin
diode would be more elegant...) It was like a new rx! In fact, in my more
paranoiac moments, I believed that Kenwood let this leakage exist to
convince you that you couldn't live without the second IF filter!
There was a guy, I forget his call, but his name was Sabin who used to write
rx design articles for QST in the late '60s and another guy, I believe W1DX,
whose mantra was "as much selectivity as early in the RX as possible, good
filters and extensive shielding". Those words still apply today. A
well-modded R4C, 75S3 or 75A4 (if your stock market holdings have done well)
can still compete with the best.
Kevin K3OX
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