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?
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
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.
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:
and here are the insides:
Looks like this one has a thermistor (the red thing) in series with
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... :-)
On Tue, 16 Feb 2010 00:56:05 +0000, Frank Erskine wrote:
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.
On Tue, 16 Feb 2010 20:26:56 +0000 (GMT), "Dave Liquorice"
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)
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
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.
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
A very well written original description IMO of the problem by the
Good luck with your project.
On Wed, 17 Feb 2010 05:43:50 -0800 (PST), Hugh wrote:
Ah it comes back now... yes it was dial switches that shunted the
bell to prevent clicks and tinkles.
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.
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
Thanks again for all your interest and invention, even all the way
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
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.
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