Pseudo-multiturn potentiometers - UK Vintage Radio Repair and Restoration Discussion Forum

TonyDuell · 31st Jan 2018, 7:31 pm

A bit of (off-topic?) background....

I am restoring an EAI1000 analogue computer at the moment. The machine I have has 3 analogue units (along with the standard control and display modules). Each analogue unit has a removeable patch panel that you wire up to connect the summers, integrators, etc. Each analogue unit also has 12 multi-turn potentiometers. 10 have one end grounded (top end and wiper on sockets on the patch panel), these are on a panel screwed to the chassis. The other 2 have all 3 terminals on patch panel sockets, and are mounted on the patch panel.

The first type are genuine multi-turn potentiometers, I assume with a helical track inside.

But the latter are something I've never seen before. They are actually normal potentiometers with rather less than one turn of wiper rotation. On the back is a ball-bearing type reduction drive between the spindle and the wiper, thus giving around 10 turns of the spindle to move the wiper from one end to the other. Has anyone ever come across those. One of mine has a broken spindle, I will try to take it apart and repair it, but if not I would need to find a replacement.

G6Tanuki · 31st Jan 2018, 7:36 pm

Photos?

Must admit, in the past I've fitted a Jackson 6:1 reduction-drive on the front of a 10-turn Helipot to do the DC-voltage varicap-tuning on a FM-band tuner. Just don't wind it hard down to either end . . .

TonyDuell · 31st Jan 2018, 7:50 pm

Here's a picture of part of the component side of the patch panel showing the potentiometer. The object to the right is a normal 1N4148 diode which will give n idea of the scale.

Notice the 3 ball bearings round the spindle, those are the reduction drive.

AC/HL · 31st Jan 2018, 9:36 pm

That system was commonly used with capacitors for slow motion tuning in the early days. I've never seen this application though, the bearings were enclosed inside the drive.

Alistair D · 31st Jan 2018, 9:36 pm

I'm pretty sure the Heathkit IM28 VTVM I have in the loft uses those pots for the zero and ohms adjusters. Will have a look tomorrow.

The parts won't be for sale but it may give you an avenue to investigate.

Al

wd40addict · 31st Jan 2018, 10:58 pm

My Heathkit IM-13u certainly uses 2

buggies · 31st Jan 2018, 11:06 pm

Quote:

Originally Posted by TonyDuell

But the latter are something I've never seen before. They are actually normal potentiometers with rather less than one turn of wiper rotation. On the back is a ball-bearing type reduction drive between the spindle and the wiper, thus giving around 10 turns of the spindle to move the wiper from one end to the other. Has anyone ever come across those. One of mine has a broken spindle, I will try to take it apart and repair it, but if not I would need to find a replacement.

Can't remember where this one came from - it was made by AB Metals and measures 6k although marked 5k. The Solartron Analogue computers used all proper 10 turn pots as far as I remember.

Radio Wrangler · 31st Jan 2018, 11:14 pm

Allen Bradley.... I remember them used in some old HP instruments, but can't remember just which.

David

joebog1 · 1st Feb 2018, 12:29 am

They are also used in "White" brand metal detectors. They used to be obtainable on eBay..

In the metal detectors they were notoriously noisy.

Joe.

TonyDuell · 1st Feb 2018, 5:50 am

Thanks for the replies. I've seen this sort of drive used on variable capacitors, either built into the capacitor or as a separate unit fitted to the spindle, but not on a potentiometer before.

The service manual on the web for an earlier version of the machine says they are 10k, I will have to check the manual and boards in my machine to check as I have found differences between the 2 versions.

As I said, I intend to dismantle the faulty one and see if I can repair it either by adding a length to the spindle or making a new spindle. If not, it is not that important to fix it, as I have several spare patch panels, but as we all know it is irritating to have something that is not right.

60 oldjohn · 1st Feb 2018, 9:13 pm

Hi Tony, I have these 1meg Lin, You may be able to use the 1 inch shaft.

John.

Radio Wrangler · 1st Feb 2018, 11:59 pm

As an aside, someone at South Queensferry christened them "Pawnbroker Pots"

David

TonyDuell · 5th Feb 2018, 6:40 pm

I feel like a total fool!

Today I thought I'd remove the old potentiometer and see if it could be dismantled. I also decided to remove the good one so I could compare the spindles. So the first job was to remove the knob from the good pot.

This I did, to reveal that both pots had very short (about 5 or 6mm above the mounting bush) spindles. In other words the potentiometer is undamaged, the spindle is not broken. All I need to get is a suitable collet-fixing knob. Annoyingly RS sell an identical knob and nut cover skirt, but only black caps (the original is red).

So thanks for the offers, but I don't need a replacement potentiometer (yet!)

60 oldjohn · 5th Feb 2018, 6:54 pm

Hi Tony can you post a picture of knob and I will have a look, no promises.

John.

TonyDuell · 5th Feb 2018, 7:33 pm

This knob from RS looks identical, even down to the design of the clamping nut. Note that there is no indicator line on it :

https://uk.rs-online.com/web/p/poten...knobs/4638457/

This is the nut cover skirt (but ignore the desciption as it seems to apply to something totally different)

https://uk.rs-online.com/web/p/poten...knobs/4638564/

Those two parts look identical to the other knobs on the machine. I might replace both knobs on the patch panel just in case there are differences, but otherwise they would be fine.

This is the cap for the top of the knob :

https://uk.rs-online.com/web/p/poten...knobs/4638536/

RS only do them in black, the ones on the machine are all red (both on the patch panels and the fixed potentiometer panels).

The other knobs that RS sell are not as good a match.

60 oldjohn · 5th Feb 2018, 7:42 pm

Hi Tony, I will not have the part in the middle link I may have a knob and cap. I will have a look tomorrow.

John.

TonyDuell · 5th Feb 2018, 7:54 pm

Thanks for the offer, but I would need the nut cover so as to match the other knobs on the machine and my experience is that you can't mix and match parts from different manufactuers. Replacing all the knobs is not an option, there are over 50 of them on the entire machine and the extra patch panels I have.

So unless I can find a supplier of the exact knob I want, I think I will just get the RS one and either put a black cap on it or try to modify some other red cap to fit.

dseymo1 · 5th Feb 2018, 8:27 pm

Would a Sifam cap fit?
They come in a very wide range of colours, including red (naturally).

Edit: I've used glued-on coloured counters in the past to substitute for unobtainable knob caps.

TonyDuell · 25th Feb 2018, 4:19 pm

Just to complete this : The RS knob and nut cover I mentioned are visually indentical to the original knob on the EAI1000 patch panel. Although I bought 2 (just in case), I only fitted one. RS also do the Sifam caps in red, with a little gentle filing they will fit the knob. I replaced both caps as the Sifam ones have a matt finish, not the gloss of the original (no, I don't intend to replace all the other knob caps on the machine, there are several styles of patch panel anyway, so having one with slightly different knob caps doesn't look too bad).

I am happy with that part of the restoration.

31st Jan 2018, 7:31 pm	#1
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Pseudo-multiturn potentiometers A bit of (off-topic?) background.... I am restoring an EAI1000 analogue computer at the moment. The machine I have has 3 analogue units (along with the standard control and display modules). Each analogue unit has a removeable patch panel that you wire up to connect the summers, integrators, etc. Each analogue unit also has 12 multi-turn potentiometers. 10 have one end grounded (top end and wiper on sockets on the patch panel), these are on a panel screwed to the chassis. The other 2 have all 3 terminals on patch panel sockets, and are mounted on the patch panel. The first type are genuine multi-turn potentiometers, I assume with a helical track inside. But the latter are something I've never seen before. They are actually normal potentiometers with rather less than one turn of wiper rotation. On the back is a ball-bearing type reduction drive between the spindle and the wiper, thus giving around 10 turns of the spindle to move the wiper from one end to the other. Has anyone ever come across those. One of mine has a broken spindle, I will try to take it apart and repair it, but if not I would need to find a replacement.

31st Jan 2018, 7:36 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: pseudo-multiturn potentiometers Photos? Must admit, in the past I've fitted a Jackson 6:1 reduction-drive on the front of a 10-turn Helipot to do the DC-voltage varicap-tuning on a FM-band tuner. Just don't wind it hard down to either end . . .

31st Jan 2018, 7:50 pm	#3
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: pseudo-multiturn potentiometers Here's a picture of part of the component side of the patch panel showing the potentiometer. The object to the right is a normal 1N4148 diode which will give n idea of the scale. Notice the 3 ball bearings round the spindle, those are the reduction drive. Attached Thumbnails

31st Jan 2018, 9:36 pm	#4
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: pseudo-multiturn potentiometers That system was commonly used with capacitors for slow motion tuning in the early days. I've never seen this application though, the bearings were enclosed inside the drive.

31st Jan 2018, 9:36 pm	#5
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: pseudo-multiturn potentiometers I'm pretty sure the Heathkit IM28 VTVM I have in the loft uses those pots for the zero and ohms adjusters. Will have a look tomorrow. The parts won't be for sale but it may give you an avenue to investigate. Al __________________ I won't tell you how I discovered that.

31st Jan 2018, 10:58 pm	#6
wd40addict Octode Join Date: May 2009 Location: Stevenage, Herts. UK. Posts: 1,518	Re: Pseudo-multiturn potentiometers My Heathkit IM-13u certainly uses 2

31st Jan 2018, 11:14 pm	#8
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,896	Re: Pseudo-multiturn potentiometers Allen Bradley.... I remember them used in some old HP instruments, but can't remember just which. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

1st Feb 2018, 12:29 am	#9
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Pseudo-multiturn potentiometers They are also used in "White" brand metal detectors. They used to be obtainable on eBay.. In the metal detectors they were notoriously noisy. Joe.

1st Feb 2018, 5:50 am	#10
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Pseudo-multiturn potentiometers Thanks for the replies. I've seen this sort of drive used on variable capacitors, either built into the capacitor or as a separate unit fitted to the spindle, but not on a potentiometer before. The service manual on the web for an earlier version of the machine says they are 10k, I will have to check the manual and boards in my machine to check as I have found differences between the 2 versions. As I said, I intend to dismantle the faulty one and see if I can repair it either by adding a length to the spindle or making a new spindle. If not, it is not that important to fix it, as I have several spare patch panels, but as we all know it is irritating to have something that is not right.

1st Feb 2018, 9:13 pm	#11
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,988	Re: Pseudo-multiturn potentiometers Hi Tony, I have these 1meg Lin, You may be able to use the 1 inch shaft. John. Attached Thumbnails

1st Feb 2018, 11:59 pm	#12
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,896	Re: Pseudo-multiturn potentiometers As an aside, someone at South Queensferry christened them "Pawnbroker Pots" David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Feb 2018, 6:40 pm	#13
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Pseudo-multiturn potentiometers I feel like a total fool! Today I thought I'd remove the old potentiometer and see if it could be dismantled. I also decided to remove the good one so I could compare the spindles. So the first job was to remove the knob from the good pot. This I did, to reveal that both pots had very short (about 5 or 6mm above the mounting bush) spindles. In other words the potentiometer is undamaged, the spindle is not broken. All I need to get is a suitable collet-fixing knob. Annoyingly RS sell an identical knob and nut cover skirt, but only black caps (the original is red). So thanks for the offers, but I don't need a replacement potentiometer (yet!)

5th Feb 2018, 6:54 pm	#14
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,988	Re: Pseudo-multiturn potentiometers Hi Tony can you post a picture of knob and I will have a look, no promises. John.

5th Feb 2018, 7:33 pm	#15
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Pseudo-multiturn potentiometers This knob from RS looks identical, even down to the design of the clamping nut. Note that there is no indicator line on it : https://uk.rs-online.com/web/p/poten...knobs/4638457/ This is the nut cover skirt (but ignore the desciption as it seems to apply to something totally different) https://uk.rs-online.com/web/p/poten...knobs/4638564/ Those two parts look identical to the other knobs on the machine. I might replace both knobs on the patch panel just in case there are differences, but otherwise they would be fine. This is the cap for the top of the knob : https://uk.rs-online.com/web/p/poten...knobs/4638536/ RS only do them in black, the ones on the machine are all red (both on the patch panels and the fixed potentiometer panels). The other knobs that RS sell are not as good a match.

5th Feb 2018, 7:42 pm	#16
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,988	Re: Pseudo-multiturn potentiometers Hi Tony, I will not have the part in the middle link I may have a knob and cap. I will have a look tomorrow. John.

5th Feb 2018, 7:54 pm	#17
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Pseudo-multiturn potentiometers Thanks for the offer, but I would need the nut cover so as to match the other knobs on the machine and my experience is that you can't mix and match parts from different manufactuers. Replacing all the knobs is not an option, there are over 50 of them on the entire machine and the extra patch panels I have. So unless I can find a supplier of the exact knob I want, I think I will just get the RS one and either put a black cap on it or try to modify some other red cap to fit.

5th Feb 2018, 8:27 pm	#18
dseymo1 Retired Dormant Member Join Date: Dec 2007 Location: Shropshire, UK. Posts: 3,051	Re: Pseudo-multiturn potentiometers Would a Sifam cap fit? They come in a very wide range of colours, including red (naturally). Edit: I've used glued-on coloured counters in the past to substitute for unobtainable knob caps. Last edited by dseymo1; 5th Feb 2018 at 8:32 pm.

25th Feb 2018, 4:19 pm	#19
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,213	Re: Pseudo-multiturn potentiometers Just to complete this : The RS knob and nut cover I mentioned are visually indentical to the original knob on the EAI1000 patch panel. Although I bought 2 (just in case), I only fitted one. RS also do the Sifam caps in red, with a little gentle filing they will fit the knob. I replaced both caps as the Sifam ones have a matt finish, not the gloss of the original (no, I don't intend to replace all the other knob caps on the machine, there are several styles of patch panel anyway, so having one with slightly different knob caps doesn't look too bad). I am happy with that part of the restoration.