Wiring old BT phone 746F

Discussion in 'Electricians' Talk' started by FagCardFreddie, Dec 4, 2018.

  1. I have an old BT dial phone which worked perfectly till we moved and someone cut the wire going into the wall leaving about a metre coming out the back of the phone.
    I have established that it needs a BT431A phone plug to connect to the wall socket BUT.........
    The cable is round, not 'D' shaped, which I do not think is a problem, but the colours are: there are four wires, Green/Blue/Red/White.
    Does anyone know which order they should be fitted into the phone jack, and would it matter if they are wrong?
    If not, does anyone know where I could find someone who does know?
    BT have got no idea what I am talking about!!!!!!!!
    Is there an 'Old' (with respect) BT engineer out there who knows about these things?
    Many thanks for any advice offered.
  2. KIAB

    KIAB Super Member

  3. Bob Rathbone

    Bob Rathbone Screwfix Select

    The old phones have a 500 ohm line impedance, especially for the bell. The new phones have a line impedance of 2000 ohms. The bells on the older types were connected in series when more than one phone is used, the mew types have the ringers connected in parallel. So you see it is more than just a matter of connecting a plug.
  4. KIAB

    KIAB Super Member

  5. MGW

    MGW Screwfix Select

    I can remember 3 wires to telephone, 2 from GPO which when into a box with resistor and capacitor the latter stored energy for bell.

    What I found was polarity mattered, and if incoming where wrong, you could ring out OK, but bell would not work. However with cordless phone the polarity didn't matter, so you could get a socket that worked A1 with a mains powered phone, but no bell with GPO powered phone.

    There were 4 connections to phone, but from memory 4th only used with party line. These no longer exist, however the old 1/4" Jack plug did have 4 connections. I never had a Jack socket phone was hard wired, the phone originally belonged to GPO the phone latter could be rented from GPO or you could use your own, with RJ14 socket, the phone often had RJ11 socket, and fax machines would often swap polarity as phone fed through fax machine, so there were polarity swapping cables around.

    I think finding wiring for 1/4" plug today will be hard
  6. KIAB

    KIAB Super Member

    Last edited: Dec 4, 2018
  7. Thank you very much for you help guys I shall try it when I get the BT431A phone plug.

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