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Old 25th Aug 2020, 7:45 pm   #1
WayneL74
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Default Tele 740 original use?

Hi all.

I have recently purchased a Tele 740 to use an an internal extension on a Plan 105. It got me wondering what the original use of this phone was.

As you can see is has 4 buttons marked 1 - 4. These individually latch if off-hook & are restored when going on-hook.

The phone was connected with an 18-core cable which has sadly been cut off. 15 cores are in use. Additional terminal blocks have been added so all cables can be connected.

From what I can deduce from the wiring, the butons are be used to connect 1 of 4 lines to the phone at a time. These come in as isolated pairs. There is also a "signal" wire for each button which connects a common wire to a seperate output wire. Guess this was used maybe to light a lamp to show the line was active?

There is an additional gravity switch which closes when off-hook. This seems to be wired so the signal wire mentioned above connects to another wire.

Finally there is a thermistor which is simply in series with the bell.

Any thoughts anyone?

Thanks
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Old 25th Aug 2020, 8:25 pm   #2
AndiiT
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Default Re: Tele 740 original use?

Hi, possibly extension plan 2A or a variant of it, diagram N1448 here

Andrew
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Old 25th Aug 2020, 9:22 pm   #3
WayneL74
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Default Re: Tele 740 original use?

Hi Andrew - thanks for that.

Found another diagram - N4505 - which shows more details & explanations.

Doesn't show the additional gravity switch & thermistor, but I think we are on the right track.

Regards,

Wayne
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Old 26th Aug 2020, 9:59 pm   #4
AndiiT
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Default Re: Tele 740 original use?

Quote:
Originally Posted by WayneL74 View Post
....Doesn't show the additional gravity switch & thermistor, but I think we are on the right track....
Hi, I have just taken a look at N4505 and had some thoughts on the original use of your phone although it's pretty much all guess work.
In the Plan 2A diagram there is an "anti - tinkle" wire, my suspicion is that the Thermistor is fitted to prevent tinkle (remember that this phone would be used in conjunction with three other phones) instead of using a third wire for anti tinkle when dialling.
The adiitional gravity switch may well have been for C wire signalling to a PMBX (with no recall), it's unlikely that it lit a lamp on other phones to indicate line in use as there is no lamp on your phone.

I guess it will be a mystery as to the original use unless someone comes along who can confirm for definite which system the phone was used on, it may well even be a Plessey plan system variant.

Regards

Andrew
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