On 16 Feb, 23:59, Frank Erskine wrote:
On Tue, 16 Feb 2010 20:26:56 +0000 (GMT), "Dave Liquorice"

wrote:
On Tue, 16 Feb 2010 00:56:05 +0000, Frank Erskine wrote:

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.

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.

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.

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?

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!



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.

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.

--
Frank Erskine

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