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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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9th Feb 2021, 5:29 pm | #1 |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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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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9th Feb 2021, 7:11 pm | #2 |
Triode
Join Date: Jun 2015
Location: Ipswich, Suffolk, UK.
Posts: 49
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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 |
9th Feb 2021, 7:59 pm | #3 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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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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10th Feb 2021, 12:50 am | #4 |
Octode
Join Date: Jun 2018
Location: Essex, UK.
Posts: 1,307
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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 |
10th Feb 2021, 1:20 am | #5 |
Octode
Join Date: Mar 2011
Location: Sheffield, South Yorkshire, UK.
Posts: 1,453
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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. |
10th Feb 2021, 1:45 am | #6 |
Octode
Join Date: Jun 2018
Location: Essex, UK.
Posts: 1,307
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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 |
10th Feb 2021, 9:44 pm | #7 |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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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
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10th Feb 2021, 9:46 pm | #8 |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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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.
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10th Feb 2021, 9:47 pm | #9 | |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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Re: BT 8746G DFM Phone not ringing
Cheers will do tomoz This has the 2000 ohms inside.
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10th Feb 2021, 9:50 pm | #10 | |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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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
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11th Feb 2021, 10:02 am | #11 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: BT 8746G DFM Phone not ringing
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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11th Feb 2021, 10:07 am | #12 | |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: BT 8746G DFM Phone not ringing
Quote:
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11th Feb 2021, 10:10 am | #13 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: BT 8746G DFM Phone not ringing
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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Regds, Russell W. B. G4YLI. |
12th Feb 2021, 2:03 pm | #14 | |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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Re: BT 8746G DFM Phone not ringing
Its the "G"
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12th Feb 2021, 2:04 pm | #15 |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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Re: BT 8746G DFM Phone not ringing
yes bell lever swinging fine Ohms reading of 2k on each
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12th Feb 2021, 2:08 pm | #16 | ||
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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Re: BT 8746G DFM Phone not ringing
Just tried and still no ring dial tone is fine
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12th Feb 2021, 2:16 pm | #17 | |
Diode
Join Date: Feb 2021
Location: Spalding, Lincolnshire, UK
Posts: 9
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Re: BT 8746G DFM Phone not ringing
yes I do get that pulse of 75vac on my digital meter be in brief
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12th Feb 2021, 2:39 pm | #18 | |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: BT 8746G DFM Phone not ringing
Quote:
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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12th Feb 2021, 3:20 pm | #19 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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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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Mending is better than Ending (cf Brave New World by Aldous Huxley) |