Dinging Telephone Bell

[Hugh]

We have an old magneto telephone bell connected into the telephone wiring which we've found useful as the sound carries better than the modern phone sounders. I've got it shunt connected between pins 3 and

5 on the conventional telephone socket, that is between the "white with blue markings" wire (Pin 5) and the "orange with white markings" wire (Pin 3), i.e in series with the ringing capacitor.

It has always worked fine until we got a couple of new phones a year or two back. These have the automatic 1571 message detection which means the phone goes off-hook every now and again to check for the special dial tone. On one phone, BT Relate 3000, you can switch off the 1571 detection, whereas the other phone, a BT Decor 1200, has the

1571 feature permanently on.

The problem is, when the phones go off-hook, the bell goes ding, presumably because the sudden change in line voltage goes through the capacitor and into the bell.

The bell is an older lower resistance unit (about 1000 ohms total for the coils). I've wired in extra resistance and gone as high as 12k ohms. The bell still rings quite well, but the ding is also still there although reduced in level.

Does anybody have any ideas if this can be fixed, or do I just need to replace the bell with an electronic sounder?

Thanks,

Hugh

[James Salisbury]

Hi, Ask uk.telecom I'm sure someone will help.

[JohnW]

You need to put a thermistor in series with the bell. These were very common when I worked for the Post Office and looked like this:

quick Ebay doesn't show any for sale, but there were so many used, there must be some around

John

[Hugh]

Thanks very much for the tip, John. I can see how that would work. I'll keep a watch for one.

[Tinkerer]

Any good?

[Hugh]

Thanks for that. Maybe the 15k thermistor might be worth a try.

[Frank Erskine]

The GPO "Rate Book" (a sort of catalogue of stores) specified the Thermistor 1A (the 'innards' of the 1A-1) as having a cold resistance of 200,000 ohms +/- 20%, and a hot resistance (with 100mW) of 500 ohms.

[Dave Liquorice]

Keep the lovely proper old bell that you can hear anywhere and dump the crappy modern user unfriendly BT Decor 1200.

[Hugh]

:

Thanks Frank, that is very useful information. The 1A thermistor has a very wide range. I've ordered a a couple of the two suitable thermistors Maplins sell:

The FX22Y is 1.19k at 100C, 15k at 25C and 45k at 0C The FX42V is 2.94k at 100C, 47k at 25C and 155k at 0C

I'll report back how these perform with the old bell.

Cheers, Hugh

[Hugh]

te:

[Hugh]

Dumping the Decor 1200 is a possibility, however my wife has it in her room and she likes its appearance and the features, so I need to find an "engineering solution"!

The old bell dates back to the first installation in the house back in the days when telephones were new fangled things. Here's a picture of what it looks like:

here are the insides:

like this one has a thermistor (the red thing) in series with the wiring?

Cheers, Hugh

[Frank Erskine]

The Thermistors 1A-1 that I recall were encased in a black rubber sleeve with a yellow label saying - 1A-1.

Originally the only 'official' application for thermistors in subs' apparatus was on shared-service (party) lines, to prevent 'tinkling' when the other party used their 'phone. Proper design of a telephone and/or an extension plan on an exclusive line would try to avoid such unreliable things as thermistors by arranging, say, to shunt a bell as appropriate to stop it tinkling.

Nowadays it seems 'de rigeur' to simply stick anything across an exchange line and expect it to work faultlessly... :-)

[Dave Liquorice]

That's a good point Frank. I have a recollection that proper phones shunt the "bell wire", when off hook to prevent tinkling the bell when pulse dialling. Seems like this Decor thing isn't doing that or the switch timeing is incorrect, it goes off hook before it has shunted the anti tinkle if it does that at all. Can it pulse dial, what happens if you set it to do that?

It might be worth checking that the wiring is correct, though IIRC you either get bells that ring non stop or don't ring at all.

[Frank Erskine]

If you study the circuit diagram for a traditional dial telephone, and an actual dial, you'll find that on the dial, as well as the clever pulsing mechanism there are two additional pairs of contacts which 'make' when the dial is 'off normal', ie away from its rest position.

One of these is used to short the handset earpiece to prevent/reduce clicking in the receiver; the other is used to shunt (short-circuit) the bell.

On an extension arrangement all the bells (which in the traditional configuration are wired in series via the 'bell/anti-tinkle' wires) are shunted by these same 'off-normal' springs.

I'm not familiar with these newfangled BT-badged things (although I do have a BT something-or-other DECT phone); I still prefer my trusty Plan 107 (master/slave arrangement with intercom and call transfer) using 700-type phones!

Sadly, I think that the current NTE5 (modern plug-socket system) is Fairly Bad News for enthusiasts of electromechanical bells and pulse 'dialling', since there's no provision for series bells with their positive anti-tinkle circuitry. At least my Plan 107 is correctly 'hard-wired' to the master socket and uses 8-wire cable to the extension.

[Hugh]

Very interesting discussion about possible wiring options. I couldn't see that the Decor 1200 would allow shunting of the bell. I imagine that kind of facility on a new phone is now seen as prehistoric, and basically everything wires up in parallel on the two wires (5 and 2). Many years ago, BT installed a new local loop connection and installed a NTE-5 and a new DTMF phone. They left the old bell wired in series and there were the occasional dings, but of course it wasn't really a issue until the new models started 1571 detection. As I mentioned at the begining of this thread, I've now got the bell connected between 3 and 5 on the NTE-5.

Following up on the thermistor, I got the two types of componenet I mentioned earlier from Maplins. The 47k thermistor (FX42V) just had too much resistance and the bell hardly rang at all. The 15k thermistor (FX22Y) allows the bell to ring consistently but not at full power and has markedly reduced the strength of off-hook ding but not eliminated it. Based on the characteristics of the FX22Y compared to the figures quoted by Frank for the GPO 1A-1, it certainly wouldn't be as effective. I'll keep looking for a 1A-1 and will resist demands to disconnect the old 1A bell!

Thanks again for all your helpful discussions and advice.

Cheers, Hugh

[terry]

Over here (Eastern Canada) everything just goes across the tip and ring of the pair from the telephone exchange.

Years ago I did hear that sometiems reversing the connections to the bell itself sometimes cured that problem.

And/Or: if the bell rings strongly it might be possible to add some tension to the bell striker? Along those lines (but not sure am now familiar with magento phones although worked with them some 50+ years ago) in some ringers the 'clapper' or striker could rest or could be made rest, to one side or the other rather than 'hover' someplace between the two gongs. So tensioning it in one direction or the other might help?

A very well written original description IMO of the problem by the way.

Good luck with your project.

[Dave Liquorice]

clever

position.

prevent/reduce

(short-circuit)

Ah it comes back now... yes it was dial switches that shunted the bell to prevent clicks and tinkles.

IIRC

pulse

The bells are in paralle between the anti tinkle/bell wire and one of the line wires. The anti tinkle is feed via a C from the other line wire. If you short the anti-tinkle to the line wire that has the other side of the bells they won't tinkle and the C maintains the DC line conditions for pulse dialling.

I used to have a proper bell and didn't have trouble with it tinkling. I think that Decor 1200 isn't shorting the bell wire as it should. The mechanical solution may well be the best option. See if you can work out which way the hammer moves when a phone goes of hook and bias it that way against the gong. Not too much or you'll damp the gong to much when a real ring finishes. Or as the gongs have offset holes just try rotating them at little to adjust the relative posistions of them and the hammer.

[Hugh]

prevent clicks and tinkles.

Want to follow-up on the excellent suggestions from Terry and Dave. Your ideas certainly tie up with what happens in practice, in that the size of the ding varies quite a bit, and is obviously dependant on where the striker is resting relative to the two gongs when the pulse comes on the telephone line.

Brilliant idea to bias the striker against the gong that will sound when the phone goes off-hook. The two gongs have very slightly different pitch so it's easy to identify the direction. The only problem I could see is that while you might be able to supress the ding when the phone goes off-hook, during the time the phone is checking for 1571, the ringing capacitor will charge to the new line voltage, and when the phone goes on-hook, a pulse will be produced in the oposite direction so a ding will sound from the other gong.

Anway, I biased the stricker to the first gong with a rubber band, initially I had too much force and the bell wouldn't ring at all. With a gentler tension applied near the bottom of the stricker arm near the pivot, the results were much better. I took out the thermistor and replaced it with a 2.2k ohm resistor (roughly the right value I think for a REN of 1 with this old bell) so I now have a better ring. With the elastic band, there is now no sound at all as the phone goes off-hook and only the faintest ding when the phone goes on-hook. Obviously the rubber band is sufficient to resist the limited energy stored in the ring capacitor.

The rubber band is a sort of temporary fix, as these things perish and change resilience, so I'll need to look around for a weak coil spring to replace it now that we seem to have come up with a satisfactory solution.

Thanks again for all your interest and invention, even all the way from Canada!

Cheers, Hugh

[terry]

to prevent clicks and tinkles.

Thanks Hugh. We used to have the ding problem, occasionally with dial phones. Today touch-tone is just about universal (see anecdote). As mentioned there are extra contacts on the dial to shunt out the 'speech portion of the telephone circuit' and the perhaps disconnect the bell, within the same phone, while dialling takes place. When however there is a separate magneto phone bell; as mentioned in the original posting, and not being familiar with today's GPO phone connections (Oops that's Brit.Telecomm these days isn't it?) it seemed like the 'mechanical' approach might be appropriate? Also I have two of those big magneto phones downstairs; always meant to modify one so as to answer the touch tone phone line when in the workshop. Anecdote: Grandson to school friend visiting briefly after school one day! "Hey have you seen this cool phone! It's got this round thing to dial numbers!". Then; "Hey Pop can I call somebody on it?". The same grandson, when many years younger, was seen pressing the numbers on calculator while pointing it at the TV! Today he can load and/or programme a computer faster than I can.