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
[EMAIL PROTECTED]
http://www.wd8das.net/
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