I'll start a thread on this topic as it is a separate project and will go in fits and starts.

I have done some preliminary work on a prototype and seen enough to procede from a mock up to a working unit.
This is an auxilliary unit to plug into the valve RF signal generator to enable frequency sweeping of radio sections such as the IF strip.
As the project proceeds I'll feed the usual PDF's to Brad.
Working on Part 1 now.

I have never actually seen a Mechanical wobbulator in any book or magazine as a piece of test equipment.
I imagine that the technology even in the 1930's would have been to use a reactance valve as the principles of this were well described in text books even then.
I used a 6AC7 as a reactance valve augmented with a big fat diode in my valve based wobbulator.

Has anybody ever seen, (i should call it a frequency sweeper) a wobbulator with a motor driven reactance such as a spinning tuning gang?
any comments welcome.

Fred

Mechanical Wobbulator - Part 1
Mechanical Wobbulator - Part 2
Mechanical Wobbulator - Part 3

Hey Fred,

I had one of these in another life but foolishly gave it away. I hope this link helps you.

https://youtu.be/bKv95nRTWcY

Cheers.

Thanks BBTV.
Wow, that is a real work of art.
I see they used a flat twin gang just like the one I cannot find!

I'm persisting with the vertical twin gang at the moment and should be able to balance it well enough to procede.
Part one coming up shortly.

Fred.

Document uploaded.

A commercial mechanical sweeper turned up at our last radio club meeting: Have photo.

I advanced the project to a rough working build and will stop at this point for a while and swap over to the 6Y9 superhet project.
The wobbulator needs a bit more refining working in conjunction with the RF signal source.
I'll send a part 2 to Brad to pin next to part 1 at the top of the topic.

Fred.

Document 2 uploaded.

I picked the project up again and got the spinning gang running and driving the RF generator.
After a lot of scratching about I have to conclude the gang, I am using is just wrong for the application, as it is.
In part 3 I documented how I went wrong and a possible way forward.
Any technical suggestions welcome.
I'll send a part 3 to Brad to pin to the front of the topic.
Fred.

Document 3 uploaded.

Straight line frequency tuning capacitors did exist, I used to have one in my cabinet of curiosities.

Picture sent to Brad for inclusion, showing the distinctive shape of both sets of plates.

Harold

Photo uploaded to Post 10.

That is one hell of a 'condenser' Harold. Task specific! Brilliant piece in any vintage electronics collection, thanks for sharing.

Fred. I just had a crazy vision--- a motor is driving a rheostat which is biasing a varicap diode??

BBV, varicap? not crazy!

When you think of it there are ways of producing a varying capacitance.
In a previous wobbulator I used a Puckle sawtooth oscillator to swing a large power rectifier across the tuning coil.
That is a BIG varicap!
A power rectifier has a large internal capacitance that measures 100pf or more.
That produced a sweep up to 50Kc wide and the linearity was controlled by the shape of the oscillator voltage ramp.
I like using parts to do something that has little to do with the original use!

Harold's condenser plate shape is exactly what I would have to do to my rotor plates to straighten the frequency change.
I would have to re-shape the concentric circular rotor plates to a snail shape.
When you look at a normal tuning gang in a shortwave valve set that's exactly how the plates are shaped.

I'll make up the eccentric pulley idea and see if it works.

Fred.

I would wonder at something like the "Rotary Butterfly caps" that you see in some comms receiver's, may be the go. They come in a variety of sizes. Albeit for constant rotation there may be a need to reconsider its bearings.

Marc

Hi Marc, yes, you are spot on, I have/had one of those and was my first choice.
Being a butterfly, a "flat twin", it is inherently in balance.
It has bronze bush bearings and is the ducks' guts to spin slowly.
I simply cannot find it, yes, it's in a box somewhere!!
I'm not sure it is a linear response however, as the plates are straight sided on both rotor and stator.
So, I would have the same sweep linearity problem.
I am just playing around so will press on with the "vertical twin" and counterbalance against the wobble and eccentric.
It is just engineering in miniature.

Cheers, Fred.