[Collins] DDS again.

[Gerald]

On Mon, 2005-05-23 at 22:22 -0500, cory wrote:
> First this:  "Is this for the 75S series. I have the 75S3 and would
> like to add more crystals."
> 
> This is for 75S-1 serial number 55.  One of the first modern
> radios off the assembly line.  
> 

That low a serial number was probably a prototype. Some time after the
first were produced the serial numbers were issued in random order (by
executive decree) so one can't depend on serial number to determine
vintage.

Collins generally started production radios with SN 100 or 101.

There's no difference in frequency control from the first 75S-1 to the
last 75S-3B/C. The receiver differences are in the AGC, filter, BFO, and
detector selections.

> A list member, Glen, sells an inboard crystal switch.  It's a mod
> that adds another crystal bank under the lid.  
> 
> I'm still thinking about it. 

I've been using a switch like that since about 1965. Didn't take much to
make it, a little piece of aluminum, a crystal socket, a miniature SPDT
switch, and some miniature coax.

> 
> However, in addition to the AOR DDS-2A, there's a website in Italy
> that shows a direct digital synthesizer hooked onto an S-Line.  It
> replaces the crystal bank as does the DDS-2A.
> 
> The DDS-2A does the crystal bank and the PTO, and memories, and so
> on. 
> 
> I also found a outfit, S&S Engineering, that makes the DVFO-II DDS
> VFO.  They say that they can generate frequencies up to 50 MHz,
> include an display with programmable offset, and can set the step
> rate. 

You could use the DDS with nearly continuous coverage and leave the PTO
setting at 100 on the dial for fine analog tuning occasionally. Just the
control program for the DDS would have to select frequencies 3.055 MHz
above the desired receive frequency. Crystals don't go in convenient 100
KHz round frequencies for S-Line. They could, I suppose, then the PTO
dial would need to be turned to '.045 to be at some '.100 frequency.

> 
> I understand the issue of spurs but ignoring that for the moment, a
> VFO that is crystal stable, steps 200 kHz (or 100 kHz) seems to be
> a possible replacement for the crystal bank. 
> 
> Of course, if I wanted a general coverage receiver, why not just
> buy one.  

Back in the spring of 1964, my boss at Collins, Lloyd Winter caught me
drooling over a 51S-1 in the lab. I admitted I was shopping for a
receiver. He had been in charge off both 51S and 75S designs. He said
the 75S with crystal pack was the superior receiver of the pair. I
bought a 75S-3B the next month or two.

> 
> I don't know the answer to that.   
> 
> de ah6gi/4  or just use the SX-100.  
> 
One of the really great benefits to the 75S family of receivers is low
phase noise from crystals and PTO that keeps down spurious responses and
makes copying really weak signals next to strong signals easier and
using any kind of a synthesizer can hurt that capability. Though the
mixers may still be the main limitation.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer