Downham Potentiometer

Hello I have a Downham jig borer with variable drive which has run well for the the last 20 years. But now the variable drive is playing up the smallest movement on the variable drive pot with make go crazy it will just about hold a single constant speed but takes a second or two to fire up the spindle

Unable to source an exact replacement pot I got the closest I could find of the same make and same 1K0K value but it is much bigger item now I am hopeful because the new pot gives a steady constant speed and fires up the spindle quickly but does not change the spindle speed at when turned.

So I think I need an equivalent potentiometer but I am well out of my comfort zone so could someone help me source a better pot please here some pictures

Original failing pot

replacement pot

hi, to test the pot, if you have access to a resistance meter, dvm, dmm, just clip the leads onto the centre and one end contacts and see if the resistance changes from zero to 1000 ohms as you turn the shaft. you can take the rear cap off these old pots they are glued on, the resistance is often wire wound so can be cleaned. if not just buy a pot from maplins for £ 1.00 or so, should work. The colvern brand was quality British gear, I have a similar pot in an old audio signal generator of mine which went noisy the other month, a quick clean with ipa and it works well.

These pots are linear wirewound and are rated at 3W, if you squirt some switch cleaner or WD 40 into the works you might cajole it back into life. As its passed its prime you might want to lever off the back cover to allow a better inspection. Its only a dust cover. I have found one likely candidate, though its a trimmer with a short shaft - 1/4"? what length shaft would you require?

The other thing is to measure the voltage from end to end. Then you can work out the power and see is a lower wattage pot could be fitted.

Frank

If you are absolutely sure you wired it up the same way it is just possible one of them is not the standard configuration of the centre connection being the wiper. Do you have a multimeter with resistance range that you can use to check?

Other possibilities are a short circuit of one of the wires against the case or a dry joint.

I've seen old pots that do not have the wiper as the central terminal. I would do as Bazyle suggests and find out which terminal on each pot is the wiper and make sure the cable from the original wiper terminal is on the new pot's wiper terminal.

Martin

I'm pretty sure the wiper is the red from your pic. If the three terminals are not equidistant or one is at a higher point on the body the odd one out is usually the wiper. As already suggested disconnect and check with a meter.

regards Martin

On most pots with the connections to the edge, the two outer connections are/were to the ends of the pot track ie max resistance and as previous, centre connection wiper. Should be obvious with cover flipped.

Regards Ian.

I agree with you Circlip when the tags are equally spaced. On other layouts there is an obvious 'pair' and an odd man out which is the wiper. The pic looks a bit unequal to me but it's difficult to tell from that angle. As the speed does not vary I would say that the wiper is connected to the wrong tag, but we will soon know when he checks with a meter.

regards Martin

Hi Stephen

normally you will find the centre terminal is the wiper that is the variable contact and the outer 2 terminals the ends of the resistive track the only time I have found a different arrangement was on surplus militry gear !

you may be able to find the potentiometer on line but the exact Colvern potentiometer could be expensive

some thing like this for example from RS components - http://uk.rs-online.com/web/p/potentiometers/5028609/

at £30 + VAT !!!!

as a more realistic replacement could be a

http://docs-europe.electrocomponents.com/webdocs/1449/0900766b81449ba7.pdf wirewound potentiometer at £5.50

or http://uk.rs-online.com/web/p/potentiometers/5225276/

1K cermet potentiometer at £4.24

a 1K linear potentiometer with a carbon film track from maplin can enable you use the machine short term

but its not going to last 30 odd years like the Colvern wire wound original

John

 PS - 

http://docs-europe.electrocomponents.com/webdocs/1449/0900766b81449ba7.pdf

has a diagram showing the mechanical details including the terminals

Edited By john swift 1 on 26/07/2016 15:55:41

I know very little about the workings of these potentiometer's, but having worked in the toolroom of Colverns in the 60's I can confirm that they are very well made, they needed to be as a lot of the production went to Military equipment.

Thanks guys, brilliant suggestions the wiper on both is definitely the centre managed to file the original and break of part of the dust cap to spray in some detox but did not help looking at it the copper seems worn away from wiper blade so I guess it is goner. Not sure why the replacement does not work but I will buy the recommended pots from RS although the price was not £30 but £5 thanks John just what I needed 

Edited By Stephen Benson on 26/07/2016 16:46:41

Edited By Stephen Benson on 26/07/2016 16:50:17

Before you lash out and place an order ,I have a used bet in working order 1 K ohm you can have for the postage, wire wound slightly smaller diameter and you will have to extend the shaft is little. not a big job for some one with a lathe. Send me a PM with your address. John. PS.For any vintage component you may like to join Vintage Radio where you will find some very knowledgeable persons

Thanks John for your very kind offer but I had already ordered from RS hopefully this will sort it thanks again

Right I thought I owned you very helpful people an update well I ordered and tried the recommended pots and got the same result even tried swapping wires around even though I was certain I had wired them correctly so my problem is circuit board related I would guess my chances of fault finding / fixing a Thyristor PCB as non existent.

So spoke to George at Newton Tesla Electric Drives and he was very helpful as always so I think that a three phase variable drive is the way forward because at the moment my mill has two speeds 1000rpm and 4000rpm OK if you are milling with 12mm-16mm end mill or drilling with a 0.5mm carbide drill but a bit annoying if you want to ream or bore I have a very nice Cowells mill so all is not lost and a Cowells lathe with a Newton Tesla drive as well as my South Bend with could do with a Newton Tesla drive.

Hi Stephen,

If you give some idea or which area of the UK you are in it is just possible there may be a member of the forum with enough knowledge that is near enough to help you fix the problem. I can't find any information on the speed controller used so I have no idea how complex the controller board might be.

Les.