Topband: Good Receivers for Top Band

[Kevin Olson]

>Any R4C for serious CW use should have a narrow roofing filter
>installed, otherwise you will be at IC-751, FT-1000 and TenTec
>performance levels except you'll have more filter blowby. Without a
>narrow filter you'll hear a higher pitched tone for a few kHz up and
>down from strong signals.

I had a R4C many years ago, and found that much (not all) of the filter
blowby in this rx was actually signal leakage around the filters.  So I
spent an evening with copper pc board and shield material and empirically
found where the shields "needed" to be.  As a rule of thumb, input and
output filter pins should not be able to "see" each other. I also remember
finding significant leakage between circuit boards.  There was one inductor
in particular I remember that radiated quite well, defeating the filters
for strong signals!  When I was done, I wasn't at tricked-out Sherwood level
but got a significant way down that path, for the cost of some shield
material! 

Over the years, I found a lot of receivers have problems in this area.
Collins 75A4 and 75S3 both had input and output pins too close together.
The A4 required a replacement filter (stock filters were built in vacuum 
tube-like sockets), the S3 required rebuilding the filter switching
wafer switch for greater separation.  A Kenwood TS 450 was almost unusable
due to blowby.  In that case, more isolation was required in the diode
switching system.  It is easy to just blame the filters...but quite often,
they aren't allowed to do their job. One rx actually had blowby through
the cw filter but I recognized the "shape" of the leakage mimicked the ssb
filter...that was back leakage through the diodes going to the ssb filter!

Some time spent logically troubleshooting can reap big dividends...

Kevin K3OX




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