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Old 22nd Jul 2017, 8:15 pm   #1
boombox
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Default 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!)
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Old 22nd Jul 2017, 9:01 pm   #2
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Default 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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Old 22nd Jul 2017, 10:09 pm   #3
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Default Re: How to tell if an old GPO phone is in need of conversion

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Check the links present against the instructions at http://www.britishtelephones.com/pstconv1.htm
Sorry - I should have said (doh) - I had a look there and that's kind of what prompted the question - the phone seems to be a mixture of both? Is that possible?

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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Old 22nd Jul 2017, 10:27 pm   #4
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Default 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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Old 22nd Jul 2017, 10:34 pm   #5
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Default 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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Old 22nd Jul 2017, 10:42 pm   #6
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Default 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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Old 23rd Jul 2017, 7:12 am   #7
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Default 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).
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Old 23rd Jul 2017, 8:00 am   #8
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Default 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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Old 23rd Jul 2017, 11:48 am   #9
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Default Re: How to tell if an old GPO phone is in need of conversion

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it's up to you whether to go for conversion or return it to the original wiring shown in the diagram N841.
Thanks very much, Dave. I'm not sure whether to go for conversion or not. I plan on using the telephone with Asterisk when I get up and running. And possibly (only possibly) with my BT Versatility before that. I will probably go for it because I need a colour matched line cord anyway and they seem to go on eBay for about a fiver with the other components it looks like I'll need. Thanks
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Old 31st Jul 2017, 9:00 am   #10
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Default Re: How to tell if an old GPO phone is in need of conversion

Just out of interest...

Quote:
Insert a 3.3K resistor between T4 - T5 (object with coloured stripes on it).
What does the resistor do?
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Old 31st Jul 2017, 9:09 am   #11
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Default 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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Old 31st Jul 2017, 9:12 am   #12
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Default Re: How to tell if an old GPO phone is in need of conversion

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What does the resistor do?
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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Old 31st Jul 2017, 9:48 am   #13
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Default 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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Old 31st Jul 2017, 10:26 am   #14
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Default Re: How to tell if an old GPO phone is in need of conversion

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Originally Posted by boombox View Post

What about if the telephone is one of a few connected to a PABX? Is it needed then?
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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Old 31st Jul 2017, 11:03 am   #15
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Default 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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Old 31st Jul 2017, 11:30 am   #16
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Default Re: How to tell if an old GPO phone is in need of conversion

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One telephone on one extension of a PABX.
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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Old 31st Jul 2017, 1:43 pm   #17
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Default 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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