Super C3 mini lathe- Fix For Speed Control Problem

Just in case anyone gets a similar problem, this might save them some time.
I have a Super C3 mini lathes from ARC Euro Trade.
http://www.arceurotrade.co.uk/Catalogue/Machines-Accessories/Lathes/Model-C3-Mini-Lathe It developed the following motor speed control problem a few weeks after purchase.
When the speed control knob was set to zero the chuck continued to turn when it should have been stationary. The speed of rotation was sometimes very erratic, particularly when set near or at zero. I tried a replacement potentiometer (pot) circuit board from ARC but unfortunately it was worse: the motor would only run at full speed independent of the speed knob setting!
Eventually I traced the problem to a fault in the speed control pot itself. A simple solution would be to unsolder and replace the pot with another 10k (10,000 Ohms) pot. I chose one with a linear (Lin) taper and this seemed to match the response of the original as far as my memory served.
Unfortunately I was disappointed to find my nice new (and surprisingly expensive) pot didn't have any 'Kilopoise' grease on it that makes for a slow smooth quality feel with controls, and I didn't much fancy trying to add the grease myself in case it leaked inside and caused fresh electrical contact problems. Apart from the aesthetics this would mean a light brush against the control with my elbow would be enough to send the motor flying up to full speed - annoying and possibly hazardous. In addition, my new replacement pot ideally needed a smaller through panel hole and the pip intended to stop it's body rotating when you turn the spindle didn't match with the hole in the panel. All a bit annoying considering I'd just spent about 5 on the thing!
Instead I decided to investigate the original pot further on the off chance it was easy to fix as then everything would fit back together nicely. I was lucky. It turned out that the problem with both my original pot and the faulty replacement were due to a loss of electrical contact between one of the connecting pins and a printed electrical track that led into its metal can. The three electrical contacts appear to have been riveted onto a piece of red board that protrudes from the metal can/body of the pot and these connect to three printed tracks that disappear across the red board into the metal can.
Using a multimeter I found that one riveted connecting pin had a poor contact with its printed track and quickly proved this was the source of the trouble.
I solved it by painting a small amount of silver loaded conductive paint across the join between the riveted pin and its associated printed track. For good measure I did the same on the other two pins/tracks. Despite the small size of the area available for applying the paint I found the two outside pins easy to deal with. The one in the middle was far more cramped and my paint job messier, but I checked afterwards with a jewelers loupe and I hadn't unintentionally bridged any conductors, so all was well. I used a sharpened match to apply drops of conductive paint rather than an extremely tiny brush, which I didn't have.
The good news is I've had no problems since :-)
Scrim
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I cured that problem on a model railway conroller using a wave spring washer between case and knob to give extra friction eg see: http://atopfasteners.tradeindia.com/Exporters_Suppliers/Exporter28910.593336/Wave-Spring-Washer.html
Thanks for the info re the lathe's controller - I have an earlier version of the mini lathe bought in 1998 and still going strong - despite the plastic gears it has withstood all my efforts to destroy them by jamming up the machine - the electronic control just blows a fuse and saves the gears - perhaps I've been lucky - I keep a pack of spare fuses from Maplin next to the machine.
Alan
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I used to do the same till I discovered that RS sell a 2 amp push button circuit breaker that fits in the same hole. Now I just have to press a button to reset it, it's quite surprising how often it goes and seems to be a common feature having spoken to other owners at shows etc.
Graham
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I wish I'd thought of that yesterday when I sent the order to RS.
On Thursday I let the magic smoke out of the Variac on the carriage drive of the Hardinge HLV that I'm rebuilding :-(
I've ordered a step-down transformer and a new Variac, but hadn't thought of replacing the fuse with a circuit breaker. The 5A fuse was protected by the 0.8A Variac. The maximum Variac output voltage was also more than the motor was rated for..
Mark Rand RTFM
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I would have thought soldering would be a more robust fix for this problem.
Regards, Tony
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Soldering is generally preferable, I agree, but in this case you'd be soldering from a pot terminal rivet to a printed track leading into the pot that appears to have been literally printed using some sort of conductive ink. If it is ink, I don't think you'll be able to solder to it.
Scrim
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