[Collins] SSEng DVFOII

[Dr. Gerald N. Johnson]

On Mon, 2007-04-02 at 07:45 -0400, kiyoinc at attglobal.net wrote:
> The SSEng DVFOII showed up the other day.
> 
> This is a direct digital synthesis device with LCD readout and 
> programmable steps and display offset.
> 
> The most common application is a stable VFO for a small receiver or 
> transmitter.
> 
> After a email exchange with a fellow who uses one to generate signals 
> for the 1st Mixer of his Drake R-4, I bit the bullet and ordered one, to 
> use with my 75S-1.
> 
> My goal is to "all-band" the 75S-1 with minimal, hopefully no internal 
> modifications.
> 
> Theory of operation
> 
> I'll be ordering Glen's crystal switch-set.  I still have not figured 
> out which one would work best.
> 
> I'd move the 75S-1's internal crystals to Glen's switch and use it to 
> switch between the crystals and the external "DVFO".  I don't know if I 
> need to "modify" anything to replace a crystal with a signal source.
> 
> I'm guessing that the oscillator will lock onto the DVFO's signal

I'm guessing it won't. The crystals are connected from screen to grid in
an electron coupled modified Pierce oscillator. The S-3C auxiliary
switch bank switches only the grid and reduces the screen bypass
capacitor to compensate for the extra wiring on the screen. When I put
in my auxiliary switch connected with coax I had to reduce the screen
capacitor a lot to keep the oscillator working on frequency.

If it was me, I'd add 1000 pf to the screen to make it a buffer stage.
Then I'd hook a lower value resistor from grid to ground but with a
series 1000 pf to load the grid if the tube oscillated. Since the tube
gets its bias from grid current, with no drive it will have zero bias
and will draw lots of current where its gain is also greater than
normal. Then it needs enough drive to get the original DC grid voltage
to control the plate current.

Otherwise, being a lazy character, I'd make a special 6EA8 with pin 7
shortened so it didn't touch the socket and drive the crystal oscillator
out jack with a 50 ohm source.
> 
> The DVFO fits perfectly in the Collins-look, small LMB box.  There's 
> about 1/16 of an inch on each side.
> 
> It should look just fine sitting on the 75S-1.  Now if it'll just work.

It will receive, but you will loose some of the superb phase noise
performance of the crystals and its intermod from phase noise and DSS
artifacts will be worse.
> 
> de ah6gi/4
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer