[R-390] Re: Trim Caps Science Experiment

[2002tii]

> Its the old adage a picture is worth a 1000 words.
> I was hoping someone has the equipment to do the test.
>
> A nice bandpass sweep on the scope to look at as one varies a cap.
> We do not need exact values from the test.

I have swept mechanical filters many times in the past to select
termination caps, though never in an R-390A.  The caps adjust the
transition region of the filter -- how much amplitude ripple there is near
the passband edge and how much of a peak before descending the skirt.
Unfortunately (to these ears, anyway), Collins went for maximum skirt
steepness at the expense of peaks and group delay errors, so mechanical
filters always sound strident to me.  You can calm them down with
additional resistive loading, but that destroys the passband shape.  As
many folks have commented, the R-390 sounds better than the -A by virtue of
its LC IF filters, but you *really* need sweep equipment to get them set up
properly if someone's been playing with them.  (If they've never been
touched since the day they left the factory, they're probably fine.)
Plug-in mechanical, crystal, and ceramic filters basically work or they
don't -- any adjustments are just for fine-tuning the passband edges.

IMHO, the best sounding plug-in IF filters are the Murata ceramic filters.
If anybody knows of a source for Murata Series CFS, CFR, CFK, CFX, or CFZM
filters in hobbyist quantities, I'd be very grateful to know about it.

Best regards,

Don