GPO 746 Fault - UK Vintage Radio Repair and Restoration Discussion Forum

andyw25 · 29th May 2013, 3:07 pm

Hi all, just finished converting a 746F and there appears to be an issue.

There is dial tone when you lift receiver but upon dialling a number the tone doesn't disappear; each digit I dial performs the correct number of pulses but then the tone comes back before I have even started to dial the next digit.

Does anyone know what could be causing this issue?

Many Thanks, Andy

Station X · 29th May 2013, 4:30 pm

Could it be that your exchange doesn't accept pulse dialling?

andyw25 · 3rd Jan 2014, 12:25 am

Been very hectic, but I've been reminded of this thread due to getting another GPO phone, a brown 8746. It has the same problem, upon dialling out I am only able to dial so far and the dial tone comes back. I've managed occasionally to dial 1471, but attempting to dial numbers of further length has not been achieved.

My line does support pulse dialling as I have a green 8746 which works perfectly, and dials any number. Does anyone have any suggestions, I've still got the same problem with the 746F. The wiring has been checked on the brown 8746 dial and I have even cleaned the contacts but to no avail.

russell_w_b · 3rd Jan 2014, 12:36 pm

Does the dial on your brown 8746 return to rest at the same time as the dial on your green 8746 if zero is dialled on both at the same time? If you have an analogue multimeter on ohms, you can put it across your dial contacts and check the 'dwell angle' when zero is dialled. Should be 10 pulses per second ideally.

You could compare the mean position of your multimeter pointer across your 'good' dial with that of your suspect dial.

russell_w_b · 3rd Jan 2014, 3:40 pm

Out of interest, I set up a simple experiment...

12V d.c. supply in series with an AVO7 analogue multimeter and a telephone socket, into which I plugged either an 'Ascom Berkshire' set to LD, or an 8746 instrument with the mic shorted out. I used the AVO7 because:

a): it has a faster-responding movement than many analogue multimeters with a similar scale plate size, and:-

b): It has the useful 'Q' knob to set the pointer to 100% FSD.

And I drove current through the telephone because the Ascom Berkshire is electronic and won't work without any!

The figures I got when I dialled 0 - 1 were as follows (as accurate as I could detect):

Ascom Berkshire - - - - 8746

0 = 22% FSD --------- 28% FSD
9 = 24% FSD --------- 30% FSD
8 = 26% FSD --------- 32% FSD
7 = 28% FSD --------- 33% FSD
6 = 33% FSD --------- 35% FSD
5 = 36% FSD --------- 40% FSD
4 = 44% FSD --------- 46% FSD
3 = 52% FSD --------- 55% FSD
2 = 62% FSD --------- 68% FSD
1 = 73% FSD --------- 84% FSD

Now... The BT standard for LD signalling is 10 PPS, as said (+ / - 1) and the ratio of break-time to overall pulse period is 67% +5% -4% with a pulse-shape determined by that produced with a shaping circuit of a 100R resistor in series with a 1u8 capacitor across the pulse contacts with between 1V and 12.5V applied.

From the above I conclude that my dial 8746 is more accurate than the electronic Ascom Berkshire! I realise that what I set up is only a kludge, and that proper instruments are available for dial timing. Perhaps the easiest way, as I suggested earlier, is simply to compare a working dial telephone with a suspect dial telephone.

I don't know the LD tolerances of other network operators.

Dave Moll · 3rd Jan 2014, 3:54 pm

Andy, I notice that you are located in D & G - across the Solway Firth from me. If you happen to travel to West Cumbria at some time, you would be welcome to pay me a visit, when I would be more than happy to plug your telephones into some of my test equipment (such as pulse timer, digit checker etc.)

russell_w_b · 7th Jan 2014, 12:01 am

Quote:

Originally Posted by andyw25

Thanks for everyone's help and offers. iI have managed to get them working finally.

Excellent! What did you need to do to get it to work?

Quote:

Originally Posted by andyw25

I do have another phone iI am stuck with, a 782R Push button. It is completely dead...'

I have an 8782, and it's just like a 746 but with a PB DTMF unit rather than a dial. Try connecting your standard BT plug as you would a 746, then link T8 to T10 and T19B to T19A (if you have these terminals). You should then be able to pick up the handset and hear dial tone with the MF dialling unit out of circuit. If you hear dial tone, the fault will likely be on your keypad.
http://www.samhallas.co.uk/repositor.../0000/N882.pdf

AC/HL · 7th Jan 2014, 3:35 pm

Several posts which changed to a different phone mid thread moved here:
https://www.vintage-radio.net/forum/...d.php?t=102604

andyw25 · 8th Jan 2014, 10:48 pm

Just a quick update to the 746. I fixed it by adjusting the two contacts that close during dialling. I gently moved them closer together and the phone now dials any length of number perfectly.

Thanks again for all the help

Andy

Dave Moll · 9th Jan 2014, 8:56 am

I'm glad to know that you now have it sorted.

29th May 2013, 3:07 pm	#1
andyw25 Pentode Join Date: May 2009 Location: Dumfries & Galloway, UK. Posts: 101	GPO 746 Fault Hi all, just finished converting a 746F and there appears to be an issue. There is dial tone when you lift receiver but upon dialling a number the tone doesn't disappear; each digit I dial performs the correct number of pulses but then the tone comes back before I have even started to dial the next digit. Does anyone know what could be causing this issue? Many Thanks, Andy Last edited by Mike Phelan; 29th May 2013 at 4:14 pm.

29th May 2013, 4:30 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: GPO 746 Fault Could it be that your exchange doesn't accept pulse dialling? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

3rd Jan 2014, 12:25 am	#3
andyw25 Pentode Join Date: May 2009 Location: Dumfries & Galloway, UK. Posts: 101	Re: GPO 746 Fault Been very hectic, but I've been reminded of this thread due to getting another GPO phone, a brown 8746. It has the same problem, upon dialling out I am only able to dial so far and the dial tone comes back. I've managed occasionally to dial 1471, but attempting to dial numbers of further length has not been achieved. My line does support pulse dialling as I have a green 8746 which works perfectly, and dials any number. Does anyone have any suggestions, I've still got the same problem with the 746F. The wiring has been checked on the brown 8746 dial and I have even cleaned the contacts but to no avail. Last edited by AC/HL; 3rd Jan 2014 at 12:38 am. Reason: Single long sentance split up.

3rd Jan 2014, 12:36 pm	#4
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: GPO 746 Fault Does the dial on your brown 8746 return to rest at the same time as the dial on your green 8746 if zero is dialled on both at the same time? If you have an analogue multimeter on ohms, you can put it across your dial contacts and check the 'dwell angle' when zero is dialled. Should be 10 pulses per second ideally. You could compare the mean position of your multimeter pointer across your 'good' dial with that of your suspect dial. __________________ Regds, Russell W. B. G4YLI.

3rd Jan 2014, 3:40 pm	#5
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: GPO 746 Fault Out of interest, I set up a simple experiment... 12V d.c. supply in series with an AVO7 analogue multimeter and a telephone socket, into which I plugged either an 'Ascom Berkshire' set to LD, or an 8746 instrument with the mic shorted out. I used the AVO7 because: a): it has a faster-responding movement than many analogue multimeters with a similar scale plate size, and:- b): It has the useful 'Q' knob to set the pointer to 100% FSD. And I drove current through the telephone because the Ascom Berkshire is electronic and won't work without any! The figures I got when I dialled 0 - 1 were as follows (as accurate as I could detect): Ascom Berkshire - - - - 8746 0 = 22% FSD --------- 28% FSD 9 = 24% FSD --------- 30% FSD 8 = 26% FSD --------- 32% FSD 7 = 28% FSD --------- 33% FSD 6 = 33% FSD --------- 35% FSD 5 = 36% FSD --------- 40% FSD 4 = 44% FSD --------- 46% FSD 3 = 52% FSD --------- 55% FSD 2 = 62% FSD --------- 68% FSD 1 = 73% FSD --------- 84% FSD Now... The BT standard for LD signalling is 10 PPS, as said (+ / - 1) and the ratio of break-time to overall pulse period is 67% +5% -4% with a pulse-shape determined by that produced with a shaping circuit of a 100R resistor in series with a 1u8 capacitor across the pulse contacts with between 1V and 12.5V applied. From the above I conclude that my dial 8746 is more accurate than the electronic Ascom Berkshire! I realise that what I set up is only a kludge, and that proper instruments are available for dial timing. Perhaps the easiest way, as I suggested earlier, is simply to compare a working dial telephone with a suspect dial telephone. I don't know the LD tolerances of other network operators. __________________ Regds, Russell W. B. G4YLI.

3rd Jan 2014, 3:54 pm	#6
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: GPO 746 Fault Andy, I notice that you are located in D & G - across the Solway Firth from me. If you happen to travel to West Cumbria at some time, you would be welcome to pay me a visit, when I would be more than happy to plug your telephones into some of my test equipment (such as pulse timer, digit checker etc.) __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

7th Jan 2014, 3:35 pm	#8
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: GPO 746 Fault Several posts which changed to a different phone mid thread moved here: https://www.vintage-radio.net/forum/...d.php?t=102604

8th Jan 2014, 10:48 pm	#9
andyw25 Pentode Join Date: May 2009 Location: Dumfries & Galloway, UK. Posts: 101	Re: GPO 746 Fault Just a quick update to the 746. I fixed it by adjusting the two contacts that close during dialling. I gently moved them closer together and the phone now dials any length of number perfectly. Thanks again for all the help Andy

9th Jan 2014, 8:56 am	#10
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: GPO 746 Fault I'm glad to know that you now have it sorted. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)