Problems with GPO 746 Phone - UK Vintage Radio Repair and Restoration Discussion Forum

Lady-Bea · 28th Apr 2015, 10:14 am

Hello, I am new to the forum and looking for some help please.

I recently purchased a GPO 746 phone with the redial button. The lady told me it has already been converted. When I plug it in at home I have a dial tone,
but I have two problems.
The dialler does not dial any numbers when you spin the dial but you can hear it clicking. Also it will not hang up, after pressing the receiver up and down,
it will not go back to the dial tone, it just asks you to replace the handset and try again and then does a horrible screeching sound.

I have enclosed 3 photo's of the wiring inside, to see if anybody can help me to see what the problem could be ?

Many Thanks in advance
Tracey

Dave Moll · 28th Apr 2015, 3:09 pm

Inability to dial: if you have a multimeter (preferrably non-digital), set it to ohms and connect it between T8 and T10 with the telephone disconnected from the telephone line. When you turn and release the dial, the meter's pointer should quiver as the pulse contacts open and close. If the reading remains steady at low resistance, either the contacts are not opening properly or there is something shorting them out.

Not going on-hook: still disconnected from the telephone line, check reading between T2 and T6. This should be high-resistance with the handset in its cradle (or the cradle contacts manually pressed) and low-resistance with the contacts released. There is a second pair of cradle contacts, but only one side goes to a terminal (T19 and its linked terminals), the other side being internally connected and difficult to access without dismantling the 'phone. Others may be able to advise if you need to check its function.

By the way, if you press the cradle contacts while connected to the telephone line and listening to the earpiece, does the sound in the earpiece go dead with the contacts pressed? If so, the cradle switch is probably working and the bell is shorting out the line.

Incidentally, the button isn't a "redial" function. This facility isn't available available with the 746, as it has no means of storing the digits dialled. It is intended as "recall" to re-establish dial tone with certain exchange functions, but yours appears not to be wired up anyway (if the three wires in a black sleeve are its connections), as two are connected together and the third is connected to a spare terminal.

Lady-Bea · 29th Apr 2015, 9:14 am

Hi Dave,

Thanks for your reply. I only have a digital meter, but here are my results:-
When metering between points T8 & T10 and turning the dialler, you do get a resistance change going up and down when dialing.

However when I meter across T2 & T6 it's 0.3 ohms when the receiver is up and no resistance at all when the receiver is down ?

And when the receiver is down the sound in the earpiece does go dead, but does not cut the phone off as when i pick it up again, it's telling me to replace the handset

Any more pointers or things I could test ?

Also I am on Virgin Media. Is that an issue or not?

Thanks Tracey

AndiiT · 29th Apr 2015, 11:32 am

Hi,
I would also check the on hook/off hook resistance between T8 and T18 (Red and White wires of the line cord. On hook you should have an open circuit reading and off hook should be in the region of 300 and 600 ohms (certainly less than 1k, but as you can obtain dial tone the exact reading isn't important in this case)
The cradle switch has more than one set of contacts and it's likely that not all of these are being released in your phone.

Are you familiar with the location of the "Linesmans latch" (if one is fitted) as this will enable you to place the phone on hook whilst the cover is removed and may also allow you to see if the hook switch operating levers are pushing the cradle switch down far enough. Some information about the latch can be found here and a picture shown in fig.10

Your dialling out issue may be as a result of you having a Virgin media line see this thread for more information.

Regards

Andrew

Nickthedentist · 29th Apr 2015, 2:06 pm

Further to what Andrew says, could you post a picture of the microswitch that's operated when you put the receiver down, and its associated lever and spring, taken from the left hand side of the phone with its covers off, please?

ThePillenwerfer · 29th Apr 2015, 3:21 pm

The wiring on the circuit board looks OK. It may be worth checking the the dial is correctly wired as well, there's a diagram here http://telephonesuk.co.uk/documents/700+WIRING+INSTRUCTIONS.pdf. You'll notice that there's a 3.3k resistor on it which your 'phone doesn't have. This WON'T be causing your current problems so don't worry about it.

- Joe

Dave Moll · 29th Apr 2015, 7:19 pm

Quote:

Originally Posted by Lady-Bea

no resistance at all when the receiver is down ?

By "no resistance" do you mean zero ohms or infinity? If the latter, that sounds correct, as there should be no DC path when the handset is on-hook.

28th Apr 2015, 10:14 am	#1
Lady-Bea Retired Dormant Member Join Date: Apr 2015 Location: Surrey, UK. Posts: 2	Problems with GPO 746 Phone Hello, I am new to the forum and looking for some help please. I recently purchased a GPO 746 phone with the redial button. The lady told me it has already been converted. When I plug it in at home I have a dial tone, but I have two problems. The dialler does not dial any numbers when you spin the dial but you can hear it clicking. Also it will not hang up, after pressing the receiver up and down, it will not go back to the dial tone, it just asks you to replace the handset and try again and then does a horrible screeching sound. I have enclosed 3 photo's of the wiring inside, to see if anybody can help me to see what the problem could be ? Many Thanks in advance Tracey Attached Thumbnails

28th Apr 2015, 3:09 pm	#2
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,130	Re: Problems with GPO 746 Phone Inability to dial: if you have a multimeter (preferrably non-digital), set it to ohms and connect it between T8 and T10 with the telephone disconnected from the telephone line. When you turn and release the dial, the meter's pointer should quiver as the pulse contacts open and close. If the reading remains steady at low resistance, either the contacts are not opening properly or there is something shorting them out. Not going on-hook: still disconnected from the telephone line, check reading between T2 and T6. This should be high-resistance with the handset in its cradle (or the cradle contacts manually pressed) and low-resistance with the contacts released. There is a second pair of cradle contacts, but only one side goes to a terminal (T19 and its linked terminals), the other side being internally connected and difficult to access without dismantling the 'phone. Others may be able to advise if you need to check its function. By the way, if you press the cradle contacts while connected to the telephone line and listening to the earpiece, does the sound in the earpiece go dead with the contacts pressed? If so, the cradle switch is probably working and the bell is shorting out the line. Incidentally, the button isn't a "redial" function. This facility isn't available available with the 746, as it has no means of storing the digits dialled. It is intended as "recall" to re-establish dial tone with certain exchange functions, but yours appears not to be wired up anyway (if the three wires in a black sleeve are its connections), as two are connected together and the third is connected to a spare terminal. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

29th Apr 2015, 9:14 am	#3
Lady-Bea Retired Dormant Member Join Date: Apr 2015 Location: Surrey, UK. Posts: 2	Re: Problems with GPO 746 Phone Hi Dave, Thanks for your reply. I only have a digital meter, but here are my results:- When metering between points T8 & T10 and turning the dialler, you do get a resistance change going up and down when dialing. However when I meter across T2 & T6 it's 0.3 ohms when the receiver is up and no resistance at all when the receiver is down ? And when the receiver is down the sound in the earpiece does go dead, but does not cut the phone off as when i pick it up again, it's telling me to replace the handset Any more pointers or things I could test ? Also I am on Virgin Media. Is that an issue or not? Thanks Tracey

29th Apr 2015, 11:32 am	#4
AndiiT Octode Join Date: Aug 2005 Location: Saltburn-East, Cleveland, UK. Posts: 1,786	Re: Problems with GPO 746 Phone Hi, I would also check the on hook/off hook resistance between T8 and T18 (Red and White wires of the line cord. On hook you should have an open circuit reading and off hook should be in the region of 300 and 600 ohms (certainly less than 1k, but as you can obtain dial tone the exact reading isn't important in this case) The cradle switch has more than one set of contacts and it's likely that not all of these are being released in your phone. Are you familiar with the location of the "Linesmans latch" (if one is fitted) as this will enable you to place the phone on hook whilst the cover is removed and may also allow you to see if the hook switch operating levers are pushing the cradle switch down far enough. Some information about the latch can be found here and a picture shown in fig.10 Your dialling out issue may be as a result of you having a Virgin media line see this thread for more information. Regards Andrew

29th Apr 2015, 2:06 pm	#5
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,858	Re: Problems with GPO 746 Phone Further to what Andrew says, could you post a picture of the microswitch that's operated when you put the receiver down, and its associated lever and spring, taken from the left hand side of the phone with its covers off, please?

29th Apr 2015, 3:21 pm	#6
ThePillenwerfer Octode Join Date: Mar 2011 Location: Sheffield, South Yorkshire, UK. Posts: 1,453	Re: Problems with GPO 746 Phone The wiring on the circuit board looks OK. It may be worth checking the the dial is correctly wired as well, there's a diagram here http://telephonesuk.co.uk/documents/700+WIRING+INSTRUCTIONS.pdf. You'll notice that there's a 3.3k resistor on it which your 'phone doesn't have. This WON'T be causing your current problems so don't worry about it. - Joe __________________ "Experts are only good at one thing: explaining why something will not work!"