Re: {Collins} 75S-1 Modifications



Regarding the CQ article to add gain between the mixer stages:

(1)  Not needed in the first place

(2)  Generally a bad idea to add any more gain than needed to assure overall noise figure in wide bandwidth stages.  The triode first mixer and the pentode second mixer stages (halves of separate 6U8's) are not terribly "strong" mixers by current standards.  

(3)  Comparing 75S-1, 75S-3B/3C (1964) and 75S-3B/3C (late production) schematics, Collins left the receiver front end pretty much alone over the product lifetime.  Through most of the product life, the output (plate) of the second mixer relied on the mechanical filters to block other than the 455 IF mixer product from getting through.  However there is a blowby from really strong off-channel signals that gets through.  This was addressed by changing the second mixer output from a shunt choke feed to an IF transformer very lightly coupled to subsequent stages.  This change can be readily made to all units.

Scott Reaser, K6TAR





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