[Collins] Fw: Re: 75A1 Modifications or inhancements

[Glen Zook]

You really need to take a look at the Collins 75A-1.  It is NOT single conversion on the 80 meter band.  Just like the later "A" Line receivers the tunable i.f. is 2500 kHz to 1500 kHz for 80 meters through 15 meters and like the 75A-2 and 75A-3 the tunable i.f. is 5.5 MHz to 3.5 MHz for the 11 meter and 10 meter bands.  The injection for all of the bands on the 75A-1 is on the "high side" and therefore the i.f. tunes "backwards".  That is for the lowest frequency of the received bands the tunable i.f. is at the high side of the 1st i.f. frequency and for the lowest frequency on the received bands the tunable is at the low side of the 1st i.f. frequency.

The first difference between the 75A-1 and the later "A" Line receivers is that the 2nd i.f. is 500 kHz and in the later receivers the 2nd i.f. is 455 kHz.  In the 75A-2, 75A-3, and 75A-4 receivers Collins added the 160 meter band switch and the r.f. amplifier circuits to allow reception of the 160 meter band using the tunable i.f.  This band tunes "backwards" from the other bands because the other bands use "high side" injection.  The 75A-4, since it doesn't multiply the PTO frequency for the 11 and 10 meter band like the 75A-1, 75A-2, and 75A-3 receivers there are 3 each 1 MHz bands, 26.5 MHz to 27.5 MHz for the 11 meter band, and then 28.0 MHz to 29.0 MHz and 29.0 MHz to 30.0 MHz for the 10 meter band.  

As mentioned before, the 75A-1, 75A-2, and 75A-3 receivers double the PTO frequency for the 11 meter and 10 meter bands.  In those receivers the 11 meter and 10 meter bands cover 2 MHz per band switch position rather than 1 MHz per the lower frequency bands.

The injection crystal for the 80 meter band on the 75A-1 is 5700 kHz.  Remember, the 80 meter band on the 75A-1 is 3200 kHz to 4200 kHz.

It is possible to couple an antenna to the tunable i.f. and then tune to the high end of the calibration of any band from 80 meters to 15 meters.  At that point it is possible to receive AM broadcast stations at the "high end" of the BC band.

As for using a TV balun to improve the sensitivity:  For the article that appeared in Electric Radio a while back go to:

http://k9sth.com/uploads/TV_baluns.pdf

Glen, K9STH

Website:  http://k9sth.com


--- On Mon, 4/5/10, Glen Zook <k9sth at sbcglobal.net> wrote:


Its single conversion on the lowest band, so make that work first.
 
With it open, put a finger on the wiper of the volume control. If the speaker buzzes, the audio section is working. If not, be sure the speaker is hooked up and go from there. Check the power supply voltages with a meter, not a wet finger. Remember tubes are in sockets because they are expected to need changing. But when you change IF and RF tubes, you need to do an alignment which can get touchy with old ceramic trimmer capacitors.
 
As for upgrades there probably were many published in the ham magazines back then, things like different mixers.