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Old 18th Dec 2018, 2:56 pm   #1
Kar1may
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Default Caroling Christmas bells - coil is weak.

I have a set of 12 Christmas bells that play up to 25 Christmas songs. One of the bells will not strike as it seems too weak to get the striker to hit the bell. The striker is connected to a small coil controlled by a controller. There are no numbers on the coil. The set was made by Capricorn Electronics in Hong Kong many years ago. Is it possible to get the coil to push the plunger in the middle, out further? Last year these were fine, but I think this one may have been dropped
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Old 18th Dec 2018, 3:37 pm   #2
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Default Re: Caroling Christmas bells - coil is weak.

I'd measure the resistance of the suspect coil and compare it with the others. It may have shorted turns.

If it tests OK, then some kind of mechanical problem such as stiffness or lack of lubrication is indicated.
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Old 21st Dec 2018, 12:16 am   #3
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Default Re: Caroling Christmas bells - coil is weak.

Quote:
Originally Posted by Station X View Post
I'd measure the resistance of the suspect coil and compare it with the others. It may have shorted turns.

If it tests OK, then some kind of mechanical problem such as stiffness or lack of lubrication is indicated.
Would a shorted turn affect the pulse if it is long enough? The current should increase.

Also measure the inductance if you can.
Measure the force compared to an adjacent bell with a separate power supply. You may need to use a new driver transistor if the current one is tired.
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Old 21st Dec 2018, 12:44 am   #4
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Default Re: Caroling Christmas bells - coil is weak.

A shorted turn on a dc operated solenoid is an interesting case. What effect does it have? I suspect little on the ultimate field in the core but an interesting effect on the rate of rise of the field, but what exactly? Will the field rise faster or slower? Slower would certainly upset the hammer operation, but faster?

Over to any electromagnetics experts for comment......
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Old 21st Dec 2018, 4:35 am   #5
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Default Re: Caroling Christmas bells - coil is weak.

I doubt that shorted turns are the problem. Shorted turns on a DC solenoid would increase the drive current and the force produced.
Also shorted turns usually result from insulation breakdown caused by overheating or over voltage.
Neither sounds very likely in a presumably low voltage and low current device.

I would suspect mechanical problems such as mis-alignment, lack of lubrication etc.
Or a marginal supply voltage caused by a weak battery or a fault in a mains derived power supply. Weak or open circuit smoothing cap, or one diode open circuit in a full wave bridge.
Or as above a failing driver transistor.
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Old 21st Dec 2018, 9:59 am   #6
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Default Re: Caroling Christmas bells - coil is weak.

In the extreme case if half the number of turns were short-circuit the resistance would halve, the current would double and the number of ampere turns would remain the same. That's only with a steady current flowing though. We don't actually know whether the coil is energised by AC or DC.

If DC then when the coil is initially energized and de-energized inductance will come into play. The shorted turn/turns MAY act as a slug giving a slow operate or slow release effect. I'm not saying that is the case, just putting forward the possibility. If AC then any change in inductance will have an effect all the time the coil is energized.

I reckon a mechanical problem is more likely and mentioned this possibility in my original post.
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Old 21st Dec 2018, 3:31 pm   #7
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Default Re: Caroling Christmas bells - coil is weak.

Just flick it with a teaspoon when it's moment comes up in the tune. Great Christmas fun for the kids, a bit like those bent wire and hoop toys that rang a bell if the two made contact while you gingerly followed the course. John.
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Old 27th Dec 2018, 7:20 pm   #8
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Default Re: Caroling Christmas bells - coil is weak.

There was a set of these running in my local DIY shop one Christmas, years ago.
The Twelve Days of Christmas Festive Musical Bells (or some such) at Eighteen Quid, a lot in 1980s money.
They were close to the cafe, where you could enjoy a hot mince pie and brandy butter. Trouble was, they kept going into a spasm of playing the same five notes over and over and over again, until one public-spirited customer, with scant regard for his own sanity, traced the lead back through the tangle of wires, and dragged the plug out, to rousing cheers.
I doubt they sold many.
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Old 27th Dec 2018, 7:54 pm   #9
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Default Re: Caroling Christmas bells - coil is weak.

Another possibility that has not been mentioned so far is that the power supply for the bells is not working as well as it should be and effecting the least efficient of the coils. A lot of this sort of equipment use class X capacitors as capacitive droppers and class X capacitors are notorious for losing capacitance valve. It is certainly worth checking.
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Old 27th Dec 2018, 9:34 pm   #10
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Default Re: Caroling Christmas bells - coil is weak.

Swapping the weak bell with a good one will establish if the solenoid or the driver is at fault. I have a set of "Carolling Bells" that I must have bought 20 years ago, where the outputs of the tune-generating IC drove the bells via DIL drive ICs and one of the drivers was weak. As there was plenty of room in the case I bypassed that stage and used a BC 107 and a couple of resistors as the driver. We eventually got fed up with the sequence and now just use them as a static decoration along the stairwell.

Last edited by emeritus; 27th Dec 2018 at 9:41 pm.
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