[Collins] kwm-2a dial speed

[Dr. Gerald N. Johnson]

On Wed, 2008-08-13 at 11:10 -0400, yash at aol.com wrote:
> Have been playing with my Kwm-2a and have really enjoyed using the rig. 
> Mainly on 20 and 17 meters.It was a lot of work ,but paid off ,flawless 
> operation and the rig looks as new.
>     Any way ,my question is this. Collins stayed with a 1/1 tuning 
> ratio in the main dial of the kwm-2a ,this can make the rig a bit 
> touchy for fine tuning and since there is no Rit for the Collins, I was 
> wondering. Has anyone ever fitted a vernier reduction drive to a kwm-2. 
> Say 4/1 or 6/1. I have one on my sb-102 Heath and it works very well. 
> So much easier to tune in, than the Collins.

In that day, 100 KHz per turn was very good resolution, and with a
little tenderness applied to the outer edge of the knob, it wasn't hard
to set the dial to the nearest Hz. (I once achieved 1 ppm frequency
measurement with my 75S-3B with such careful tuning and use of
calibrator leakage and images). Except for the 75A-4 with the optional
vernier reduction knob (these days costing at least $100 if you can find
one) it was the Collins standard for amateur, commercial, and military
HF dials to cover 100 KHz per revolution.

Since the rack and PA tuning require similar precision for proper
operation, it behooves the operator to develop that fine tuning
facility.

>         I have looked thru the archives  and can't find anything 
> relating to this. If anyone could give me some direction about this ,I 
> would be grateful.
> thanks
> dale  wt4t

Except for the epicyclic vernier used in some Heathkit gear (prone to
having a sloppy knob) the verniers with friction drive that could have
been applied have been all used up and their friction losses made tuning
unfriendly. I'm sure had Collins designers like Arnie Spielbauer who did
the S-line and KWM-2 dial wanted finer resolution they could have
achieved it with a finer thread on the PTO shaft. But they didn't.

Even at 100 KHz per turn cranking a 51J nearly a MHz is a pain. The
mechanical verniers didn't allow two speeds so making that 40 turns
instead of ten turns isn't a nice option. Of course, the M-2 and S-line
only cover 200 KHz per band segment so there aren't so many turns.

The other thing achieved with the direct drive to the PTO shaft in
Collins gear is a complete lack of backlash. That's hard to keep out of
a vernier, whether friction or geared.

The best vernier for such a project was probably one sold by HP for use
on the HP606 signal generator. I have no idea where to find one today.
Otherwise Jackson made some pretty good ones, and there might be some
available at Dan's Small Parts or Fair Radio or Surplus Sales of
Nebraska or some British (since Jackson was a British company) surplus
stores. It will be hard to mount without having the tuning knob stick
out and without carving up the escutcheon and spare engraved escutcheons
are not a common item anymore.

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