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Old 10th Aug 2022, 3:06 pm   #1
Telephone Guy
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Default Converting a Telephone 746 for use on a plug and socket system.

Hi folks,
Sorry to drive you all mad, but this is one of those very simple questions if you know the answer!
I'm trying to help someone else to do a 746 conversion from the old spade connectors ( at least, I think it's a 746 - haven't actually set eyes on it yet ). I'm pretty sure you can easily buy leads with spade connectors for the phone on one end and an RJ11 plug on the other ( am I right? - is it an RJ11 I'm thinking of? ), but for the life of me I can't seem to find one. I don't know if I'm just searching under the wrong thing ... but anyone know where you can get hold of a good quality, fairly inexpensive lead of this kind?
Also, am I right in saying that assuming the phone is otherwise in good working order, and assuming you wire the new lead up correctly, and assuming the line itself is in good working order and works with pulse-dialling, that's all that should be needed to convert the phone?
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Old 10th Aug 2022, 3:47 pm   #2
winston_1
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Default Re: converting a 746

RJ11 is an American plug and is not the one you want. You can buy leads though with spades to a BT plug.

Instructions are here. I don't bother with the 3.3K resistor and leave the link as it does not do what people think. It is suggested by those that don't understand the difference between impedance and resistance


https://telephonesuk.org.uk/conversion/
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Old 10th Aug 2022, 3:58 pm   #3
AC/HL
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Default Re: converting a 746

This supplier has them: https://telephonelines.net/index.php...roducts_id=416
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Old 10th Aug 2022, 4:48 pm   #4
Pellseinydd
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Default Re: converting a 746

Quote:
Originally Posted by AC/HL View Post
One shown on that website doesn't appear to have the grommet at the telephone end of the lead which fixes lead to phone.-

This one does -
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Old 11th Aug 2022, 10:24 am   #5
Telephone Guy
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Default Re: converting a 746

Thank you all for those suggestions. I think the phone in question is black, but I don't suppose the colour will matter that much.
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Old 11th Aug 2022, 10:39 am   #6
198 kHz
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Default Re: Converting a Telephone 746 for use on a plug and socket system.

Various colours available here:

https://www.britphone.net/gpo-700-se...cables-4-c.asp

Incidentally the flat BT plug is a BT431A.
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Old 16th Aug 2022, 5:49 am   #7
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Default Re: Converting a Telephone 746 for use on a plug and socket system.

All information gratefully received and dutifully passed on.
Thanks, everyone.
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Old 16th Sep 2022, 9:44 pm   #8
Oldcodger
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Default Re: converting a 746

Quote:
Originally Posted by winston_1 View Post
RJ11 is an American plug and is not the one you want. You can buy leads though with spades to a BT plug.

Instructions are here. I don't bother with the 3.3K resistor and leave the link as it does not do what people think. It is suggested by those that don't understand the difference between impedance and resistance


https://telephonesuk.org.uk/conversion/
The reason for the resistor is simple. On the GPO system , phones ( other than a plan four- one permanently wired with a bell , where all phones had a plug and could be moved to other sites ) were speech in parallel and bells in series ( up to four 1000 ohm , resistive,in parallel, so the curent in bell circuit was V/( 4000-Jx). Now we've up to four phones in circuit , resistance drops to 250 ohm so the impedance ( resistance + inductive element has to be increased to stop ring trip . another reason for only having one capacitor in circuit is ring trip, another result of having two phone+ two capacitive bell circuits in parallel . So solution is simple - increase the inductive effect of 250 -x j to reduce the bell circuit current to that of V/(1000-xj , by adding a series resistor in bell circuit. ( x is inductive element = 2*PI*F*L)( where pi= 3.14, F= frequency of ringing and L= inductance of bell circuit).
And telephone guy- plenty of phone shops sell leads with a modern plug ( RJ ?) one one end. If yu still have the old phone cable, it's simple to cut the grommet off cable at where the it enters the phone by cutting across in parallel to line of cord and carefully priseing away and then glueing grommet to new cable. All you have then to do is plug BT plug into a socket and identify which wire goes where, solder terminals to the wires and connect. OR , cut the old cord and cut /solder and heatshrink to the new cable. Many of the old cables were copper with a plastic/ cloth inner . We found best way, was to strip a portion of sheath then remove another portion , slide this to end and tin the bit between, then cut off the end .

Last edited by Oldcodger; 16th Sep 2022 at 10:01 pm.
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Old 18th Sep 2022, 2:18 pm   #9
winston_1
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Default Re: converting a 746

Adding a resistor does little as the resistive part of the bell circuit impedance is minor compared with the inductive part. Having phones or capacitors in parallel does not cause ring trip as there is no DC path.
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Old 18th Sep 2022, 9:08 pm   #10
rambo1152
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Default Re: Converting a Telephone 746 for use on a plug and socket system.

Ring trip isn't the same as line seizure, as I discovered doing my schoolboy experiments in the late 1960s. Probably only needed a few hundred μs and there was a line reversal to provide an edge for triggering one. Phone Phreaking discussions are probably verboten here so I'll leave out what my "discovery" led to.
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