[Collins] T9

[Glen Zook]

Collins issued SB-2 on the 75S-3B and 75S-3C receivers.  This is for the moving of T-4 from the AM filter (which reduces that filter to a single i.f. can) to the plate of the 2nd mixer (V-4).  This Service Bulletin can be downloaded from

http://www.collinsradio.org/archives/manuals/75S-3B_3C_SB_2.pdf

Since the 75S-3B and 75S-3C receivers were the only receivers still in production when the Service Bulletin was issued the only units referenced are the 75S-3B and 75S-3C.  However, the modification can be made to the 75S-1, 75S-2, 75S-3, and 75S-3A receivers using basically the same information.  I have modified my 75S-3A and both 75S-1 receivers, as well as a 75S-3 that I owned for several years and then traded off.  The modification serves two very useful purposes:  First of all, it definitely improves the CW and SSB performance.  Then, by reducing the AM filter to a single i.f. can, this "broadens" the bandwidth when in the AM position which, in turn, improves the "sound" of the AM signal.

Glen, K9STH

Website:  http://k9sth.com


--- On Sat, 5/22/10, Dr. Gerald N. Johnson <geraldj at weather.net> wrote:

> From: Dr. Gerald N. Johnson <geraldj at weather.net>
> Subject: Re: [Collins] T9
> To: collins at mailman.qth.net
> Date: Saturday, May 22, 2010, 10:50 AM
> An IF transformer in place of a
> single tuned circuit probably increases 
> the IF gain while it improves the inherent selectivity and
> reduces the 
> effects of signals leaking past the mechanical filters
> outside the pass 
> band.
> 
> There was also the T4 mod applied to the later S-3B/C
> receivers which 
> moved one of the AM IF transformers from the AM filter
> position to the 
> plate of the second mixer to accomplish more of the same
> while giving 
> even better protection keeping DC off the mechanical
> filters.
> 
> In S-3B/C receivers, these mods being factory made would
> indicate a 
> later production which can't be told by serial number since
> the serial 
> numbers were issued at random. Changes in emblem also hint
> at late 
> productions, first the meatball in place of winged, then
> the Rockwell 
> emblem.
> 
> There are service bulletins for these mods to the 3B/C, I
> didn't spot 
> them immediately in my Collins data for the 75S-3 but they
> probably exist.
> 
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio
> Association.
> 
> On 5/22/2010 6:50 AM, Master Lee wrote:
> > Hello All
> >
> > Every now and then I see a Collin 75S-3 receiver for
> sale and there is mention
> > of the T9 transformer.
> > What is so good about this option?
> >
> > 73 Lee
> >
> >
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