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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment

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Old 9th May 2017, 5:14 pm   #1
davidm123
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Default GPO 741 help

Not sure if anyone might be able to advise me in some way.
I have a GPO 741 wall mount phone, but i want to make it so i can manually make it ring, and when the receiver is picked up the ringing stops automatically and a message is played. When the receiver is placed back down it stops automatically, until i make it ring again. If i can use what's already in the phone that would be good, and i have an MP3 player i can use for playing the message. Its one of these: http://www.electronics123.com/shop/p...al-blocks-8267 Thank you in advance.
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Old 10th May 2017, 2:43 pm   #2
julie_m
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Default Re: GPO 741 help

Unfortunately, most of the bits that make a telephone work are in the exchange. There is not much inside an old 'phone besides the microphone, speaker, bell, dial, switches and induction coil -- and even in a more modern analogue land line phone, the functions of the bell, dial and induction coil are performed by ICs.

Your best bet might be to build a PABX using a scrap PC, an analogue FXS adaptor and the free Asterisk software (which can definitely do what you want).
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Old 10th May 2017, 5:01 pm   #3
TonyDuell
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Default Re: GPO 741 help

Would you object to modifying the internal wiring of the Telephone 741?

You seem to want to use 3 parts of the telephone : the bell, the gravity switch (handset rest switch) and the receiver (earpiece). You presumably do not need the dial, induction coil, regulator bits, etc. Now a telephone is designed to have only 2 wires back to the exchange (for economy, running more wires would be expensive). So there is circuitry at the exchange end to detect off-hook, to send an AC signal to ring the bell, to couple the speech signal to the line, etc. You could duplicate that (it's acutally not that complex), or you could seprate the bits in the telephone and have 6 wires, 2 for the receiver (red and green wires in the handset cord), 2 for the bell, 2 for a contact on the gravity switch.

The gravity switch is a simple mechanical switch. The receiver can be driven as a fairly high impedance speaker. The worst part is the bell. Telephone bells ring on AC, they do not have the trembler/interrupter contact of, say, a doorbell. A reasonable kludge, in the absence of a properly designed ringing supply, is 30V AC at 50Hz (from a small transformer) and an 8uF non-polarised capacitor in series. Apply that to the bell coil wires and it will ring, you should then adjust the gongs (off-centre mouting holes) for the best ring. I will not sound right to an expert, but I guess you don't need that.

Your actually problem seems to be a state machine. From the idle state, sending the trigger signal would go into state one and energise the bell. Closure of the gravity switch would then go into state 2, turning off the bell and sending the audio from the MP3 player to the receiver. Opening the gravity switch would then go back to state 0.

These days. based on the idea that you should never use a dozen components when a million would do, I guess you do this with a microcontroller. I'd do it with a handful of gates and flip-flops, or even relays.
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Old 10th May 2017, 5:22 pm   #4
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Default Re: GPO 741 help

Given there are a number of contacts on the hook switch it gets easier (more wires though)...

1. Press manual button to latch a relay for the ringing, relay drops out when handset lifted.
1a. Manual button "rewinds" the message thingy.
2. Second pair of contacts on hook (handset) switch starts replay.
 
Old 10th May 2017, 9:56 pm   #5
Oldcodger
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Default Re: GPO 741 help

BT used to sell a small 2 line system ( I've even seen them sold in Curry's etc) some years ago for not a lot. And Panasonic sold a 206, at not a lot. I'd be surprised if there's not a few around second hand going cheap( as well as ringing phones). I'd suggest the BT one would be a better idea ,as it did not need a fancy phone for programming. Add in something like an old Modem .with a built in answer phone and it would be easy and cheap. ( I remember an old Azteck 56k MODEM that had an answer MC built in ).
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Old 10th May 2017, 10:35 pm   #6
julie_m
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Default Re: GPO 741 help

But a hardware PABX needs something at the other end to initiate the call; now you need a way to dial a number and play the MP3 output into the line. At least with the Asterisk method, you have a handy computer there to do that for you!

I've an idea, though, for a pure-analogue solution that would not require modifying the phone ..... Be back once I've scribbled out a few ideas .....
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Old 11th May 2017, 8:10 pm   #7
davidm123
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Default Re: GPO 741 help

Thank you everyone for replying
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Old 24th Jun 2017, 11:15 am   #8
davidm123
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Default 741 Telephone advice/Help

Hi I have got a wall mounted 741 rotary dial phone and a BLACK PowerDsine PCR-SIN03V12F20-C RINGING GENERATOR 3W 12VDC,TO 70V 20Hz. I have wired up the ring generator to a master BT socket and it works great only thing is i am not sure what to do with the yellow wire that is not connected to anything. as the ring generator works ok what ide like now is once i pick up the handset that a message is played and stops when i put the handset down Thank you and hope it makes seance to you
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