[AMRadio] 75A-2 project


sbjohnston at aol.com sbjohnston at aol.com
Tue Jan 22 19:46:41 EST 2008


The project to restore my old Collins 75A--2 receiver has gone very 
well.  After replacing the paper/wax caps, the filter caps, checking 
and replacing the weak tubes, cleaning the sockets and controls, and 
replacing quite a few failed (due to corrosion) front end trimmer 
capacitors, and a "by-the-book" alignment, the unit meets or exceeds 
original specs.

The widest bandwidth is 4 kHz (crystal filter out, just the tuned 
circuits in the IF stages) and I would like to add the capability for a 
wider choice for Hi-Fi AM reception.  Anyone know of any articles on 
such a modification?  Maybe in the context of "updating" the 75A-2 or 3 
to mechanical filters?  Or have any of your own ideas?

I'm thinking perhaps I could replace the back-to-back IF transformers 
with a 10 kHz BW 455 kHz ceramic filter I have here, but I'm not sure 
about the issues of impedance matching into and out of the filter, 
which was designed for low-Z solid-state circuits I'm sure.  Or maybe I 
could increase the values of the small caps coupling the top of the 
primaries to the top of the secondaries.  But that would be hard to 
switch in/out.

A block diagram of the 75A-2 can be seen at

http://www.wd8das.net/Collins/75A-2blockDia.pdf

and a schematic is posted at

http://www.wd8das.net/Collins/75A-2schematic.pdf


Thanks...

Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
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