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Old 12th Nov 2011, 9:32 pm   #1
Dave Moll
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Default interconnection of multiple field telephones

I hesitate to describe the simple device for interconnection of my collection of field telephones as an "exchange", as that would be far too grand a name for it. It consists as a number of jack sockets, each connected to a line with a field telephone at the far end, plus a collection of patch leads with a jack plug at each end enabling any two sockets to be connected together.

To enable a field telephone with magneto signalling to request connection, each socket has a suitably rated neon indicator connected across it, thus the "operator" can connect to the flashing socket and enquire the connection required prior to patching through to it. If, as requested on some units, the magneto is used again to "ring off" at the end of the call, the connection can then be unmade.

Now we come to the problem for which I am seeking ideas - those telephones that use a buzzer for signalling rather than a magneto. I would appreciate any suggestions for a simple device that can be connected to each socket to alert the "operator" of buzzer signalling - preferably a device that would not be damaged by receiving a magneto-originated signal.
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Old 12th Nov 2011, 9:55 pm   #2
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Default Re: interconnection of multiple field telephones

A telephone earpiece with some back to back diodes across it (enough not to short out normal speech voltages)?

Would need to be out of circuit for normal operation with a magneto phone otherwise the neon wouldn't work, but presumably apart from accidents, you would know which line had which sort of phone on it?
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Old 12th Nov 2011, 10:47 pm   #3
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Default Re: interconnection of multiple field telephones

Slight suggestion --

If all field telephones -then they've all got ringer facility .If so ,this might be an upgrade -based on rural telephone system in use in thir world countries ,where there could be a set number of subscribers on each line.To call someone on your line ,you oicke up phone ,made sure circuit was clear and gave the correct number of fings -eg 2 3
Conect all phones together ,do away with operator an any two can call each other direct .
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Old 12th Nov 2011, 11:08 pm   #4
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Default Re: interconnection of multiple field telephones

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A telephone earpiece with some back to back diodes across it ...

Would need to be out of circuit for normal operation with a magneto phone otherwise the neon wouldn't work, but presumably apart from accidents, you would know which line had which sort of phone on it?
This could be connected to a spare plug that is left in the appropriate socket when the 'phone is not in use - and yes, the desire for magneto survivability was to protect against accidents. It would be immediately obvious if a magneto is accidently used in this scenario, as it would be difficult to turn when feeding into a short circuit.

As a traditional receiver would be rather bulky in this application, I'll see what I've got in the way of more modern (but dead) telephones in the stores.
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Old 12th Nov 2011, 11:31 pm   #5
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Default Re: interconnection of multiple field telephones

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Slight suggestion --

If all field telephones -then they've all got ringer facility
Unless it's a type 'D', which has a magneto bell and a buzzer, but no magneto, and so can't ring out! The Type 'D' signals out by key and buzzer, but inward calls may be received from an exchange magneto (or magneto instrument).

Audible monitoring might work, as suggested by Herald1360, with clipping diodes across the receivers. It ought to be easy enough to discriminate between 17Hz rattle from a magneto instrument (Type 'F', etc...) and the HF buzz of a Type 'D'.

Do you know which type of telephone will be connected to whichever socket?
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Old 12th Nov 2011, 11:39 pm   #6
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Default Re: interconnection of multiple field telephones

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Quote:
Originally Posted by DAVEHALL View Post
Slight suggestion --

If all field telephones -then they've all got ringer facility
Unless it's a type 'D', which has a magneto bell and a buzzer, but no magneto, and so can't ring out! The Type 'D' signals out by key and buzzer, but inward calls may be received from an exchange magneto (or magneto instrument).

Audible monitoring might work, as suggested by Herald1360, with clipping diodes across the receivers. It ought to be easy enough to discriminate between 17Hz rattle from a magneto instrument (Type 'F', etc...) and the HF buzz of a Type 'D'.
Apologies -field tele to me would mean linesmans type .Only known two types -the old fiel one with magneto and the green one with a ringer generator ,both giving 17 hz out .
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Old 13th Nov 2011, 12:15 am   #7
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Default Re: interconnection of multiple field telephones

OK, I've rigged up the receiver as described above. The result was a rather strangulated buzzer output - from both the 'phone and the external receiver.

To check that it was the receiver straining the buzzer generator, I tried again without the receiver plugged in. I was amazed to discover that the buzzer signal lights the neon! No doubt, it is generating a high enough voltage to activate a "rectifier no. 205" - if it is high enough to fire the neon.

As the buzzer activates the neon, the problem is solved - without any further complication. It never occurred to me that the buzzer would be powerful enough to fire a neon (rated at 95V and with a 27K series resistor). It is even possible to tell whether the neon is lit by a magneto or a buzzer, as it is visibly flashing with the former and steady (to the human eye) with the latter - owing to the higher frequency.

I have checked that it works with both a Type "D" (rightly guessed by Russell as the source of the supposed problem) and a Type "F".

As an aside, the output from the ringer generator of my 704 linesman isn't sufficient to fire one of these neons. I add this merely as a curiosity, not as a problem, as the 704 is not intended to be part of the "network".
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Old 13th Nov 2011, 12:28 am   #8
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Default Re: interconnection of multiple field telephones

Can you use a battery and lamp between one of the calling wires and earth?

The telephone then applies an earth to the calling wire to complete the calling circuit.

For more involved, have a look at Central Battery Signalling System here:
http://www.johnhearfield.com/Telephone/CBS2.htm
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Old 13th Nov 2011, 1:02 am   #9
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Default Re: interconnection of multiple field telephones

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I was amazed to discover that the buzzer signal lights the neon! No doubt, it is generating a high enough voltage to activate a "rectifier no. 205" - if it is high enough to fire the neon.
I ought to have spotted that.

The Type 'F' in my office (house-shed link) has a buzzer-coil in it. I once put my 'scope on it and observed a horrible spikey back-ee-emm-effy waveform that made my moistened fingers tingle. Have you tried your neon on a buzzer telephone via a resistor equivalent to a decent length of telephone cable, to see if it still works?
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Old 13th Nov 2011, 10:22 am   #10
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Default Re: interconnection of multiple field telephones

As I say, I now have a working solution - or rather, my perceived problem simply doesn't exist. I will, however, do some experiments to see how much additional resistance I can introduce before the neon fails to fire.

Incidentally, I did notice that the buzzer only fires the neon when no other unit is connected - thus, apart from the neon itself, the signal is being sent into an open circuit. The increased loading of a receiving 'phone keeps the voltage below firing - so the buzzer cannot be used to "sign off". This would have to be done by a magneto - which does still generate enough voltage.

Returning to Dave Hall's suggestion of an alternative approach with all 'phones connected together and a pre-agreed signalling code, there is presumably a limit to the number of 'phones (equivalent to the REN system) before even the magnetos start to struggle - another area for some experimentation. This could form the basis of a "night service" for the system.
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Old 22nd Nov 2011, 3:01 pm   #11
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Default Re: interconnection of multiple field telephones

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Have you tried your neon on a buzzer telephone via a resistor equivalent to a decent length of telephone cable, to see if it still works?
It would appear that 100K (in addition to the 27K already in circuit) is about the limit. At this value, the neon strikes but then extinguishes. With 47K rather than 100K the neon is still going reasonably strongly.

Answering this question was delayed by the set "D" developing a fault that I failed to identify, but which went away after taking out the batteries to check them and putting them back again when they checked out OK - presumably a poor battery connection.
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Old 22nd Nov 2011, 4:33 pm   #12
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Default Re: interconnection of multiple field telephones

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Quote:
Originally Posted by russell_w_b View Post
Have you tried your neon on a buzzer telephone via a resistor equivalent to a decent length of telephone cable, to see if it still works?
'...At this value, the neon strikes but then extinguishes. With 47K rather than 100K the neon is still going reasonably strongly.'
Thinking about it, a length of cable has distributed L and C as well as simply R, which might 'tame' the back EMF voltage spikes that would otherwise light your exchange neon. I guess the best way to find out is to try it!
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Old 29th Nov 2011, 12:04 pm   #13
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Default Re: interconnection of multiple field telephones

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Returning to Dave Hall's suggestion of an alternative approach with all 'phones connected together and a pre-agreed signalling code, there is presumably a limit to the number of 'phones (equivalent to the REN system) before even the magnetos start to struggle - another area for some experimentation. This could form the basis of a "night service" for the system.
"we would recommend the 5-Bar Generator and 1600 ohm ringer; these will be found to give excellent service with any number of subscribers up to thirty - or even beyond that number - bridged on a long distance line, and we have no a hesitation in stating that this equipment is the most powerful that can be obtained. Efficient service will be given under the most severe conditions found in party-line working."

http://www.britishtelephones.com/gec/peelcompact.htm

Probably field telephones have quite small magnetos but if you condensered the bells you could fit them remote to the telephones; e.g. Bedrooms 1 and 2 could be served by a bell mounted in the hall between then, thus reducing the number of bells to be code rung by the magneto.

You could separate the magneto calling wire from the ringing wire, parallel the magnetos to a calling wire, use the calling wire as the input to a BT REN Booster and the output of the REN Booster on a ringing wire to ring all the bells.

Our American friends over at http://www.classicrotaryphones.com/forum/index.php probably have more knowledge of rural party line working.
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Old 29th Nov 2011, 8:32 pm   #14
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Default Re: interconnection of multiple field telephones

I could just make enquiries about a proper field telephone exchange if you were interested Dave....
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Old 30th Nov 2011, 10:29 am   #15
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Default Re: interconnection of multiple field telephones

Sean, you have a PM.
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