Author Topic: UK GPO 746 line polarity confusion  (Read 1405 times)

Offline royalbox

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UK GPO 746 line polarity confusion
« on: January 09, 2017, 10:32:17 AM »
Hello,
Can anyone tell me the correct polarity of the white and red wires connected to a UK 746 telephone please? I'll go into more detail of why I'm asking later.
Many thanks,
Barry.

Offline andy1702

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Re: UK GPO 746 line polarity confusion
« Reply #1 on: January 09, 2017, 02:24:15 PM »
I'm not entirely sure what you're asking exactly here. I can tell you the red one should go to T8 in the phone and the white one to T18 although I don't think it really matters too much under normal circumstances.

If you're asking which way the round they go in relation to the DC speech circuit then I have no idea other than a guess that the red one carries +v.

Assuming you're using plug & socket then it also depends which way round the line connections have been wired in there.

If you need me to have a look inside a 746 just let me know.

Andy.
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Offline royalbox

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Re: UK GPO 746 line polarity confusion
« Reply #2 on: January 09, 2017, 04:32:08 PM »
Hello Andy,
Here's what I've found here: The white (T18) is negative and the red (T8) is positive. If I swap those 2 wires then when you speak or blow into the transmitter, you no longer hear yourself in the receiver.

All the phones I've checked including modern ones send some of the transmitter to the receiver, otherwise it feels like the phone is not connected.

There is a reason for all this, honest! I just don't want to get over-complicated explaining why straight away or it'll get confusing.

I'll keep searching online to see if I can find any more on this.

Thanks for your help,
Barry.

Offline andre_janew

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Re: UK GPO 746 line polarity confusion
« Reply #3 on: January 09, 2017, 06:05:05 PM »
On rotary phones made in North America, polarity doesn't matter.  On GPO phones, I have no idea.  For all I know, one polarity could cause a problem with bell tinkle whereas the other would not.  It could be a matter of using the polarity that works best for you.

unbeldi

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Re: UK GPO 746 line polarity confusion
« Reply #4 on: January 09, 2017, 06:11:37 PM »
Hello Andy,
Here's what I've found here: The white (T18) is negative and the red (T8) is positive. If I swap those 2 wires then when you speak or blow into the transmitter, you no longer hear yourself in the receiver.

All the phones I've checked including modern ones send some of the transmitter to the receiver, otherwise it feels like the phone is not connected.

There is a reason for all this, honest! I just don't want to get over-complicated explaining why straight away or it'll get confusing.

I'll keep searching online to see if I can find any more on this.

Thanks for your help,
Barry.

Does the phone have a diode (=rectifier) installed ?
I recall that this was often practiced.

Offline Jack Ryan

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Re: UK GPO 746 line polarity confusion
« Reply #5 on: January 09, 2017, 06:21:49 PM »
I just don't want to get over-complicated explaining why straight away or it'll get confusing.

Too late.

Generally some background is needed or everyone who reads the question will have a different understanding of it.

T18 is the A line.
T8   is the B line

A = Ground = 0 volts
B = Battery = -50 volts nominal

Reversing those leads will have no effect on any telephone I can think of except:

- Older DTMF signalling telephones. If the supply is reversed, the dial won't work.
- DC signalling telephones.  If the supply is reversed, the dial won't work.
- Phones that use a reversal such as payphones

A standard (as supplied) 746 is not polarity sensitive. If yours is, you have a rectifier in circuit that shouldn't be there.

If there is a good reason that your 746 is polarity sensitive, I'll think of it after I press "Post".

Regards
Jack



Offline Jack Ryan

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Re: UK GPO 746 line polarity confusion
« Reply #6 on: January 09, 2017, 06:22:54 PM »
Does the phone have a diode (=rectifier) installed ?
I recall that this was often practiced.

What was it for Unbeldi?

Jack

Online twocvbloke

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Re: UK GPO 746 line polarity confusion
« Reply #7 on: January 09, 2017, 06:40:46 PM »
I recall that this was often practiced.

I believe you're thinking of the Rectifier 205A, which wasn't related to the line polarity... :)

The Rectifier No. 205A was placed across the receiver connections inside the phone to reduce the loud pop produced when pressing the hookswitch for a follow on call to reduce the "Acoustic shock" issue that can occur (and acoustic shock is not a nice thing, having done it to myself a few times!) with the user still holding the handset to their ear. It's just two parallel diodes wired in opposite polarity to each other, and can be made using cheaply available Schottky diodes (such as the 1n400x types)... :)

Offline Jack Ryan

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Re: UK GPO 746 line polarity confusion
« Reply #8 on: January 09, 2017, 06:42:41 PM »
I believe you're thinking of the Rectifier 205A, which wasn't related to the line polarity... :)

The Rectifier No. 205A was placed across the receiver connections inside the phone to reduce the loud pop produced when pressing the hookswitch for a follow on call to reduce the "Acoustic shock" issue that can occur (and acoustic shock is not a nice thing, having done it to myself a few times!) with the user still holding the handset to their ear. It's just two parallel diodes wired in opposite polarity to each other, and can be made using cheaply available Schottky diodes (such as the 1n400x types)... :)

But that doesn't affect the polarity of the phone.

Jack

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Re: UK GPO 746 line polarity confusion
« Reply #9 on: January 09, 2017, 07:23:12 PM »
But that doesn't affect the polarity of the phone.

Jack


Hence why I said that it didn't affect line polarity, in the first line.......  ;D

There is also the regulator in there, which is a series of diodes wired in a full wave rectifier manner, but, again, this doesn't affect the polarity (in essence it actually makes it unaffected!!)... :)

Offline Jack Ryan

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Re: UK GPO 746 line polarity confusion
« Reply #10 on: January 09, 2017, 08:10:44 PM »
Hence why I said that it didn't affect line polarity, in the first line.......  ;D

There is also the regulator in there, which is a series of diodes wired in a full wave rectifier manner, but, again, this doesn't affect the polarity (in essence it actually makes it unaffected!!)... :)

Sorry, I thought you said "unrelated" which leaves open the possibility of unwanted side effects. Point taken

Perhaps the diode(s) that Unbeldi was thinking of were in the 312 (and equivalent 700) which *is* polarity sensitive. That won't work properly on a normal phone line though and the diodes didn't affect side tone.

Regards
Jack

Offline royalbox

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Re: UK GPO 746 line polarity confusion
« Reply #11 on: January 10, 2017, 05:42:53 AM »
Thanks for the replies everyone.
Here's the full details I was trying hard not to post.

I've build a pulse to DTMF converter from an open source project. The device is very similar to rotatone.

The wiring is the same and so I followed the advice on the rotatone site for fitting to a 746 and added a bridge rectifier polarity guard as they describe. The converter worked but I noticed I wasn't hearing myself in the receiver like all the phones I have tried including the converted one before converting.

After a lot of testing I found that it was due to the polarity guard. It sent positive to T18 and ground to T8. I re-wired the phone to normal and I could hear my voice in the receiver once more. I then swapped the T18 and T8 wires so the polarity would be the same as if the rectifier was installed, and again I could no longer hear my voice in the receiver. Hence why I wanted to know if the polarity was important (on the phone, not the converter) as I had read somewhere that it wasn't.

It seems strange that either rotatone got it wrong, or if the polarity doesn't matter, strange that I'm getting this problem when reversing it. Hence why I'm asking.

T18 is the A line.
T8   is the B line

A = Ground = 0 volts
B = Battery = -50 volts nominal

With the phone wired correctly (white to T18, red to T8) and with the negative lead of my multi-meter on T18 and positive on T8, I read just under 49VDC. (not minus 49).

With the polarity guard fitted as per the rotatone guide, I get -48VDC (minus).

So adding the rectifier as per the rotatone guide reverses the polarity of my phone and has the effect of not sending any signal from the transmitter to the receiver.

That's what happens here anyway.

Thanks all,
Barry.

Offline Jack Ryan

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Re: UK GPO 746 line polarity confusion
« Reply #12 on: January 10, 2017, 07:12:59 AM »
Well, there is a wiring problem somewhere but I'm not very good with the tab A slot B type instructions that the WEB site gives - I need a circuit diagram.

There is nothing in the phone that is sensitive to polarity so inserting the polarity guard and other associated wiring has just broken the phone.

As a matter of curiosity, why isn't the bridge rectifier and the zener part of the pulse to tone convertor? It would simplify installation.

Sorry, not very helpful I know.

Jack
« Last Edit: January 10, 2017, 07:29:05 AM by Jack Ryan »

Offline andy1702

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Re: UK GPO 746 line polarity confusion
« Reply #13 on: January 10, 2017, 10:07:12 AM »
Hi Barry,

I just did a bit of testing here. I got a phone (basically a 706) with red to T8, white to T18 and blue to T6. This is a normal conversion for plug & socket working. I blew down the handset and could definitely hear myself in the receiver.

Next I swapped the red and white line cord terminals around and repeated the test. I could still hear myself as expected.

I'd say take all your modified wiring off and just test the phone like this for a start. It's not beyond the realms of possibility that you might have a fault in the original phone. It could be a crack on the circuit board or even some metallic debris of some kind (often the nut off the back of the base securing screw on a 746) that's lodged itself between the board and the base and is shorting something it shouldn't be.

Another thing that might be worth doing is chacking how your sockets are wired. When you look at a standard plug you'll see it has teh capacity for 6 contacts but most likely only the middle 4 are used. Of these middle 4 the outer 2 should be the red and white line. As you look at the pins of the plug from above with the gold contacts at the top the left one should be white, next green (generally unused at the phone end), next the blue 'ring' wire and finally the red side of the line. If you trace this back through the SOCKET it's not beyond the realms of possibility that someone might have connected red and white up backwards either on the back of the socket or even done by BT on the back of your master socket where the drop cable comes into the house.

Personaly I'm not a fan of the Rotatone. If you must have outgoing tone then the Dialgizmo is better because it's in a box which you plug in between your outside line and your phones. So you can have lots of extensions with lots of phones but just one box. However, having said that, for less than the cost of either option you can pick up a cheap PBX (mione was £10) that will convert pulse to tone for dialling out, which means LOTS of phones, and inter-phone dialling!  :)

Of course if you're doing the converter for fun then carry on. I just didn't want you to be doing something you thought you needed when there are better ways to connect up old telephones.

Andy.
Call me on C*net 0246 81 290 from the UK
or (+44) 246 81 290 from the rest of the world.

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Offline royalbox

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Re: UK GPO 746 line polarity confusion
« Reply #14 on: January 10, 2017, 05:36:12 PM »
Thanks for all the replies and help, much appreciated.

Now that I know polarity shouldn't affect it I'll check all the wiring as you suggest Andy and also test with another phone. Tomorrow hopefully.

Quote
As a matter of curiosity, why isn't the bridge rectifier and the zener part of the pulse to tone convertor? It would simplify installation.

Actually, I used a jump wire where the diode is shown. Rotatone say use the diode if you don't have an electronic transmitter as it will guarantee enough voltage to run the converter. I should have mentioned that. Also, there is a 5.1V zener used to regulate the voltage in the circuit of the converter project.

I agree though, I think it would make more sense to have the rectifier as part of the circuit. I probably will on the next one then.

Here's what rotatone say about it:
Quote
When Rotatone was being developed, a decision was made not to incorporate a polarity correction circuit within Rotatone module. This decision was made for a number of reasons, the chief of which is that the polarity protection was best applied to the entire phone, and not just to the Rotatone module.

Doesn't say what other reasons.

Here's something else the rotatone instructions says:

Quote
Voltages up to 35 volts are acceptable, as the module is fitted with its own voltage regulator. Voltages however must not exceed 40 volts, so do not connect the unit on the exchange side of the hook switch where it would be exposed to ringing voltages (it would cause the line to go ‘off hook’ but this can’t be
relied on to protect the circuit). For additional surge protection, a 24v Zener diode can be installed across the line after the switchook as shown in the generic diagrams below.

Do you think it would be a good idea to add a voltage regulator like rotatone has instead of the 5.1V zener? I have a couple of 7805s here already.

Andy, PBX is something I know nothing about although I've heard it mentioned here and there. Will look it up.

I built the project as I like doing things like that and had most of the components in my bits box. Also, I have never flashed a micro-controller before and was interested in trying that. I was quite impressed that it all worked! Also wanted the 7 quick-dial numbers and ability to dial * and #.

I've posted an image of the circuit diagram of the converter in case anyone's interested in looking it over.

Thanks again for all your help,
Barry.