[Collins] 75A2 / 75A3 mixer modification

[Dr. Gerald N. Johnson]

On Tue, 2006-06-27 at 12:41 -0400, Jim Brannigan wrote:
> I changed both mixers in a 75A3 many moons ago, 
> 
> QST 56,57,58?
> 
> Jim
> 
> 
> > On Mon, 2006-06-26 at 18:56 -0700, Glen Zook wrote:
> >> I had an inquiry about a 75A3 that has had the first
> >> mixer changed out from the 6BA7 to a 12AT7.  Since I
> >> had not seen this change before I thought that I would
> >> see if anyone had any information on the modification
> >> so that I could pass along the information.
> >> 
> >> Of course the same modification would work on the 75A2.
> >> 
> >> Glen, K9STH
> >> 
> >
The only 73 magazine reference I see was October 61, p 32, an article
about a dual triode mixer (as I recall, I've not found that magazine in
the pile here yet). The index shows nothing specific to the 75A family.

My CQ collection is limited because I came to distrust the editor's
technical expertise, but I found no references to 75A receivers in '56,
'57, or '58. I didn't look at the few '59 issues I have.

David Edsall <dedsall at sover.net> found a reference to an article in QST,
July '55 p 25 on the A2 and A3. It suggests mixer changes, but not to
12AT7. It should be authoritative being written by Ed Andrade and Ernie
Pappenfus at Collins. It suggests changing the RF stage and shows some
AGC mods. Should be very useful.

WALLY <k5op at yahoo.com> suggests an article by W6ZO, ham radio magazine,
April '70, p. 42. Its on the 'A4, uses a 12AT7 for the first mixer and a
6DJ8/6922 for the second mixer. It references only the book on SSB by
Bruene, Pappenfus, and Schoenike. The circuits will probably work fine
in 'A2 and 'A3.

I didn't find every issue or December for QST of '56, '57, or '58,
though I still have my dad's bound volumes here somewhere. The question
is which box are they in? Here or in the storage unit?

So, its closer, but not sure yet.

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer