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Old 4th Sep 2007, 9:31 pm   #1
DoctorWho
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Default Can anyome identify this GPO Magneto Wall Telephone Please?

I recently bought this rather nice vintage magneto wall telephone which is complete except for a mouthpiece. There is no model number anywhere, the only number I can find is "4101A" on the inside of the door. Sadly the majority of the circuit diagram is missing, there is no information there to allow you to identify the model.

If anyone can identify the model do they know anywhere that you can obtain a circuit diagram please?

The telephone has an interesting history, it was installed in a manor house in Norfolk in the 20's I believe and everything seems in pretty good shape.

I just now have to track down a mouthpiece (any suggestions?) and get it up and running.

What voltage batteries do I need to use for the transmitter? Can it be converted without any major "surgery" to allow it to run for the voltage present on the line?

Thanks for any information.

Peter.
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Old 4th Sep 2007, 9:46 pm   #2
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

I hope the vandal who added a smiley mouth is suitably ashamed of themselves
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Old 4th Sep 2007, 9:47 pm   #3
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

Some similarities with the GEC K8055...

http://www.britishtelephones.com/gec/gecm.htm#k%208055
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Old 4th Sep 2007, 10:28 pm   #4
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

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What voltage batteries do I need to use for the transmitter? Can it be converted without any major "surgery" to allow it to run for the voltage present on the line?

Thanks for any information.

Peter.
It looks like it'll take a couple of 'Flag' cells, so 3 Volts, I would say. Too much and you're liable to 'fry' the carbon-granule transmitter (when you obtain one...).
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Old 4th Sep 2007, 10:39 pm   #5
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

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If anyone can identify the model do they know anywhere that you can obtain a circuit diagram please?

Peter.
It might just be similar to the Western Electric model shown attached (from 'Telephone Erection and Maintenance' by Herbet G. White, 3/6 net, October 1922). I don't suppose it will be a million miles away...
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Old 5th Sep 2007, 10:11 am   #6
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

Nick - I know what you mean. Thankfully that was easily removed with white spirit. What a vandal indeed! At least it hadn't been hacked about inside. You may well be right about the model number, however there are no holes or signs to suggest that there was ever a writing slope on the front. Aside from that, it looks identical. Maybe the writing slope was optional?

Russell - Thanks for that, thankfully the original transmitter is intact and working, I've tested that amd it's very clear actually. Is there any way to modify this to run from the line current? Thanks for the circuit, that's very helpful.

By the way, I found the date of 1929 on the transmitter housing.

Best regards,

Peter.
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Old 5th Sep 2007, 12:01 pm   #7
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

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Russell - Thanks for that, thankfully the original transmitter is intact and working, I've tested that amd it's very clear actually. Is there any way to modify this to run from the line current? Thanks for the circuit, that's very helpful.

Best regards,

Peter.
To run it on line current, you'd be modifying the telephone from a local-battery instrument to a central-battery instrument, ie: the mic will need to be line-powered rather than from a local battery. I'd be inclined to make up a little board to sit inside with an ASTIC (Anti-Sidetone Induction Coil) circuit, as found on a 200 or 300 series instrument with the dial strapped out, and connect receiver and microphone to it. Use the drawing from a 300 series instrument as a guide. You could mount a 3k3 resistor for the bell on the board as well (assuming it is 1000R overall, or REN 4).

Have you traced out the wiring and drawn it? It might be interesting to compare with the LB magneto telephone on the drawing I posted.

I would leave the magneto disconnected!
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Old 6th Sep 2007, 5:56 pm   #8
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

Thanks Russell, that's very useful information. I will have a think about whether or not to modify it from LB to CB, not sure about that yet as I do like to keep things as original as possible. The bell is indeed 1000 ohms.

I haven't traced the wiring, this is something I really ought to do. Out of interest why would you leave the magneto disconnected?

Thanks again,

Peter.
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Old 6th Sep 2007, 6:20 pm   #9
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

You could certainly wire it to ring easily enough with virtually no mods.

Nick
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Old 6th Sep 2007, 8:00 pm   #10
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

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I haven't traced the wiring, this is something I really ought to do. Out of interest why would you leave the magneto disconnected?

Thanks again,

Peter.
I don't suppose it would do any damage if you did leave the magneto connected, as (and I'm assuming it's a 'silent ringing' type where operation of the magneto pulls the contact into position and drops it out when the handle is released) it wouldn't normally be in cct.

All that happens when you turn a magneto onto a CB line is that the generated voltage has to overcome the DC line voltage and stiff resistance is felt. The AC waveform from the magneto may simulate dial pulses and a spurious call might be initiated.
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Old 6th Sep 2007, 10:20 pm   #11
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

Nick - Yes indeed, I have tried this and it works well, you can also hear dial tone clearly, I guess the 3 volts are just needed for the transmitter. Any suggestions as to the best batteries to use?

Russell - It is indeed only in circuit when turned. I tried as you suggested and it made the other telephones in the house ring.
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Old 10th Sep 2007, 9:35 pm   #12
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Default Re: Can anyome identify this GPO Magneto Wall Telephone Please?

Here it is, all finished and working and fixed to the wall.

Thanks for the advice in getting it up and running again.
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