I've been passed an old phone - one of these:
It has a BT plug on it so I presume someone's had it working, though that's not necessarily the case of course.
Any idea how these models ought to be wired for BT lines?
Daniele
I'm sure that there will be something here.
Tim
Ace, thanks.
Daniele
IIRC its nothing more fancy than the bell in series with a big capacitor. Bell resistance stops it messing with the audio.
NT
Except that forum post says it's a 1930's Belgian phone. The terminal block isn't likely to match any of the GPO ones. Thought there was gold in the forum in the shape of a link to the schematic but it doesn't work. B-) Might be enough clues for google to find it though.
The gotcha is probably the impedance of the bell coils, these days they are expected to be high, in the days of yore they where significantly lower. The frequency of the UK ringing signal might be wrong for the resonance of the clapper, the UK uses 25 Hz I have a sneaky suspicion that europe uses 17 Hz but could be wrong.
UK used to use 17Hz (actually 16 2/3Hz) but that won't matter, you cna even make a phone ring on 50Hz. ISTR bell impedance was 500R, you could run up to 4 in series. Problem is most likely to be the bell circuit isn't connected on the socket. many of them are 2 wire since that's all that modern phones need.
This linke might be more helpful
If it has already been converted but doesn't ring check that you have pin 3 connected at your extension socket.
Owain
It was still handy though, and I've got the phone to work nicely.
There was a schematic in the inside of the phone that I hadn't noticed, which also helped.
That indicated the phone should have three connectors to the phone socket, but it worked with just two.
BT phones seem to use three.
What are they all for?
Daniele
Only two wires come in from the outside, signal wires that include power.
The third wire is for the bell and connects to the junction of a resistor and capacitor connected between the two incoming wires.
The capacitors used to keep dc off the bell were rather large in the 30s, s o the cap was in a box feeding the phone connection, enabling the phone to be a fair bit smaller. Since then the bell cap has tended to move into the phone itself, so some phones work on 2 wires or 3, some need 3. And there a re feeds about lacking the 3rd connection.
NT
If there was only a single instrument on the circuit (pretty rare these day s, but common in the GPO days) only the two wires coming from the exchan ge were used. (At that time one would have been the earth return, the other connected to -48V through a relay; if you held both wires, shorted them to gether then broke the connection the back emf from the relay inductance cou ld give you a fair shock - the chap I saw do this was quite surprised). The bell circuit used a capacitor in series with the bell to block DC but pass the ringing signal. If you had an extension phone, which would have been hard wired by the GPO engineer, the third wire was connected to the bell in the extension phone, and the capacitor removed or disconnected, so that both bells used the one capacitor in the master phone instrument. I think the bells may have been w ired in series, but I can't remember how they did things if you wanted more than one extnsion. The intention was to reduce the load of the bell circui t, and to prevent the bell in one phone tinkling when the other one was use d. this of course is just based on my memory, since a colleague has borrowed m y copy of Atkinson, the definitive manual on how the GPO used to do things.
I once, or twice, borrowed a copy from the public library
Well you can make a phone rattle with 50 Hz but it won't be as effective or sound as nice as the correct frequency. B-)
It's a very long time ago, I thought each bell coil was 1 k connected in parrallel to give 500 R, with "bells" connected in series. Then the modular system came in with high impedance coils and "bells" in parallel. The bell coils in old phones had to be chnaged from parallel to series to avoid loading the line too much and producing ring trip.
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