Malfunctioning external bell unit

[trsomian]

A colleague has a large telephone bell, suitable for external use which does not work, apparently because the central permanent magnet has lost some of its magnetism. Is this diagnosis plausible, and if so, are there any suggestions as to how to remagnetise it?

[russell_w_b]

Originally Posted by trsomian

Is this diagnosis plausible, and if so, are there any suggestions as to how to remagnetise it?

It is plausible, as it is a polarised bell (magnetically speaking) and requires a permanent magnet. It doesn't have to be very strongly magnetised though, so check for other reasons why it isn't ringing first. Are there a.c. volts across the bobbins when it's meant to ring? 30 - 70V RMS. Do the armature and hammer move freely between the gongs when moved by hand? Is there muck fouling the gap(s) between the pole-pieces and the armature? Are the gongs (which are mounted eccentrically) preventing the hammer moving between them? Can you measure 1000 Ohms across two bobbins in series or 500 Ohms across each with the bell disconnected from the rest of the telephone circuit??

Has the wiring been tampered with? The left-hand terminal of one bobbin should be connected to the left-hand terminal of the other with the bell-wires connected to the right-hand terminals. Or the other way round, NOT linked left-to-right or right-to-left.

I suspect it will be a type 59A bell motor, which are pretty common if you wish to replace it. The modern type 59A bell-motor has a small bar-magnet mounted under a clamp and the older unit has a cylindrical magnetic bar between the bobbins. See here.

If you've tried all the above and feel you need to remagnetise the cylindrical bar, remove it from the yoke, wrap about twenty turns of 1.5mm wire around it and 'flash' it on a car battery for a second or two (but wear eye protection!).