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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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10th Feb 2022, 10:23 am | #1 |
Diode
Join Date: Feb 2022
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 2
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GPO 746 Capacitor Problems
Hi All, Firstly wanted to say thanks for all the amazingly useful information on this forum, it has helped me identify why my 746 doesn't seem to ring, but I am now stuck on how to resolve the issue...
The suspect 746 doesn't ring on an extension socket in a recently built house (2015), however it does ring when plugged directly into the master socket. As the Master Socket is in a cupboard under the stairs this isn't the ideal location for a beautiful 746! I have tried using a selection of microfilters I have with the phone but none seem to let the phone ring. Opening up the phone I can see that C3 (0.9uF) exists but C1/2 is empty (see attached photo). From a previous thread I understand that a few 746's were modified to exclude this for modern phone lines. The Bell Coils are 2 x 2000ohm, so no need for the 3k3 resistor. I have also noticed when I plug the phone in through a microfilter into the extension socket I don't hear a dial tone? My questions are as follows: 1. Can someone recommend a Microfilter which has a decent capacitor which would allow the phone to be used on the extension socket? - I had a Sky Microfilter which I dismantled and looked to have some capacitors installed but couldn't see the value and the filter doesn't help the phone ring anyway. 2. What type of capacitor would I need to install between T7 and T9 to make the capacitor internal? I know its a 1.8uF but what voltage and design of capacitor would suit. (I am an electrician by trade, but mainly power systems, so not an expert on electronics!) Thanks in advance for any help / guidance you could provide. |
10th Feb 2022, 10:52 am | #2 |
Dekatron
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 17,853
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Re: GPO 746 Capacitor Problems
You could try replacing the extension socket with a “master”, which would contain the required 1.8uF capacitor to ring the bell. Just connect the two wires to 2 and 5, either way round.
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10th Feb 2022, 11:03 am | #3 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,129
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Re: GPO 746 Capacitor Problems
I assume from the above that only terminals 2 and 5 in the extension socket are wired back to the master socket. If there is a second pair in the cable (normally coloured orange/white and white/orange, with the blue/white and white/blue connected to 2 and 5 respectively), check whether these are wired to the master socket - they should be wired to 3 and 4 respectively at each end. This should save the need to acquire a second master socket.
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10th Feb 2022, 11:47 am | #4 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,273
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Re: GPO 746 Capacitor Problems
if it works on the master but not on an extension then the extension socket must be wired wrong, should be 3 wires from master to extensions, connecting 2,3,5 in each.
(crossed with previous post)
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Kevin |
11th Feb 2022, 10:34 pm | #5 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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Re: GPO 746 Capacitor Problems
Beware of more than one master socket in any one installation( except as in my circuit below) as the additional capacitance can cause ring trip ( and I've also heard of transmission problems). The wiring between sockets (should be) blue pair from master socket to slave terminals 2 & 5( with same colour =i.e white with blue marker or blue with white marker on same pin of both sockets for line ) and either orange with white marker or white with orange marker to pin 5. Reversal of the speech pair can cause ring problems.
it's quite simple running phone extensions- ( pedantics will quote wire colours as a "must" between sockets, but as long as the same coulor is used from the same numbered terminals in each socket ,then all is well) The microfilter is only needed to seperate the broadband and line as the BB will cause noise problems in the reciever and the capacitors in the telephone circuit will slow down the BB. Personally I only use one filter in my home. I install a filter into the BT master socket . I then plug my router into the BB output of the filter and make up a lead with a telephone socket to wire to another master, on pins 2 & 5. From this I extend using 3 wires ( as above) to the other slave sockets in house. |
12th Feb 2022, 10:32 am | #6 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,289
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Re: GPO 746 Capacitor Problems
There was no circuit attached to your post.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
12th Feb 2022, 8:50 pm | #7 |
Dekatron
Join Date: Jan 2003
Location: Bradford on Avon, Wiltshire, UK.
Posts: 3,310
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Re: GPO 746 Capacitor Problems
The phone is obviously OK since it rings when plugged into the master socket. I would also suspect a fault with the extension wiring or socket.
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13th Feb 2022, 8:39 am | #8 |
Heptode
Join Date: Jan 2008
Location: Hakadal, Norway
Posts: 643
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Re: GPO 746 Capacitor Problems
Does another phone work in the socket you want to use for your 746?
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15th Feb 2022, 12:08 am | #9 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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Re: GPO 746 Capacitor Problems
Apologies. For the pedantic, should have read " as per my description following", where the BT 2w line is fed to a filter. The BB side is fed to a router. The phone side is fed to a master socket and hence to the house wiring as extension sockets ( or for us old GPO blokes, a simple Plan 1A)
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16th Feb 2022, 9:30 pm | #10 |
Moderator
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Location: Ipswich, Suffolk, IP4, UK.
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Re: GPO 746 Capacitor Problems
Oldcodger.
You've described how phone lines should be wired up several times in the past. I think posting some sort of diagram would be useful to members. Sorry to use a cliché, but "a picture is worth a thousand words". I know what a Plan 1A is, but do vintage telephone newbies? I doubt many Plan 1A's have been installed in the last 30 years, except of course in enthusiast's homes. A combination of master and slave sockets is not a Plan 1A.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
20th Feb 2022, 6:33 pm | #11 |
Diode
Join Date: Feb 2022
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 2
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Re: GPO 746 Capacitor Problems
Hi all, thanks for the advice. My slave socket has twin cables in terminals 2,3 & 5, nothing in 4. I have used digital phones in the socket previously and they have rang, so its only a problem because the phone is an old 746. Any other advice?
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20th Feb 2022, 7:44 pm | #12 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
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Re: GPO 746 Capacitor Problems
Go to this webpage and scroll down to the bottom.
Then download the document "UK Telephone Socket Information". It may help. https://www.vintage-radio.info/data
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
20th Feb 2022, 9:25 pm | #13 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,129
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Re: GPO 746 Capacitor Problems
Don't worry too much about 4, as this is spare in most circumstances. Personally, I prefer to have the fourth wire in the cable anchored at each end rather than flapping about loose, but if there isn't a fourth wire, that's fine.
When you refer to "twin cables" in the terminals, is that one wire from the master and the second to another slave? You also need to check that each wire in the slave(s) is wired to the same terminal at the master.
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Mending is better than Ending (cf Brave New World by Aldous Huxley) |
4th Mar 2022, 10:54 pm | #14 | |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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Re: GPO 746 Capacitor Problems
Quote:
Whether or not a single master or extensions by adding slaves could be called plan 1 (not 1A as there's no bell prevention) is possibly pedantic. Plan 4 ? where additional teles were plugged into sockets. I could happily create diagrams of how to wire additional sockets, , and try to pitch my advice at those with no technology background. |
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