[Boatanchors] Collins 75A-1 Mod info?

[Geoff]

----- Original Message ----- 
From: "Roger Shultz" <nj2r at verizon.net>
To: <boatanchors at mailman.qth.net>
Sent: Thursday, February 09, 2012 10:16 AM
Subject: [Boatanchors] Collins 75A-1 Mod info?


> I'm working on the restoration of a reasonably good condx 75A-1 that has a
> couple mods which I have yet to find any documentation about.
>
> The front end has been changed to a 6BZ6, the mixer is a 7360 and the HF
> oscillator is now a 6AH6.

Those are all fairly common mods. The 6BZ6 is far better than trying to run 
AGC on a sharp cutoff 6AK5 and was readily available unlike the 6DC6. 
Collins didnt understand ham requirement AGC until the 75S3B.

The 6SA7 mixer is extremely noisy. A 6SB7Y is a direct replacement but the 
7360 mod is one of the best for low noise and lack of overloading. The 
noisest mixer ever is the 6L7 and the stages preceeding it now hopefully 
have overcome most of it. The early mechanical filter is also very lossy.


The 6AH6 is a known excellent stability VFO tube, perhaps the 6SJ7 was 
giving the owner drift problems.


 The first RF amp has a 6SG7 in place of the 6SK7
> though I don't see any socket changes there.

A direct swap and used in later A1 production in both IF's.

>
> The second mod is the addition of a fast/slow AVC switch neatly mounted to
> the right of the Phone jack.


Sounds good if its done right. SSB wsnt given any consideration in 1946.



>
> I'm inclined to leave well enough along though the s-meter does not seem 
> to
> work as it should however I have yet to check out anything about why that
> may be.


All the paper and oil caps should be replaced before doing any 
troubleshooting. Also check for out of tolerance resistors.
Im also inclined to leave good mods alone.

> I've seen mods for the 7360 mixer for the 75A-4 and suspect the front end
> 6BZ6 which is close to the 6DC6 may be a copy of that in the 75A-4.

I modified my A4 to 7360's in both mixers in 1965 and still use it.
The 6DC6 was replaced with a 6GM6.
Both above were 60's era mods, published and unpublished by W2VCZ and others 
except in obsure bulletins and club monthlys.

I havent dug much into my A2 and A3 yet except for caps and resistors 
however a 6ES8 Pullen mixer mod is on the horizon as 7360's are $20+ these 
days and Im saving my NOS spares for the A4.



I've
> searched Electric Radio, on line, Collins archives etc but have found
> nothing close so far. Has anyone seen this before?
>
> The really good thing about this radio is that is has the 353C31 
> mechanical
> filter which is not very common as I recall.


Very scarce.

Carl
KM1H



>
> Thanks for any info you may have.
>
> 73, Roger, NJ2R
>
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