Converting a UK GPO746 for US/Canada

[Tribune]

Probably stating the obvious for most people on the forum here, but as I'm still something of a telephone novice I thought I'd post it up for other novices like me. And besides, I felt quite proud of myself figuring it out!

Adapting a UK PST-converted GPO 746 phone for North American use.

While the UK British Telecom system runs on a tip-and-ring 2 conductor system (wires A "earth" & B "battery") outside, once the drop cable (central office line) enters a property it becomes a three-line system. Also, UK phones don't typically have a ringer capacitor inside. Instead, the 2-conductor drop cable terminates in the house at a BT Master Socket (currently the split-face NTE5). Inside the Master Socket is where the 1.8mF 250v ring capacitor is housed as well as other bits and bobs such as a surge suppressor (for lightening protection) and a 470,000 ohm test resistor, and this is where the 2 conductors (A & B) become 3. Phones are connected by a series of either daisy-chained or spidered slave sockets that superficially resemble the Master Socket but lack the internal components. Nor does the modern UK network use RJ-11 connectors, rather they use their own system (PST) with the BT 431A or 631A plug. Interestingly in Ireland, they use the RJ-11 system but still with an externalized central ringer capacitor. Other countries such as New Zeeland and some Middle East states have adopted the BT system.







So, up until now I've had to use a BT Master Socket as an adapter to run my UK GPO 746 phones here in Canada. Basically just ran a two-conductor line with a RJ-11 on the end out the back of the Master Socket to plug into the normal tip-and-ring system at the wall. My PST-fitted British phones then plugged into the Master Socket as if they were back home on the other side of the pond. Did make for a big nasty white box kicking around on the line cord though. And by the way, altering, modifying, or installing a Master Socket yourself in the UK is illegal, but you can still buy them on the internet.

Sick of the awkward Master Socket adaptor, it was time to internalize the ring capacitors in the GPO 746s and replace the BT 431A plug with a RJ-11 spade-to-modular adaptor. No need for the surge arrestor or the test resistor, just the ring capacitor.

1)   Identify the colours of your line cord conductors. With a BT 431A plug the four conductors are 2,3,4, & 5, but 4 is not connected.
2 connects to the incoming B (battery/ring) line and 5 to the A (earth/tip)line.
Unlike on a North American 4-conductor RJ-11 plug, the main conducting pair are the outside pair on a BT 431A. The order of conductors can be identified by the diagram below. Note that colours used for the conductors over the years have varied so it is essential to correctly determine their order for your specific phone.
On my two late 1970s era 746s 2=White, 3=Green, 4=Blue, 5=Red.



2)   Identify the line cord connections inside the telephone. On my two 746s they were connected as follows:-
       No.2 (white) to T18
       No.3 (green) to T15
       No.4 (blue) to T6
       No.5 (red) to T8

3)   Disconnect No.3 (green) from T15 and isolate
Disconnect No.4 (blue) from T6 and isolate, or leave in place as it isn't used

4)   Attach a 1.8mF 250v capacitor between T6 and T8. If using a polarized capacitor the - side (typically the short lead) connects to T8. I didn't have any spare 1.8mF capacitors and didn't want to cannibalize the Master Socket I had, so used a 2.2mF 250v one instead – works fine.

5)   Slice off the BT plug from the end of the line cord and replace with a spade-to-modular RJ-11 adaptor. Connect No.2 (white) to the red (ring) terminal. Connect No.5 (red) to the green (tip) terminal. No need to connect No.3 (green) or No.4 (blue) as they aren't used, just isolate. Note that conductors in 746 line cords are very thin. I attached spade connectors only after stripping the insulation and folding back the bare conductor before crimping to ensure a connection.

Hopefully that should work on a GPO 746! If not, check you have correctly identified you line cord conductors in the right order and have the polarity of you capacitor correct. If that fails, your line cord may be attached differently inside. Just make sure that if it is, that you leave your No.2 and No.5 conductors in place, disconnect and isolate the other two conductors as necessary, and connect your capacitor between where your No.4 used to be and your No.5. That should replicate the wiring in the Master Socket. This approach would probably work on other UK phone models, but as I haven't attempted that yet I can't say with certainty. Next is to do this to my British Telecom Tribune set.

Cheers
Mark

[bingster]

An excellent post, Mark, and one that I'm sure will come in very handy for many of us.  Thanks!

[Dennis Markham]

Yes, excellent post.  I have "stickied" the topic.

[Tribune]

Well, I hope it proves useful.

Most of the background for understanding the UK wiring and master socket conventions came from UK Telephone Extension Wiring. And of course Wikipedia!

For purists who don't wish to add the capacitor to the inside of their phones, another option is to recreate the pre-1980s with the old-style hard-wired GPO 52A "soap-box" connection box at the end of the line cord. Connect the capacitor to the correct terminals in (if you can find room!)there and then run a cord out to an RJ-11. For me though, that just re-introduces another awkward box back into the line cord.

[gpo706]

Mark asked me and I hadn't clue.

Nice work Mark, I knew you'd get there.

BTW all my 500 sets work fine without too much tinkering here.

[AE_Collector]

BTW all my 500 sets work fine without too much tinkering here.

So there is an alternative to all the messy time consuming modifications. Send your GPO sets back to GPO706, where they came from!

Nice work Mark!

Terry

[gpo706]

If they get through the snow, then you're more than welcome to!

[Tribune]

Ha ha GPO! Cold enough for ya?

I guess poor Blighty's been getting a taste of real winter! Even my family down in Cornwall go snow! Well, here in Alberta , Canada, there's a good foot of snow in places, daytime highs of -15C/+5F and dropping some nights recently to -34C/-30F!!!   

[HarrySmith]

Teery, I like your new avatar, that is one nice phone! Is that in your collection?

[AE_Collector]

Teery, I like your new avatar, that is one nice phone! Is that in your collection?

Oh heck NO! I think that it is or was in Ray Kotke's collection. Other than that one which was walnut or mahogany, I have never seen a colored AE #2 desk set. They were only made between about 1928 and 1934 so must have been some of AE's very early experimenting with color. In my mind it makes sense that the brown bakelite colors would have been the first attempts at colors.

I do have a black one with chrome trim though.

Terry

[gpo706]

Ha ha GPO! Cold enough for ya?

I guess poor Blighty's been getting a taste of real winter! Even my family down in Cornwall go snow! Well, here in Alberta , Canada, there's a good foot of snow in places, daytime highs of -15C/+5F and dropping some nights recently to -34C/-30F!!!   

Mark, been up to over a foot deep, worse where its been drifting, overnight lows about -15C.

They say its the worse for 40 years, I can only recall 1979!

Post office can't deliver anything but small jiffy bags and bills, still no Moose sighted as yet  though...

[Tom B]

We've had a dusting of snow too here in Gorgeous Lancashire, and lows of -17c, but we're still mincing around in T-shirts and shorts, no problems

[migette]

Hi some good advice given I still get confused with our phones (uk) as they adopted the series ringing method. This was wiring the bells or ringers in series and using one capacitor. When one of the phones was answerd a loop was put on the line and normal tripping took place. Since the intro of modular they still used this method  and required several wires to each extn. It is interesting to note that Ireland who as far as I know did not use our 706 but adopted the Northern Electric 500 wired to UK system. Often the phones are seen at Boot Sales flee markets.  Another tip is to wire a thermister in series with the bell coil if wiring normally as this will stop bell tinkle. US phones have a tension spring which does the same thing. Hope this makes sense and adds to the topic. In uk this w/e getting warmer but they promise us more cold having to keep my greenhouses warm as another hobby is growing cacti and succulents. Seasons greetings to you all   peter

[Tribune]

I've been getting serious bell tinkle on all my British sets so adding something to stop that would be great. Excuse my ignorance, but why a thermister over a regular resistor? And what kind of rating?

[dsk]

The easiest may be to add a spring, a spring from a pen may do the job.

dsk