GPO 746 Capacitor Problems

[Oshbarg]

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.

[Nickthedentist]

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.

[Dave Moll]

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.

[McMurdo]

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)

[Oldcodger]

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.

[Station X]

Originally Posted by Oldcodger

Beware of more than one master socket in any one installation( except as in my circuit below)

There was no circuit attached to your post.

[Tim]

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.

[dagskarlsen]

Does another phone work in the socket you want to use for your 746?

[Oldcodger]

Originally Posted by Station X

There was no circuit attached to your post.

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)

[Station X]

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.

[Oshbarg]

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?

[Station X]

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

[Dave Moll]

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.

[Oldcodger]

Originally Posted by Station X

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..

If I could remember the N diagram for a plan 1/1A ( the difference to be pedantic is that on a PL1A, there is a facility on a tele 706/746 to allow for extra phones to have the bell circuit disabled) I'd post it but in modern telecomms there's no need for this as it's simply run a cable from socket to socket and connect like wire to terminals 2/3/5.
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.