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Old 23rd Apr 2020, 7:28 pm   #1
Dave Moll
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Default Bellset 44 problem with indicator

The indicator for the extension being in use on my bellset 44 has a problem in that it is not clearing after it has been set. There is nothing mechanically preventing it from returning, as moving it to the "set" position by hand allows it to return without problem. It seems more that the solenoid is remaining magnetised until its armature is manually released.

I do notice that a switch (marked "I1"?) on the diagram places a 2μF capacitor across the coil, but I'm struggling to identify what that switch is.

Any thoughts on the cause of the problem?
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Old 23rd Apr 2020, 8:47 pm   #2
OscarFoxtrot
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Default Re: Bellset 44 problem with indicator

I'd guess it's contact 1 on relay I - which is the Indicator, which is labelled I over 1, suggesting it's a relay with one contact set.
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Old 23rd Apr 2020, 9:32 pm   #3
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Default Re: Bellset 44 problem with indicator

The indicator is indeed a relay with one contact set, and the capacitor is shown as across the coil when the relay is operated, so could this be what is keeping the coil energised after the extension breaks the loop? It doesn't appear, however, that shorting out the coil releases the relay.
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Old 24th Apr 2020, 10:13 am   #4
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Default Re: Bellset 44 problem with indicator

Maybe a paper shim on the end of the magnetic core would "fix" the problem?
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Old 24th Apr 2020, 10:40 am   #5
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Default Re: Bellset 44 problem with indicator

A capacitor across a relay coil is normally used to provide a slow release function.

Many telephone relays had a residual stud, of non-magnetic material, between the pole face and the armature to break the magnetic path and prevent the armature being held in by residual magnetism. Tape as suggested by Herald1360 will also do the job.
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Old 24th Apr 2020, 11:50 am   #6
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Default Re: Bellset 44 problem with indicator

I think the capacitor (or any other electrical energising) was innocent, given that the relay had failed to release over twelve hours later.

Thank you, Chris, for the shim idea. That was successful, so presumably it was residual magnetism. The end of the core now has a small round sticky label attached to it serving the same function.

Maybe there would originally have been some such barrier that has subsequently fallen off and gone missing.
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Old 24th Apr 2020, 2:08 pm   #7
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Default Re: Bellset 44 problem with indicator

"Residual stud".

I knew there would be a proper term for it....
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Old 24th Apr 2020, 2:29 pm   #8
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Default Re: Bellset 44 problem with indicator

Sometimes took the form of a screw with a locknut. A special feeler gauge with a hole in it was positioned between the armature and pole face with the screw thread passing through the hole. The screw was then adjusted until the gauge was just gripped.
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Old 24th Apr 2020, 3:35 pm   #9
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Default Re: Bellset 44 problem with indicator

Yes, there is. I'll leave the temporary fix for the moment - until I find a suitable spanner to loosen the locknut, as there's not a lot of room - but thanks pointing out how the adjustment should be made.
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Old 15th May 2020, 12:29 pm   #10
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Default Re: Bellset 44 problem with indicator

As a slightly off-topic addendum to this, I have now finished incorporating this bellset 44 into a plan 5 with two internal extensions - the finishing touch having been applied when (with a stroke of serendipity) I managed to acquire the three bells 56 needed for internal signalling between the main and the extensions.
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Old 16th May 2020, 11:49 am   #11
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Default Re: Bellset 44 problem with indicator

Residual studs were part of relay adjustment in the day.
Gauges had a hole in to allow the bolt type to be adjusted or to check the wear on fixed studs.
This set was on my first van when I got it in '69 it dates from 1957
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