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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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22nd Jul 2017, 8:15 pm | #1 |
Pentode
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How to tell if an old GPO phone is in need of conversion
I bought a phone on eBay that was I think hard wired in. It was still working when removed and sold to me. There is no cord coming out of the phone at all. How do I tell whether it needs converting? Many thanks
(P.S. I'm creating a lot of threads at the moment, please tell me if I should either slow down or consolidate them or both!) |
22nd Jul 2017, 9:01 pm | #2 |
Dekatron
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Re: How to tell if an old GPO phone is in need of conversion
Check the links present against the instructions at http://www.britishtelephones.com/pstconv1.htm, moving them as necessary, then wire in a line cord as instructed therein.
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22nd Jul 2017, 10:09 pm | #3 | |
Pentode
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Re: How to tell if an old GPO phone is in need of conversion
Quote:
Remove strap T6 - T7. not done Remove strap T4 - T5. not done Insert strap T5 - T6. done Red wire of line cord to T8. no line cord yet Blue wire of line cord to T6. no line cord yet Insert straps T16 - T17 - T18 - T19. all strapped. 17-18 seemingly an after thought as bell wire used not the flat sheet metal used elsewhere White wire of line cord to T18. no line cord yet Green wire of line cord to T15. no line cord yet Insert a 3.3K resistor between T4 - T5 (object with coloured stripes on it). not done Insert a Rectifier No. 205 (if needed) between T1 and T2. possibly - there is a small cube shaped component which is wrapped in what looks to be electrical tape Anyway I think I know what needs to be done! |
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22nd Jul 2017, 10:27 pm | #4 |
Pentode
Join Date: Aug 2016
Location: Dunstable, Bedfordshire, UK.
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Re: How to tell if an old GPO phone is in need of conversion
You would really need to tell us what king of phone you have. A picture would really help. The problem with ebay "for spare" phones is sometimes components will be broken. I've had open ctts caps and open ctt. coils.
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22nd Jul 2017, 10:34 pm | #5 |
Pentode
Join Date: Mar 2012
Location: Rotherham, South Yorkshire, UK.
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Re: How to tell if an old GPO phone is in need of conversion
You're right. It's a 741 wall phone. I'll post a picture when my phone charges. Thanks!
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22nd Jul 2017, 10:42 pm | #6 |
Pentode
Join Date: Aug 2016
Location: Dunstable, Bedfordshire, UK.
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Re: How to tell if an old GPO phone is in need of conversion
As an fyi, if your thinking about setting up a PBX in your home you may not need to do any kind of conversion, the phone may work on a two wire set-up.
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23rd Jul 2017, 7:12 am | #7 |
Dekatron
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Re: How to tell if an old GPO phone is in need of conversion
As far as I know, any old GPO telephone, wired as originally intended, will work on the modern network if you connect the line wires to the outside 2 pins (2 and 5) of the BT socket. There are 3 issues though
1) The bell will be low-impedance (1k) and you may not be able to have it ring properly with other telephones on the line. That's the reason for putting the 3k3 resistor in series with the bell. If you do keep the telephone original inside, you can fit this resistor in the Block Terminal (junction box) in place of the strap there that you remove for extension bells. 2) Without the conversion, you may get bell tinkle if other telephones on the line use pulse dialing 3) And of course the old telephone will be pulse dialing, which some modern networks (most?) don't accept, I like to keep my old telephones original inside, so I do the above. Not that I have a landline to use them on (I have various test sets and line simulators). |
23rd Jul 2017, 8:00 am | #8 |
Dekatron
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Re: How to tell if an old GPO phone is in need of conversion
If both straps T6-T7 and T8-T9 are still in place, the internal capacitor is still in circuit, so it is effectively unconverted. As the capacitor is between T8 and T9, removing either link removes it from the circuit, thus enabling connection of the "ring" wire (blue) of a line cord to use the capacitor in a master socket.
The strapping of T16-T17-T18-T19 simply bypasses the external connection for an extension series bell. I think you can infer from the above that what has been done by any previous owner is not really one thing or the other, so it's up to you whether to go for conversion or return it to the original wiring shown in the diagram N841. If you do go for conversion, you could always use one of the straps you remove to replace that piece of wire between T17 and T18.
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23rd Jul 2017, 11:48 am | #9 | |
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Re: How to tell if an old GPO phone is in need of conversion
Quote:
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31st Jul 2017, 9:00 am | #10 | |
Pentode
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Re: How to tell if an old GPO phone is in need of conversion
Just out of interest...
Quote:
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31st Jul 2017, 9:09 am | #11 |
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Re: How to tell if an old GPO phone is in need of conversion
The modern plug and socket system was designed to work with high impedance bells connected in parallel. These have a resistance of 4k, whereas bells connected in series had a resistance of 1k. The 3.3k resistor increases the resistancece of the bell circuit to 4.3k.
This is a controversial area though with some people saying that the resistor is useless as the bell coil's impedance is largely inductive rather than resistive. They can get very hot under the collar about it. It's up to you whether you fit the resistor. Personally I do, but I've never done any detailed measurement or calculations relating to it.
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31st Jul 2017, 9:12 am | #12 |
Dekatron
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Re: How to tell if an old GPO phone is in need of conversion
It reduces the REN of the telephone from 4 to 1 by dropping approx. half the ringing voltage across the resistor whilst still providing enough current for the bell to ring satisfactorily.
It's debatable whether it's needed or not on a single instrument on the line. I've never had any bother by leaving it as a solid link.
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31st Jul 2017, 9:48 am | #13 |
Pentode
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Re: How to tell if an old GPO phone is in need of conversion
Ahh thank you. Good explanation. What about if the telephone is one of a few connected to a PABX? Is it needed then?
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31st Jul 2017, 10:26 am | #14 |
Dekatron
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Re: How to tell if an old GPO phone is in need of conversion
Do you mean one of a few telephones connected to the same extension on a PABX or do you mean one telephone on one extension of a PABX?
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Regds, Russell W. B. G4YLI. |
31st Jul 2017, 11:03 am | #15 |
Pentode
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Re: How to tell if an old GPO phone is in need of conversion
One telephone on one extension of a PABX. If it doesn't matter in that case should I just strap it?
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31st Jul 2017, 11:30 am | #16 |
Dekatron
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Re: How to tell if an old GPO phone is in need of conversion
I guess you'd need to read the specification of your PABX. I've only had experience of 'Herald' and 'INDeX' exchanges. The BT Herald exchange would supply a REN of 4 no bother: all our 746 dial instruments connected to it had 1 kilohm d.c. bells (typically 2740 ohm Z at 25Hz but reducing to 2293 Ohms Z total load with the series capacitor in circuit).
The INDeX system was digital but had analogue ports too and ran at 36V a.c. rather than the more typical 75V a.c. I think the REN allowed was 1 - must try to dig out my notes -although, in practice, it would ring a REN4 bell with no ill-effects. Or you could suck it and see. I don't think you'd damage anything and the worst that would happen would be an irregular ring, or, at worst, no ring at all; maybe just a ting or two.
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31st Jul 2017, 1:43 pm | #17 |
Pentode
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Re: How to tell if an old GPO phone is in need of conversion
If the ac voltage is a bit low you can always adjust the bells inwardly slightly. My pbx runs at 50v and will ring bells on my phones no problem, once I've spent a little time adjusting them.
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