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Old 21st Nov 2019, 3:54 pm   #1
Julesomega
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Default Synchronous clock not running

Our kitchen quartz clock stopped running recently, and about the same time I dug out an old Gents synchronous mains clock I had been given which is so handsome and legible that we decided it would nice for the kitchen. ISTR it was donated because it kept stopping, but when I hung it on the wall it worked nicely, but after a few weeks it has stopped and cannot be started (it is not self-starting and has a knob which flicks the motor in the right direction). It may have come from a school and I guess dates from the 30s or 40s.

What would stop such a movement from running? No sign of polymerised grease or mechanical sticktion if you spin the rotor by hand (finger). Should the rotor be magnetised? I can detect no remnant magnetism sufficient to attract a small sewing needle.

The coil measures ~6k ohm at DC and I suspect that inductance would dominate over resistance at 50Hz. Besides there is no buzzing from the core.
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Old 21st Nov 2019, 5:14 pm   #2
kalee20
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Default Re: Synchronous clock not running

I'd expect inductance would be significant. 6kΩ of DC resistance at 240V would imply 40mA. I'd be expecting nearer 10mA.

No buzzing or humming though - what happens if you hold a steel screwdriver next to it?

It is possible that the rotor is not magnetised. It could be a variable-reluctance motor, where a toothed rotor.
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Old 21st Nov 2019, 5:33 pm   #3
PJL
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Default Re: Synchronous clock not running

That's a good looking clock with a very solid looking movement. I would not suspect the motor as it has been running for a while, much more likely it is too much friction. It will need to be serviced by stripping down, cleaning, and oiling with clock oil but only on the pivots, it doesn't take long to do but take plenty of pics as you disassemble.
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Old 21st Nov 2019, 5:45 pm   #4
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Default Re: Synchronous clock not running

Make sure there is a small amount of rotational movement between the rotor (with the 'coggled' edge) and the plain 'flywheel' adjacent. They're coupled by a lightweight 'U' shaped spring but the flywheel can sieze on the shaft once the lube dries up.

This usually makes starting very difficult.

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Old 21st Nov 2019, 6:54 pm   #5
Leon Crampin
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Default Re: Synchronous clock not running

I have a virtually identical Gents clock with the same movement. The running torque produced by this movement is marginal, so you will probably need to strip it down and clean the pivots and plate holes carefully. A thin oil such as Moebius 8000 works well. These clocks are a bit reluctant to start even when in good clean condition, but mine never stops in normal use.

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Old 21st Nov 2019, 9:21 pm   #6
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Default Re: Synchronous clock not running

Quote:
Originally Posted by Trigon. View Post
Make sure there is a small amount of rotational movement between the rotor (with the 'coggled' edge) and the plain 'flywheel' adjacent. They're coupled by a lightweight 'U' shaped spring but the flywheel can sieze on the shaft once the lube dries up.

This usually makes starting very difficult.

Cheers
You took the words out of my mouth, that is the fault, it causes difficult starting and random stoppages. I have had a few of these with this fault any freeing up and lubing the rotor flywheel has always sorted it. While you’re at it though clean the whole movement as all the grease will be dry.
Use a light grease on the rotor pivots, not oil, or it will be noisy with that synchronous motor 50 hz whiry buzz!
Enjoy
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Old 21st Nov 2019, 10:07 pm   #7
Julesomega
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Default Re: Synchronous clock not running

Thanks so much for the observations and advice. It never occurred to me that the 'rotor' was in two parts - I can move them in opposite directions but when I let go they fly back to the same position. I also find there is some slight snagging between the 'toothed' rotor and the pole pieces.
I'll do the necessary by daylight tomorrow. It would be really good to have it running again.
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Old 21st Nov 2019, 10:51 pm   #8
Robsradio
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Default Re: Synchronous clock not running

Quote:
Originally Posted by Julesomega View Post
Thanks so much for the observations and advice. It never occurred to me that the 'rotor' was in two parts - I can move them in opposite directions but when I let go they fly back to the same position. I also find there is some slight snagging between the 'toothed' rotor and the pole pieces.
I'll do the necessary by daylight tomorrow. It would be really good to have it running again.
That is what they should do, there is a spring linking them, it is a shame they are free as the usual cause is they are not. The toothed rotor and pole pieces should be close but must not make contact. It sounds as though the rotor bearing may possibly be worn. There is nothing that is not repairable, they are a common movement so if anything too badly worn a donor should be easy to find, but don’t give up on yours until it has been cleaned and examined. It may be possible to make some small adjustment to the pole pieces using the ‘slop’ in the holes.
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Old 22nd Nov 2019, 1:43 am   #9
emeritus
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Default Re: Synchronous clock not running

I can recommend using a petrol lighter fluid for cleaning clocks and watches, and the grade of Moebius clock oil that is appropriate for your clock. Expensive, but as only a smear is needed, a 1 oz bottle can last decades (mine has). A couple of weeks ago the much used and battered Lorus wrist watch that my wife wears when gardening stopped. The battery was fine, and as it would have been binned if I couldn't get it to go, I tried flushing the mechanism with lighter fluid, after first having removed it from the case and detached the face. On re-assembly it worked fine. I had only used it on clocks before.
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Old 23rd Nov 2019, 4:21 pm   #10
Julesomega
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Default Re: Synchronous clock not running

I followed all the advice on cleaning and lubricating, photos etc, and was delighted to see it start up happily. I washed out a lot of filth (I used Turpentine Substitute which is a very thin oil) and could see that there were dribbles of oil that had become waxy and thick.

Can't work out the screw threads, do horologists have their own? Measured the nuts at 4.9 and 5.7mm A/F so was able to use 5.0 and 5.5mm nut runners.

Also found a serial No: 106400
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Old 23rd Nov 2019, 4:39 pm   #11
TonyDuell
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Default Re: Synchronous clock not running

Those could well be 6BA and 5BA nuts. BA threads are not uncommon in British electric clocks, telephones, teleprinters, radios, etc.
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