[Collins] 75A4 Roofing Filter ?

[Adam Farson]

Hi John,
 
The architecture of the 75A-4 is such that a "roofing filter" in the modern
sense is an invalid concept.
 
The 1st mixer down-converts the RF signal to a 1.5 - 2.5 MHz variable 1st
IF. The 1st LO is crystal-controlled; the bandswitch selects the appropriate
crystal. The 2nd mixer (excited by a PTO tuning 1955 - 2955 kHz)
down-converts the variable 1st IF to a fixed 455 kHz IF. The 2nd IF filter
is a 455 kHz Collins mechanical filter; this is the selectivity filter, not
a roofing filter.
 
http://www.collinsmuseum.com/75a4.html
 
I hope my explanation is reasonably accurate, and helpful (I am not a
Collins owner, but would love to have an HF9500 in my shack!)
 
Cheers for now, 73,
Adam VA7OJ/AB4OJ



  _____  

From: collins-bounces at mailman.qth.net
[mailto:collins-bounces at mailman.qth.net] On Behalf Of John Hensley
Sent: 22-Jan-09 11:59
To: Subject: [Collins] 75A4 Roofing Filter ?


Is anyone aware of a tested roofing filter mod for the A4 which actually 
improved the receiver performance?

TIA, John

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