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Old 15th Jul 2014, 1:10 pm   #1
craggus2000
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Default GPO 746 wiring query 2-wire

Hi all,
Just signed up as a member here, although have been taking advantage of the wealth of knowledge on the forum for a little while...
I am in the process of restoring and converting a GPO 746 with a new line cord with BT plug (4/3 wire, depending on what you like to refer to it as!).

I am one of these people who disconnected the ring/bell wire from my master socket to improve ADSL performance so therefore have a 2 wire system (not a problem until now, as all my other phones in the house are cordless and don't need the 3rd wire to ring).

I have a couple questions - obviously as it stands the GPO phone won't ring on the 2 wire system, so I wanted to know if I can do anything inside the phone to enable this. Have seen a thread that suggests (I think) putting a 1.8uF cap between the red and blue wires of the line cord inside the phone
(between T6 and T8?), is this right and all that would be required and what type of capacitor?

I have tested the phone with a BT - RJ45 master adapter, and when I call it and pickup the phone works fine but my other cordless phones in the house carry on ringing 4 or so more times before they stop. Is there any way to stop this?

I like to think, that whilst not an expert I'm pretty competent/confident, so any suggestions welcome!

Thanks very much...
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Old 15th Jul 2014, 1:52 pm   #2
Nickthedentist
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Default Re: GPO 746 wiring query 2-wire

Hello and welcome.

Answer 1: These phones were originally designed for 2-wire operation, so a suitable capacitor is already on the board. You just need to connect it appropriately. More on that later, hopefully.

Answer 2: No, that's a "feature" of your cordless phones, I'm afraid.

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Old 15th Jul 2014, 2:02 pm   #3
craggus2000
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by Nickthedentist View Post
Answer 1: These phones were originally designed for 2-wire operation, so a suitable capacitor is already on the board. You just need to connect it appropriately. More on that later, hopefully.
Oh right, I didn't realise that! Didn't seem immediately obvious from googling...

Quote:
Answer 2: No, that's a "feature" of your cordless phones, I'm afraid.
That's annoying, do you know the reasons for this?
Just found this on another forum, not sure if it helps, but maybe there is a solution

Quote:
The ringing circuit (the bit that makes the bell ring) should 'disconnect' when a call comes through and the call part of the line connects (when you pick up the receiver). The telephones detect that the ringing circuit has stopped and this should stop the phones ringing. You can have a fault with either the wiring to the extensions which has an affect of delaying the 'disconnection of the ringing circuit' or the phones them selves are faulty causing the same delay.
You mentioned that the old phones dont do this, i would try one old phone and one new phone and call the line to see if it happens, if it doesnt try the other new phone. if the line still rings after picking up the receiver when trying the 2 new phones (and an old one) it is probably the extension wiring. If it only does iot with one of the new phones it is probably that phone (ringing circuit faulty or set too sensistivly).
But I know there's no fault in my wiring...
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Old 15th Jul 2014, 2:36 pm   #4
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Default Re: GPO 746 wiring query 2-wire

Quick answer: Wire it basically as per N diagram 846; but connect a 3.3 kΩ resistor between T17 and T18. Then connect the outer two pins of the BT plug to T8 and T18. If there are 4 wires in the line cord, connect the middle two wires to T11 and T14 (which aren't connected to anything else).
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Old 15th Jul 2014, 3:14 pm   #5
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by Nickthedentist View Post
......a suitable capacitor is already on the board.
The capacitor is present in most cases but some later PCB's had the capacitor removed with the advent of PST (plug and socket wiring).

If there is no capacitor available on the PCB you can always salvage one from an old master socket and connect between terminals T9 and T7, linking terminals T7 - T8 in the phone to connect the 'B' leg (usually red of line cord) to the bell and transmission circuitry.

Regards

Andrew
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Old 15th Jul 2014, 3:18 pm   #6
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by craggus2000 View Post
I am one of these people who disconnected the ring/bell wire from my master socket to improve ADSL performance.
How does that work? If all your 'phones use two-wire connections (with their own ring capacitors), surely the "bell wire" will be open-circuit at the far end, so whether it is connected at the master end is immaterial.
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Old 15th Jul 2014, 3:22 pm   #7
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Default Re: GPO 746 wiring query 2-wire

The ringing wire acts as a very effective aerial, injecting noise at the BT socket. It also unbalances the twisted pair. Typically ADSL speeds increase by 20% if you disconnect it.
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Old 15th Jul 2014, 3:49 pm   #8
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Default Re: GPO 746 wiring query 2-wire

Try this.

Busy at work, so haven't read anyone else's posts, so apologies if I've crossed with anyone.

Nick.
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Old 15th Jul 2014, 4:08 pm   #9
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Default Re: GPO 746 wiring query 2-wire

Thanks, Paul. I hadn't thought of the noise aspect. That's a useful tip - especially given that the only things connected directly to my telephone line are a PABX (two-wire) and a couple of modern items that display caller identity (which is unfortunately lost on the PABX extensions), plus the fact that my third wire is quite a long aerial. Incidentally, does it still have the same effect if there is a bell at the far end?
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Old 15th Jul 2014, 4:23 pm   #10
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Default Re: GPO 746 wiring query 2-wire

I may - or may not - be barking up the wrong tree here but if the system is a master socket plus extension sockets, and with the "ringer" wire removed, are you not using ADSL filters on each of the phones? I thought that these incorporated a "ringing" capacitor?

Incidentally I use a replacement faceplate for my NTE5 master socket that incorporates a "master" filter and has sockets for the ADSL modem and a telephone. Extension wiring is connected to the rear of this as usual but the broadband signal is only available at the master socket (no issue for me). I find that connecting the "ringer" wire seemingly makes no difference to the ADSL speed since the extension phones have the ADSL signal filtered out.
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Old 15th Jul 2014, 4:33 pm   #11
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Default Re: GPO 746 wiring query 2-wire

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Originally Posted by bobbyball View Post
I may - or may not - be barking up the wrong tree here but if the system is a master socket plus extension sockets, and with the "ringer" wire removed, are you not using ADSL filters on each of the phones? I thought that these incorporated a "ringing" capacitor?.
Er, good point, Bobby

With your phone wired the normal (3-4 wire) way, it should ring if plugged in via a decent quality microfilter. Some of the cheapies have minute or non-existent ringing capacitors.

N.
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Old 15th Jul 2014, 7:24 pm   #12
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Default Re: GPO 746 wiring query 2-wire

Ah, but if the ringing wires are from filters in extension sockets, they will presumably be much less closely coupled to the ADSL circuit, therefore less efficient at introducing noise.
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Old 15th Jul 2014, 10:50 pm   #13
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by Nickthedentist View Post
With your phone wired the normal (3-4 wire) way, it should ring if plugged in via a decent quality microfilter. Some of the cheapies have minute or non-existent ringing capacitors
My phone extensions are from the back of a faceplate micro filter, with only wires 2 and 5 connected so still no bell...

Ideally I want something inside the phone so it can be connected to a 2 or 3/4 wire system (only pins 2 and 5 on the line cord would be live on the plug). I currently have it wired as attached also with a bell on/off switch (ironically) between T16 and T17. So starting from that point, and C2 doesn't exist on my PCB, would I move the blue wire to T11 and put a 1.8uF cap between T6 and T9, or am I barking up the wrong tree??
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Old 16th Jul 2014, 7:48 am   #14
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Default Re: GPO 746 wiring query 2-wire

Morning,
To clarify, this is how it's currently wired (now I've had a chance to draw it...)
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Old 16th Jul 2014, 8:45 am   #15
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by craggus2000 View Post
But I know there's no fault in my wiring
It's still a 'feature' / fault on your cordless phones.

You can test this by phoning yourself from a mobile (without answering) then hanging up. The wired phone should stop ringing immediately; the cordless will probably continue ringing for a couple of cycles. The handset ringers are not driven in cadence by the Exchange ringing current and the base unit waits for an absence of ringing current before telling the handsets to "stop ringing".

Cordless phones really are more trouble than they're worth. The batteries run down, they don't work in a power cut, you can lose the handsets or drop then down the WC, etc. Much better to get some wired extensions on a small PABX. You'd need a couple more phones of course, but it probably won't be long now ...
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Old 16th Jul 2014, 9:40 am   #16
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by craggus2000 View Post
My phone extensions are from the back of a faceplate micro filter, with only wires 2 and 5 connected so still no bell
If only two wires are connected, then no bell is to be expected.

Quote:
Originally Posted by craggus2000 View Post
To clarify, this is how it's currently wired (now I've had a chance to draw it...)
Looks good.

Now disconnect the blue wire from T6 (and "park" it on the unused T14), and connect a 1.8uF capacitor (nicked from a master socket?) between T6 and T8.

Nick.
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Old 16th Jul 2014, 9:51 am   #17
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Default Re: GPO 746 wiring query 2-wire

Or, just plug it in using a spare microfilter. The microfilter should contain the necessary capacitor to provide the ring signal, though they can sometimes struggle to provide enough current to ring a mechanical bell.
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Old 16th Jul 2014, 10:12 am   #18
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Default Re: GPO 746 wiring query 2-wire

Yes, there should be no problem with having the phone signal "filtered" twice AFAIK.

Quote:
Originally Posted by craggus2000 View Post
C2 doesn't exist on my PCB
But is C1 present, and 1.8uF? If so, you don't need to fit an extra capacitor, this one will do!
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Old 16th Jul 2014, 10:37 am   #19
craggus2000
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by Nickthedentist View Post
If only two wires are connected, then no bell is to be expected.
Sorry, should have stated, I was just confirming that was the case

Quote:
Originally Posted by paulsherwin View Post
Or, just plug it in using a spare microfilter. The microfilter should contain the necessary capacitor to provide the ring signal, though they can sometimes struggle to provide enough current to ring a mechanical bell.
I know I can do this, but wanted everything 'internal' to the phone.

Quote:
Originally Posted by Nickthedentist View Post
Now disconnect the blue wire from T6 (and "park" it on the unused T14), and connect a 1.8uF capacitor (nicked from a master socket?) between T6 and T8.
Like this? Although I've done it between T6 and T9 just because there are already 3 connections on T8.
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Old 16th Jul 2014, 10:40 am   #20
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Default Re: GPO 746 wiring query 2-wire

Quote:
Originally Posted by OscarFoxtrot View Post
You can test this by phoning yourself from a mobile (without answering) then hanging up
Will test this and report back.
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