Vintage electric clock repair - coil issue.

[KalandraJane]

Hello all,

looking forward to a look round your forum and finding out how to fix some of my vintage items!

Wondering if anyone knows what I can do about a vintage mains operated clock which appears to have a blown coil (not sure of technical term - actuator? Field coil?) anyway, I have tested the rest of the wiring with a multimeter and it's all fine, but there is definitely no current passing through the copper coil. The wire on it is very fine, not something I am capable of re-wrapping.

The clock is a 1950s Metamec wall clock with a bakelite case for the workings, made in England, and the coil is rectangular. I suspect the damage may have occurred because the previous owner fitted it with a 13A fuse rather than something lower and it has caused a break somewhere in the coil.

I have some experience of working with tattoo machines which also run on coils, albeit cylindrical ones with much thicker wire. Do you think it would be possible to re-wrap the coil with thicker wire, or do you think the number of turns or the thickness/length of wire used will have an effect on the speed at which the mechanism works and make the clock run fast or slow?

I would appreciate any help

[Darren-UK]

Hi,

The correct term is Field Coil. Your coil's probably suffered 'green spot' corrosion; a gremlin which affects copper wire. The break is almost always deep within the windings but you might be lucky and find it's near the outside. Remove the outer insulation tape and undo a few turns of wire to find out. If the break is near the surface, you can reconnect it at that point and the loss of just a few turns won't do any harm. Such a fix might only be temporary though, depending on what mood the aforementioned gremlin's in.

Typically these coils were wound with 46 or 48SWG wire which is tricky to rewire without the proper equipment. You can rewire with a heavier gauge but you'd need to drop the voltage into the coil.

The rotational speed of the motor is dictated by the mains frequency, ie 50Hz and not the length or gauge of the wire. The latter two factors effect the coil's impedance, hence the need to drop the voltage if heavier gauge wire is used. That's the 'nutshell' description anyway.

The rating of the plug fuse won't be the cause of the failure; if green spot corrosion is present the coil will fail regardless. The plug fuse is there to protect the mains lead and, for synchronous clocks such as yours, a fuse of 1A rating is adequate (if you can't get one, then use a 3A). Remember that the original mains flex on these clocks was only intended for 5A circuits at the most and wall/mantle clocks often had their own special socket, or junction box, which often contained a 1A fuse.

If you don't fancy having a go at rewinding the coil, you can always ask in our Wanted section for another. Failing that, salvage a coil from another clock. Metamec's are common so finding another wouldn't be difficult.

Meantime, have a look and see if the break in the wire is near the surface and we'll go from there.

Hope that helps.

[Leon Crampin]

Rewind the coil with 30 SWG wire filling the bobbin. Power up the clock with a variac and note the voltage at which the motor will run when manually started. Provide this voltage + 30 to 50% from a wall-wart (AC) transformer.

This scheme also avoids the possible need to earth the clock.

Leon.

[John_BS]

You've got the best possible advice available in just two posts!

If you don't have access to a variac, use a 24v ac transformer with a selection of (>2W) dropper resistors to determine the manual start as described by Leon.

John

[M0FYA Andy]

I have recommended this site a couple of times previously in response to similar questions, it provides some useful ideas - http://sound.westhost.com/clocks/ocm.html
Andy