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Old Rotary phone - GPO 746

Discussion in 'Electronic Repair' started by [email protected], May 11, 2007.

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  1. Guest

    I recently got myself a GPO 746. A very old rotary dial pulse dialling
    type telephone.

    Usually, to dial a number you just put your finger in the hole and
    turn it. HOWEVER. The thing doesn't "turn back" correctly. It'll give
    a pickup as if I just picked up the phone again. I think this is
    because it's not going back as fast as it should.

    If I pull a number and push it back into position manually, a bit
    faster. It'll dial that number! So I think the retaining springs
    inside are a bit loose. Would anyone here know how I could either
    tighten this spring or would I have to get a new spring?

    Thanks very much in advance!
    Dan
     
  2. Arfa Daily

    Arfa Daily Guest

    There is usually a 'bucket and weight' centrifugal governor on the dial
    return, which is worm driven. The end float of the shaft is adjustable with
    a screw, and I have known this to be over-tightened by people such that the
    governor is too tight for the return spring to move it. Likewise, if the
    grease on the governor drive and bearings has gone hard and sticky, this can
    also jam or restrict it.

    The dial assembly is easily removed to work on it. There is a single screw
    at the bottom, which needs only to be loosened. The dial then hinges up and
    away from its mounting cradle at the bottom, and then lifts out at the top.
    The dial return speed can be checked, once you have it on the move again, by
    dialing a zero, and then saying out loud "One-thousand-one-hundred-and-one".
    That's without any gaps in the words and just at normal talking speed.

    The dial should hit stop just as you finish speaking. The exchanges are (
    certainly used to be ) fairly tolerant of the pulse repetition speed, but if
    it is too far out, a mis-dial can occur. If the spring is broken , it
    usually goes at the looped end-anchor. By heating it in a gas flame, you
    might be able to bend a new anchor on it. The other option might be to use a
    section of an old clock mainspring.

    It's been a long time since I worked on one of these, so it's all from
    memory, but I think I'm about right. Hope it helps.

    Arfa
     
  3. John_H

    John_H Guest


    Hi Dan

    I made a pulse dialer once and this may help you.

    Return time from Zero dialing to rest position should be about 1.
    seconds in America and I think is a bit different in Europe. I remembe
    tone pulses at 10 pulses/second.

    I think you should clean the inside clutch which makes uniform retur
    rather than tighten spring and adjust the press screw to get thi
    rate.

    Hope this work..

    Regards

    John_
     
  4. Ross Herbert

    Ross Herbert Guest


    This is probabbly the best site for old UK phone info
    http://www.telephonesuk.co.uk/phones_1960-80.htm#746

    Circuit etc.
    http://www.samhallas.co.uk/repository/n_diagrams/0000/N846.pdf

    Dial type 21 (basically the type 12 dial with plastic case and clear
    finger plate)
    http://www.samhallas.co.uk/repository/n_diagrams/0000/N581.pdf

    When a rotary dial has been unused for years it will generally become
    slower in operation (or not work at all) due to lubrication drying
    out, or somebody has played around with the tensioning of the governor
    arms (or both). The only way to fix it is to totally strip it and then
    re-assemble. This means you have to know what you are doing and how to
    set it up and adjust it if it is to work properly. Techs in training
    usually spent a week performing this operation under supervision
    before they became proficient at dial refurbishment.
     
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