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Old 9th Feb 2021, 5:29 pm   #1
pigunderaroof
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Default BT 8746G DFM Phone not ringing

This phone has been in the family since the 1970s and worked fine until 2004 at my Uncles Flat in Hackney. After he died I took it home. It needed a new handset and I fitted one. It has a dialling tone, will make a call but will not ring. I have looked at this and other sites and seeing as I only touched 4 wires for the new handset, cant fathom it out. I am ex copier /fax/PC/litho technician be it a rusty vintage one, so have some lecky savvy. 2000 ohm resistors are under the dial times 2. See attached photo for how it is right now. We have had BT out to rewire the 2 in hosue sockets on a router issue and that and existing phones all work fine New master socket. This has been tried in the master socket. Many Thanks in advance
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Old 9th Feb 2021, 7:11 pm   #2
WayneL74
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Default Re: BT 8746G DFM Phone not ringing

Hi there.

Wiring looks ok.

Can you move the bell clapper from side to side quite easily?

This sometimes corrodes & sizes up.

If that is ok then we need to do some electrical tests. Do you have & know how to use a multimeter?

Regards,

Wayne
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Old 9th Feb 2021, 7:59 pm   #3
Dave Moll
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Default Re: BT 8746G DFM Phone not ringing

Does the socket into which it is plugged provide a ringing signal on that blue wire. If you have an analogue voltmeter with AC setting there should be pulses of 75VAC or so between the blue wire on T6 and the white one on T16/17/18/19 when ringing is being received.

Many modern 'phones have gone back to using their own ringing capacitor rather than using the one in the master socket, so don't require use of the blue wire, but this 'phone doesn't have one of its own (note the empty area marked C1/2 on the circuit board) so does need the blue wire for its ringing signal.
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Old 10th Feb 2021, 12:50 am   #4
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Default Re: BT 8746G DFM Phone not ringing

I'm not the most experienced in this, but doesn't T4 and T5 need a 3.3Kohm resistor, not a jumper? Not sure if that'd make much of a difference, if any

'77
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Old 10th Feb 2021, 1:20 am   #5
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Default Re: BT 8746G DFM Phone not ringing

You only 'need' the extra resistor if the bell coils are the old 500Ω ones. Instruments with a number starting with 8 are from the early-'80s after the introduction of plugs and sockets and the three-wire system and had 2,000Ω ones from new.

As 2,000Ω ones are made of more and thinner wire it'd be worth checking that one coil at least isn't open circuit.
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Old 10th Feb 2021, 1:45 am   #6
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Default Re: BT 8746G DFM Phone not ringing

Oh thats interesting, i thought they all needed it, regardless of age. Learn something new everyday!

Other than that, I'm of no help. Good luck on fixing it though!

'77
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Old 10th Feb 2021, 9:44 pm   #7
pigunderaroof
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Default Re: BT 8746G DFM Phone not ringing

Hi Wayne sorry for delay I never got notified of this. Yes can use a multimeter and will have a look tomoz Cheers for help


Quote:
Originally Posted by WayneL74 View Post
Hi there.

Wiring looks ok.

Can you move the bell clapper from side to side quite easily?

This sometimes corrodes & sizes up.

If that is ok then we need to do some electrical tests. Do you have & know how to use a multimeter?

Regards,

Wayne
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Old 10th Feb 2021, 9:46 pm   #8
pigunderaroof
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Default Re: BT 8746G DFM Phone not ringing

Hi would that matter if it worked fine in 2004?. All I have changed since then is the handset. I know tecnlgy changes over time but it did work fine When Uncle died we just unplugged it and its been in a box here since then.


Quote:
Originally Posted by 19Seventy7 View Post
I'm not the most experienced in this, but doesn't T4 and T5 need a 3.3Kohm resistor, not a jumper? Not sure if that'd make much of a difference, if any

'77
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Old 10th Feb 2021, 9:47 pm   #9
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Default Re: BT 8746G DFM Phone not ringing

Cheers will do tomoz This has the 2000 ohms inside.



Quote:
Originally Posted by ThePillenwerfer View Post
You only 'need' the extra resistor if the bell coils are the old 500Ω ones. Instruments with a number starting with 8 are from the early-'80s after the introduction of plugs and sockets and the three-wire system and had 2,000Ω ones from new.

As 2,000Ω ones are made of more and thinner wire it'd be worth checking that one coil at least isn't open circuit.
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Old 10th Feb 2021, 9:50 pm   #10
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Default Re: BT 8746G DFM Phone not ringing

Just curious but two year ago I tried this and it would not ring Since then about six months ago had new master socket and slave fitted and wiring all checked by BT. Still don't ring but will check what you say ta



Quote:
Originally Posted by Dave Moll View Post
Does the socket into which it is plugged provide a ringing signal on that blue wire. If you have an analogue voltmeter with AC setting there should be pulses of 75VAC or so between the blue wire on T6 and the white one on T16/17/18/19 when ringing is being received.

Many modern 'phones have gone back to using their own ringing capacitor rather than using the one in the master socket, so don't require use of the blue wire, but this 'phone doesn't have one of its own (note the empty area marked C1/2 on the circuit board) so does need the blue wire for its ringing signal.
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Old 11th Feb 2021, 10:02 am   #11
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Default Re: BT 8746G DFM Phone not ringing

Quote:
Originally Posted by ThePillenwerfer View Post
'Instruments with a number starting with 8 are from the early-'80s after the introduction of plugs and sockets and the three-wire system and had 2,000Ω ones from new.'
Unless you have an 8746F or an 8746R, that is. They have the low-impedance bell! The devil is in the detail.
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Old 11th Feb 2021, 10:07 am   #12
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Default Re: BT 8746G DFM Phone not ringing

Quote:
Originally Posted by Dave Moll View Post

Many modern 'phones have gone back to using their own ringing capacitor rather than using the one in the master socket, so don't require use of the blue wire, but this 'phone doesn't have one of its own (note the empty area marked C1/2 on the circuit board) so does need the blue wire for its ringing signal.
You could prove this by trying your 8746G through a plug-in ADSL filter, if you have one, which has a built-in ringing capacitor.
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Old 11th Feb 2021, 10:10 am   #13
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Default Re: BT 8746G DFM Phone not ringing

Quote:
Originally Posted by 19Seventy7 View Post
'Not sure if that'd make much of a difference, if any'
It drops about half the ringing voltage across the resistor, so it can make a difference, depending on load and line conditions. If you can get away without replacing the strap with a resistor, then do so.

Remember, the original mod was for Trimphones with the electronic warbler, not telephones with a magneto-bell. Mind, the bell will ring satisfactorily down to 12mA and sometimes less.
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Old 12th Feb 2021, 2:03 pm   #14
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Default Re: BT 8746G DFM Phone not ringing

Its the "G"



Quote:
Originally Posted by russell_w_b View Post
Quote:
Originally Posted by ThePillenwerfer View Post
'Instruments with a number starting with 8 are from the early-'80s after the introduction of plugs and sockets and the three-wire system and had 2,000Ω ones from new.'
Unless you have an 8746F or an 8746R, that is. They have the low-impedance bell! The devil is in the detail.
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Old 12th Feb 2021, 2:04 pm   #15
pigunderaroof
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Default Re: BT 8746G DFM Phone not ringing

yes bell lever swinging fine Ohms reading of 2k on each

Quote:
Originally Posted by WayneL74 View Post
Hi there.

Wiring looks ok.

Can you move the bell clapper from side to side quite easily?

This sometimes corrodes & sizes up.

If that is ok then we need to do some electrical tests. Do you have & know how to use a multimeter?

Regards,

Wayne
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Old 12th Feb 2021, 2:08 pm   #16
pigunderaroof
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Default Re: BT 8746G DFM Phone not ringing

Just tried and still no ring dial tone is fine



Quote:
Originally Posted by russell_w_b View Post
Quote:
Originally Posted by Dave Moll View Post

Many modern 'phones have gone back to using their own ringing capacitor rather than using the one in the master socket, so don't require use of the blue wire, but this 'phone doesn't have one of its own (note the empty area marked C1/2 on the circuit board) so does need the blue wire for its ringing signal.
You could prove this by trying your 8746G through a plug-in ADSL filter, if you have one, which has a built-in ringing capacitor.
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Old 12th Feb 2021, 2:16 pm   #17
pigunderaroof
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Default Re: BT 8746G DFM Phone not ringing

yes I do get that pulse of 75vac on my digital meter be in brief


Quote:
Originally Posted by Dave Moll View Post
Does the socket into which it is plugged provide a ringing signal on that blue wire. If you have an analogue voltmeter with AC setting there should be pulses of 75VAC or so between the blue wire on T6 and the white one on T16/17/18/19 when ringing is being received.

Many modern 'phones have gone back to using their own ringing capacitor rather than using the one in the master socket, so don't require use of the blue wire, but this 'phone doesn't have one of its own (note the empty area marked C1/2 on the circuit board) so does need the blue wire for its ringing signal.
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Old 12th Feb 2021, 2:39 pm   #18
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Default Re: BT 8746G DFM Phone not ringing

Quote:
Originally Posted by pigunderaroof View Post
yes I do get that pulse of 75vac on my digital meter be in brief
Can you measure similarly right across the bell bobbin terminals? Check across the wires leading from each bobbin to the circuit board.

The UK ringing cadence is only 400 milliseconds on - 200 milliseconds off then 400 milliseconds on again so it is very brief. An analogue meter might read better.
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Old 12th Feb 2021, 3:20 pm   #19
Dave Moll
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Default Re: BT 8746G DFM Phone not ringing

With the telephone not plugged in, it may also be worth checking the resistance reading across each bobbin of the ringer. It sounds horribly as though one may have gone open circuit, unless there is a dodgy joint somewhere in their wiring.
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