[Collins] T9

[Dr. Gerald N. Johnson]

Not exactly.

75S-3B/C SB-1 installs a new IF transformer in place of L9 for more 
selectivity. Calls it T10. And installs a new T9 in the Q 
multiplier/notch filter. That SB also changes the headphone jack and 
replaces the fiber mode switch shaft with a metal shaft. And that SB 
installs the magnetic shield over the mechanical filters to eliminate 
the hum modulated noise from the power transformer magnetic field. I 
used a sheet of mu metal on my receiver to achieve that effect.

75S-3B/C SB-2 for early receivers, moves T4 from the AM filter to the 
mixer plate circuit. That way it improves selectivity of the raw IF to 
reduce filter leak through outside the filter pass band. And its still 
in effect on AM, so the AM response still is set by T4 AND T5. T4 is NOT 
out of the circuit on AM. Look at the 11/75 manual schematic.

Without staggering the IF alignment the AM bandwidth and skirts will be 
narrowed after these two circuit changes. But the filter blow by will be 
significantly reduced.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 5/22/2010 11:51 AM, Glen Zook wrote:
> 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
>
>