[Collins] kwm-2a dial speed

[Jim Brannigan]

I agree with all of Jerry's post;

Long ago, I attached a Jackson Bros vernier drive to a 75A3.
This required a bracket bent at appropriate angles.  The anchor point was 
the CAL knob located below the tuning knob.
It stuck out from the front panel and suffered from some slippage and 
backlash, but it did the job.
I suppose a similar arraingement could be for the KWM-2.  The anchor point 
could be the RF GAIN.
This would not be pretty, but functional and fully reversable.

Jim


> 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