[Collins] 75A2 / 75A3 mixer modification

[Dr. Gerald N. Johnson]

On Wed, 2006-06-28 at 21:23 -0400, Jim Brannigan wrote:
> Cal etal,
> It may have been from the '55 QST article.  I have it around somewhere, but 
> have not been able to locate it.

I'm sure it wasn't the July '55 article. There they changed the 6BA7 to
6BE6. And changed the RF stage from 6AK5 to 6CB6 or 6DC6 for more linear
AVC control.

I found my bound volumes of '56 to '59 in the storage unit. Nothing in those under
H&K or reeceiving or receivers in the December indices.
> 
> I did the mod in the early 70's so my memory is kinda hazy.....

The ham radio magazine article about the 12AT7 first mixer for the 'A4
was published in April '70. Time enough to inspire you.

> I also did not have the test equipment (or knowledge) I have today.
> If I recall, my 75A3 would not hear the first RF stage noise.  I thot that 
> changing the mixers would help, it did not.
> Ultimately, I traced the tube noise to the added IF stage that compensates 
> for the mechanical filter loss and was never able to resolve it to my 
> satisfaction.
> That 75A3 was subjected to a lot of abuse by me.  At various times I added a 
> product detector (plugged into the FM socket), changed BFO's (crystal, 
> variable, internal, external) and a whole host of other changes.
> I finally realized that I would never make this receiver into a SSB/CW DX 
> killer like my friends 75S-1 and sold it to buy a Yaesu FT-101, it also had 
> a terrible receiver. The useless mods to the FT101 are another story.
> 
> All the work on the A3 did teach me a valuable lesson DON'T MESS WITH 
> COLLINS ENGINEERING!!!!
> The A3 is a nice AM radio.  There were good reasons for developing the A4 
> and 75S series.
> My late WE S-line is my backup to my TT OMNI-VI+ (a great receiver) and the 
> R-390A is a great AM RX for my Ranger.

Someday I need to collect everything I've done to my 75S-3B. And then go
undo most of the mods and find it a new owner. When used with a 2m
converter I drastically improved its dynamic range, at the cost of
poorer HF sensitivity and birdies that had been hidden by 2nd mixer
noise.
> 
> Jim
> PS 300+ DX countries on the S-line and two ARRL CW DX NLI section wins, 92 
> es 93.

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