Slow Motion drives.

[G6Tanuki]

I'm looking to build a HF VFO, covering around 200KHz at 4.5MHz - I want a really low tuning-rate to make resolving SSB easier. 10KHz per revolution of the knob is the worst I will accept; 5KHz or 2.5KHz would be better.

There's no need for any sort of tuning-dial/pointer, the frequency display will be catered for by the digital counter in the transceiver the VFO will connect to, and there's also no need for the drive to be mechanically rigorous as to whether it returns the capacitor precisely to the same position following an excursion from one end of the tuning-range to the other and back again.. So I need a slow-motion drive with good 'feel' but not something big bulky and complicated with a dial/scale like an Eddystone 898.

The best I can find is a Jackson 36:1 ball-bearing drive, but from what I remember these were a wierd dual-speed thing with the low ratio only available over a single turn of the input spindle so you had to tune beyond the intended point then tune backwards to get the slow-motion effect.

Any suggestions? My other alternative to using a capacitor and reduction-drive is to use varicap tuning, a 15-turn potentiometer, and then putting a 6:1 Jackson-type drive on the front to give 90 turns end-to-end.

[CambridgeWorks]

If anyone has the drive from the Hatfield Instruments Psophometer they were superb. Or a scrap instrument. Around 3" diameter outer knob for fine and slightly smaller inner for coarse, both aluminium. Really smooth.
Rob

[David G4EBT]

Originally Posted by G6Tanuki

I'm looking to build a HF VFO, covering around 200KHz at 4.5MHz - I want a really low tuning-rate to make resolving SSB easier. 10KHz per revolution of the knob is the worst I will accept; 5KHz or 2.5KHz would be better.

There's no need for any sort of tuning-dial/pointer, the frequency display will be catered for by the digital counter in the transceiver the VFO will connect to, and there's also no need for the drive to be mechanically rigorous as to whether it returns the capacitor precisely to the same position following an excursion from one end of the tuning-range to the other and back again.. So I need a slow-motion drive with good 'feel' but not something big bulky and complicated with a dial/scale like an Eddystone 898.

The best I can find is a Jackson 36:1 ball-bearing drive, but from what I remember these were a weird dual-speed thing with the low ratio only available over a single turn of the input spindle so you had to tune beyond the intended point then tune backwards to get the slow-motion effect.

Any suggestions? My other alternative to using a capacitor and reduction-drive is to use varicap tuning, a 15-turn potentiometer, and then putting a 6:1 Jackson-type drive on the front to give 90 turns end-to-end.

The other option if the have the space, would be to use two of those small 6:1 epicyclic drives in series using a shaft coupler, which would give you a 36:1 ratio without the 2-speed backlash drive complication.

Good luck in sorting something.

[Radio Wrangler]

Drive and capacitor from a scrap BC221?

100:1, all ball bearing and instrument grade. Almost sacrilege but there are plenty around and though some are rough, the dial drive and capacitor are very well protected.

David

[ms660]

If you can find one and rig it up, R1155 original tuning mech. = 1:1 or 100:1 (0 to 180degrees)

Lawrence.

[G6Tanuki]

Thanks for the suggestions. The idea of series-connecting a pair of 6:1 drives is intriguing. The likes of the BC221 drive are a bit physically large and encumbered by the tuning-scale [is that removable?] - the drive needs to fit on the end of a diecast box that's about 4 inches by 5 inches...

I still like the idea of the 6:1 drive ahead of the 15-turn pot; it's neat packaging and would give a good reduction. Have mocked it up using a spare 4511 drive and a 10-turn pot, which will be good enough for experimentation, currently connected to a 2N3819 series-tuned Colpitts oscillator with a big 50A stud-mounted power diode as the varicap.

[G6Tanuki]

Here's my first mockup of the varicap-tuned drive; it uses a 6:1 Jackson and a 10-turn 100K pot, shown on top of the diecast box that will be the home for the resultant VFO. [yes, I _do_ have a bit of a thing for dabbing strangely-coloured nailvarnish on screws/nuts as a threadlock].

If I do choose this as the final approach I'll be using another drive [without the flange-with-tapped-holes for attaching a dial] and with shorter standoff-pillars. And a 15-turn pot for finer resolution. I also will be including some lumps of heavy flashing-lead inside the box, which will serve both to weight it down and add thermal-mass to the enclosure, which will help reduce drift. If that's not enough to make it short-term-stable I can always do the "Huff and Puff" crystal-timing-derived sampling-gate thing as described by Klaas Spaargaren PA0KSB [now sadly silent-key, see here: http://www.hanssummers.com/huffpuff.html for a few modern versions] in the 80s, I've got all the parts necessary and there's plenty of room in the box.

[kellys_eye]

A largish diameter rubber (edged?) wheel with a simple 'edge touching' shaft will give a huge reduction - especially if the driving shaft is mm in diameter compared to a couple of centimetre diameter driven wheel.

[Chrispy57]

Two separate tuning capacitors in parallel, a kHz capacitor and a MHz capacitor, both fitted with highly geared drive ratios to suit your requirements - inspired by the Racal RA17 arrangement perhaps?

[m0cemdave]

It's possible to modify the 36:1 drive to remove the 6:1 coarse ratio completely, just break off the "pip" that constrains the movement.

[G6Tanuki]

Originally Posted by m0cemdave

It's possible to modify the 36:1 drive to remove the 6:1 coarse ratio completely, just break off the "pip" that constrains the movement.

I hadn't noted that. It might be an appropriate second-line solution if my 90-turn-end-to-end DC-tuned VFO doesn't work out.

The idea of separate coarse and fine tuning-capacitors, while interesting, would mean nasty compromises and packaging problems given the available space on the front of the case I intend to use.

[Andrew Sinclair]

I made a varicap tuned VFO with a 10 turn pot. It tuned over a 500kHz range. The resolution of the potentiometer only gave a step size of about 100Hz. It tuned like a Japanese early eighties synthesised radio! The resolution is limited by the size of wire in the pot.
Andrew

[stuarth]

Won’t 90 turns end to end with no flywheel action be a bit laborious moving quickly across the band?

Stuart

[Craig Sawyers]

Originally Posted by Radio Wrangler

Drive and capacitor from a scrap BC221?

100:1, all ball bearing and instrument grade. Almost sacrilege but there are plenty around and though some are rough, the dial drive and capacitor are very well protected.

David

There are some insanely priced BC221's on eBay at the moment (hundreds of quid) and not surprisingly just go round and round. The only one that sold recently was fifteen quid.

The most I have paid is £50, but that was for this unit https://www.royalsignals.org.uk/photos/bc221ak.htm with canvas cover, original manuals, correct and head clamping headphones and pukka power supply. A real rarity.

The other two were free, but working 100%.

So why have I got three? Well if someone offers you a free one, what are you going to do?

Craig

[G6Tanuki]

BC221-type tuning drive is much too big!

The backstory on this is that I find the standard PTO tuning on my Ten-Tec Corsair to be too coarse for easy SSB tuning. It also requires you to hold the tuning-knob between your thumb and forefinger and holdm your arm up along the length of nyour forearm, which gets painful [I have Carpal Tunnel and Arthritis].

A friend has built a control-box for his SDR; the tuning knob drives a rotary-encoder on the right *side* of the case, fitted with a knurled metal knob anout 1.2 inches diameter. There's a felt washer between the knob and the case to add some 'drag' otherwise the thing was very fiddly despite only doing 3KHz per rotation. The knob is mounted with its centre about 1.8 inches above the base/table-on-which-it-sits.

You tune it just with one finger or thumb, pushing forward and back on the upper circumference in the same way as you use a mouse scrollwheel. Your wrist and hand rest on the table. I've played around a bit with this and it's really comfortable in operation. So I want to try recreating it. Of course I have to do it all in analog rather than digital...

[MotorBikeLes]

A suggestion from me. Two 6:1 drives in series, but with a brake somewhere. (I have not thought it out yet) so that you can switch between 6:1 and 36:1 in an instant. Rough tune on 6:1 then switch over and fine tune.
I have other things to think about, so over to somebody else,
Les.

[G6Tanuki]

A 36:1 reduction - if used to drive a normal capacitor that only has 180 degrees of functional rotation - would only give 18 turns from end to end.

Which when the oscillator is aiming to cover somewhere around 300KHz gives a tuning-rate in terms of KHz-per-turn-of-the-knob that's going to make SSB operation demand the skills of a safecracker to the same extent as the present setup.

4 or 5KHz per turn of the knob seems about right to me. Yes it will mean that going from one end of the band to the other is a bit tiresome but the CW end of 80M is largely unknown territory to me, and even on 28MHz the real working is the 200KHz between 28.3 and 28.5 so a 200KHz sweep [which I can slowly tune across over a period of minutes] sounds good.

[RichardGM]

What is the objection to a second, much smaller, variable capacitor?

[G6Tanuki]

Originally Posted by RichardGM

What is the objection to a second, much smaller, variable capacitor?

See my comments upthread about the ergonomic considerations I am trying to address.

Two separate capacitors would be rather hard to operate with just one thumb connected to an arthritic hand. A multi-turn thingy like a mouse scrollwheel is easy, because you don't need strength in your thumb to grasp it. You just roll it.

[Cruisin Marine]

Two separate capacitors would be rather hard to operate with just one thumb connected to an arthritic hand. A multi-turn thingy like a mouse scrollwheel is easy, because you don't need strength in your thumb to grasp it. You just roll it.

Perhaps a slo mo drive and an added edge wise thumbwheel connected to a varicap may give the answer?

Oh, and totally cut out Margarine if you have Arthritis, I developed it and stopped Margarine- totally gone now.