[Premium-Rx] RACAL 6790/GM filter question

[Charles Steinmetz]

By way of further explication, I thought this information might be 
useful to others:


There should be no loading caps (Racal calls them "shunt caps") in
any filter Position where crystal filters are installed.  If your radio has
shunt caps in any Position(s) where crystal filters are installed, it
did not leave the factory that way.  It is because someone changed
previously-installed mechanical filters in those Positions to crystal
filters and did not remove the shunt caps.

If any crystal filters are installed in your radio, remove all shunt caps
(input and output) from those filter Positions.

Wherever mechanical filters are installed, make sure the appropriate
value shunt caps (input and output) are installed in those Positions.

Now:  *Should* you replace crystal filters with mechanical filters in a
6790/GM, "just because"?

Both crystal filters and mechanical filters have very sharp "shape
factors" (a measure of the steepness of the filter skirts -- a higher
ratio is sharper).  High shape factors create ringing at the corner
frequencies, which sounds bad (often described as "fatiguing").

As noted, *both* crystal filters and mechanical filters create this
ringing. The question is, which sound less bad (less fatiguing)?
The comparative shape factors for the filters may be some guide
(i.e., all things equal, use the filters with lower shape factors).
However, the character of the filters' sound is more complicated
than just the shape factor, so the choice should be made by
listening tests.

Best regards,

Charles