[Milsurplus] Collins 75A-4 Receiver Coverage

[Glen Zook]

The 75A wasn't actually commercially produced.  There were several prototype receivers made but the nomenclature was changed to 75A-1 for production models.  I actually owned s/n 4 but traded it off in the mid 1970s (wish I had it back!).  The 75A-1 was basically a 1947 design.

The 75A-1 covered 80, 40, 20, 15, 11, and 10 meters and had an i.f. of 500 kHz.  The 75A-2 came out in 1951 and had the 160 meter band plus the same bands as the 75A-1.  The i.f. was shifted down to 455 kHz.  Next came the 75A-3 which was basically the same receiver as the 75A-2 but had provisions for 2 mechanical filters (the receiver came with a 3.1 kHz filter "standard").  Collins came out with a modification kit for the 75A-2 which added the mechanical filters and that receiver was designated the 75A-2A.  The 75A-3 came out in late 1952.  The 75A-1, 75A-2, and 75A-3 all doubled the PTO frequency for use on the 11 meter and 10 meter band and therefore those 2 bands are 2 MHz wide rather than 1 MHz like the other bands.

Both the 75A-2 and 75A-3 had provision for an optional NBFM adapter that was switched from the front panel.  

The 75A-4 was introduced in late 1954 and was the first of the Collins receivers that was designed for single sideband operation.  The PTO frequency was not doubled on the higher bands and therefore all the bands are 1 MHz wide.  Also, the 75A-4 came from the factory with a 3.1 kHz i.f. filter and places for 2 additional filters.

The frequency accuracy of the Collins 75A- series is due to the PTO.  Of course the fact that all bands except 160 meters are a converter into the basic receiver adds to this.  The basic receiver tunes from 1.5 kHz to 2.5 kHz (the 75A-2, 75A-3, and 75A-4 are single conversion on 160 meters) and then basically a crystal controlled converter into this for the higher frequency bands.

Glen, K9STH

Website:  http://k9sth.com


--- On Sun, 2/15/09, Al Klase <al at ar88.net> wrote:

I think the answer is that the 75A was introduced in 1946. (per Moore, 3ed Edition)  This was the first commercial double-conversion superhet with a crystal controlled first oscillator.  This gave it unprecedented  frequency accuracy.  The 51J was developed from the 75A and wasn't ready until 1949. 
 
Like someone said:  The 75A was a stop-gap measure for the military.  They really wanted general coverage.