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Where to get small Electrical component?

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Martin King 201/11/2021 15:13:21
1018 forum posts
461 photos

Hi All,

I have a watchmakers lathe with a dicky motor which I think needs a new part as shown here.

motor part.jpg

I think that this is a capacitor but need to know what part number to search for and where are the best place to look online. Some letters missing I think?

Regards, Martin

mgnbuk01/11/2021 15:35:43
1211 forum posts
72 photos


A Google search for "KPB 7301" brings up what appears to be the manufacturer's website - the star symbol appears to be the same.

Nigel B.

DC31k01/11/2021 15:53:18
732 forum posts
2 photos
Posted by mgnbuk on 01/11/2021 15:35:43:


A Google search for "KPB 7301" brings up what appears to be the manufacturer's website...

...where it describes the item as a 'radio interference suppression capacitor'.

I am struggling to see how failure of this part could stop the motor working (unless it is shorted, in which case simple disconnection of it is worth trying).

Andrew Johnston01/11/2021 16:05:56
6679 forum posts
701 photos
Posted by DC31k on 01/11/2021 15:53:18

......(unless it is shorted.....).

One would hope not! The capacitor is marked X Y, ie, it should be designed so as to not to fail short circuit.


Nicholas Farr01/11/2021 16:30:59
3425 forum posts
1592 photos

Hi, and the B.S. number suggests it's nothing more than a radio interference suppressor, so shouldn't stop the motor from working properly anyway.

Regards Nick.

Dave Daniels01/11/2021 17:05:45
83 forum posts

Just what I know as a 'Delta Capacitor / Suppressor'.

Electrically about the same except the Y2s are 4700pF not 5000pF

Physically obviously different. which may be significant.




Edited By Dave Daniels on 01/11/2021 17:07:11

John Haine01/11/2021 17:50:55
4718 forum posts
273 photos

Had it failed short circuit, it would have been vapourised, partially at least! Try running with it disconnected, but almost certainly not the problem.

Do you know if the machine has a commutator or induction motor? Is it variable speed? The fact that they fit a suppressor suggests it's a commutator type. What sort of noise did it make when running? Could you give more details of the machine, photos etc, to help diagnosis?

Edited By John Haine on 01/11/2021 17:53:12

Martin King 201/11/2021 18:08:42
1018 forum posts
461 photos

Hi All,

I will post a couple of pics tomorrow but a better description may be of help here!

The motor comes with a foot pedal and what looks like very old wiring and connectors.

When I got it and plugged it in there was a very low almost subliminal hum from the foot pedal and motor did not run.

On removing the bottom cover of the pedal, there is a micro switch; a potentiometer driven by a spring loaded bit of string connected to the pedal, a small circuit board and this capacitor thingy!

There is 240V across one side of the micro switch and earth; 240V across other side of switch when pedal depressed;

Nothing across the terminals of the capacitor.

The motor itself has carbon brushes and a commutator in poor shape. I assume it is supposed to be variable speed else why the pedal and potentiometer?

Photos will certainly be of more help to explain tomorrow.

Cheers, Martin

Nicholas Farr01/11/2021 18:15:18
3425 forum posts
1592 photos

Hi Marin, a commutator in poor shape won't help and you could have some wires broken that connect to it.

Regards Nick.

Tim Stevens01/11/2021 18:29:55
1623 forum posts

If you need a replacement motor etc, you might look at a sewing machine system. Much more common than a watchmaker's lathe, but working to just the same principles.

Cheers, Tim

John Haine01/11/2021 19:23:34
4718 forum posts
273 photos

Unless there is an "authenticity" issue and given the commutator is in poor shape I suggest that everything from the mains plug to the motor pulley is ditched. Tim's suggestion is good, I, sure there was a recent posting on here that described using a modern sewing machine servo motor and controller to drive a small lathe.

Michael Gilligan01/11/2021 19:59:17
20289 forum posts
1064 photos

This may be the thread that you had in mind, John : **LINK**

… but there have been others.


John Haine01/11/2021 22:09:26
4718 forum posts
273 photos

Exactly! Well found Michael.

Martin King 202/11/2021 11:27:17
1018 forum posts
461 photos

Here are 2 pics of the pedal and commutator:

pedal 1.jpg

comm 1.jpg

John Haine02/11/2021 11:34:16
4718 forum posts
273 photos

Looks like it may be a home-made speed control board, probably thyristor or triac based. Would definitely need a suppressor then, but only for interference. Commutator looks very worn. If you have a low voltage power supply, say 12V, or even a car battery, the motor should run on it albeit slowly if it's OK.

Samsaranda02/11/2021 11:39:49
1485 forum posts
7 photos

Commutator does look a little worn! Probably best to go sewing machine motor route. Dave W

Harry Wilkes02/11/2021 11:41:57
1376 forum posts
66 photos

by the look at that com the motor as earned it's retirement


old mart02/11/2021 17:40:13
3913 forum posts
268 photos

Also, with a sewing machine motor, you can get the matching speed control pedal and modify it for other than foot operation.

Model Enginerd03/11/2021 01:27:11
19 forum posts
11 photos

Condensers are placed in parallel to commutator segments to dampen spikes that both generate EMI or could damage a controller upstream. I was taught in similar applications that if they go out, you might also get premature wear out of your carbon brushes, but some would differ on it. Looks like the brushes are gone and you were down to the metal, which might be why it's not running correctly.

I'd try getting a new motor like they suggested, or you could turn down that commutator and get some new brushes installed. I've installed brake pads on bad rotors and had them break in ok. hehe

Nicholas Farr03/11/2021 06:12:04
3425 forum posts
1592 photos

Hi, the commutator does look bad, but I've tidied up some worse looking ones and they have worked OK, but it would pay to check the continuity of all the coils and segments before wasting any time truing the them up and it will need new brushes fitted.

Regards Nick.

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