GPO Bell-set No 20 Wiring Query

[vic0239]

Following on from my other thread https://www.vintage-radio.net/forum/showthread.php?t=200232, I am now attempted to connect the bell-set to the PABX extension (Exch) and 1/232 phone (Main), however the results are not as expected.

I can dial out from the 1/232 and successfully call another extension on the PABX and can also call the 1/232. However the bell-set does not ring, but lifting the receiver of the 1/232 does connect the call. This is with the switch set to the “Speak to Exchange” position. With the switch set to the “Speak to Extn” and “Extn to Exch” positions the bell-set rings. With the switch set to the “Speak to Ext Exch held” position I get the engaged signal.

I have the red and white line wires connected to the A & B exchange terminals respectively and the 1/232 red, white and green wires to the A, B & E terminals respectively.

I’m trying to follow the Extension Plan 7 & 7A diagram, but this includes a bell-set 26 in the setup which is slightly confusing me. I’m scratching my head now, so any help would be much appreciated.

Thanks … Andy.

[Pellseinydd]

Originally Posted by vic0239

Following on from my other thread https://www.vintage-radio.net/forum/showthread.php?t=200232, I am now attempted to connect the bell-set to the PABX extension (Exch) and 1/232 phone (Main), however the results are not as expected.

I can dial out from the 1/232 and successfully call another extension on the PABX and can also call the 1/232. However the bell-set does not ring, but lifting the receiver of the 1/232 does connect the call. This is with the switch set to the “Speak to Exchange” position. With the switch set to the “Speak to Extn” and “Extn to Exch” positions the bell-set rings. With the switch set to the “Speak to Ext ; Exch held” position you will get the engaged signal.

I have the red and white line wires connected to the A & B exchange terminals respectively and the 1/232 red, white and green wires to the A, B & E terminals respectively.

I’m trying to follow the Extension Plan 7 & 7A diagram, but this includes a bell-set 26 in the setup which is slightly confusing me. IÂ’m scratching my head now, so any help would be much appreciated.

Thanks Â… Andy.

The diagram you should be wiring up to is N4309 See Page 3 for the 232 & Bellset 26 connected up as the telephone on the main and the 'external extension' is connected by a single pair with magneto rining between the 'main' and the 'extension'. Page 4 shows the setup of an 'internal' extension with fours wres between main/extension. Note that the 232 set up on the 'main' has a Bellset 26 included in its circuit. A BS26 is normally part of the set up with a Tele 232. If you do not wire one in, an incoming call will not ring any bell with the switch in some positions. The bell in the BS20 will ring if the main phone is switched through to the extension or if on a external extension with generator calls the 'main' phone.

With the switch in the “Speak to Ext Exch held” position, the exchange line to PABX will be 'looped' by the BS 20 thus causing it to be 'busy' if you dial it from another telephone not connected to the Bellset 20.

Hope that helps.

Ian J

[vic0239]

Thanks Ian, that’s the diagram I am following.

Looks like I may have misunderstood the working of the BS20.

To my (flawed?) logic as the 232 is connected to the BS20 then I assumed it’s bell would announce an incoming call if the 232 is switched to the exchange. But from what you are saying the bell on the BS20 is only used to alert an incoming call if the extension is switched to the exchange or if the extension wants to call the main, hence the need for a separate BS for the 232. Is that how it works?

I have a 232 on a BS26 so will try that or perhaps a 300 series.

Can I also ask about the difference between the internal and external extensions. From the diagram I see there is a generator needed for the external phone to call the main, but not with the internal extension. So can the internal extension call the main? What provides the ringing voltage?

Sorry if I am failing to interpret this properly, but I am going cross eyed staring at these diagrams.

PS. I did once apply for a job with the GPO as a telephone engineer and almost accepted, but my hesitation was due to my red/green colour blindness. Imagining the rats nest of this wiring in real life makes me glad I turned the job down!

[Pellseinydd]

Originally Posted by vic0239

Thanks Ian, that’s the diagram I am following.

Looks like I may have misunderstood the working of the BS20.

To my (flawed?) logic as the 232 is connected to the BS20 then I assumed it’s bell would announce an incoming call if the 232 is switched to the exchange. But from what you are saying the bell on the BS20 is only used to alert an incoming call if the extension is switched to the exchange or if the extension wants to call the main, hence the need for a separate BS for the 232. Is that how it works?

I have a 232 on a BS26 so will try that or perhaps a 300 series.

Can I also ask about the difference between the internal and external extensions. From the diagram I see there is a generator needed for the external phone to call the main, but not with the internal extension. So can the internal extension call the main? What provides the ringing voltage?

Sorry if I am failing to interpret this properly, but I am going cross eyed staring at these diagrams.

PS. I did once apply for a job with the GPO as a telephone engineer and almost accepted, but my hesitation was due to my red/green colour blindness. Imagining the rats nest of this wiring in real life makes me glad I turned the job down!

An 'internal' extension is basically a extension within the same building where a 4 wire/2 pair cable . The 'main' used the generator on the BS20 to call the extension using the bell in the extension telephone. There was a separate pair from the speech pair with a DC bell adjacent to the BS20 and a push button at the extension.

An external telephone was normally either in another building on the same site with an overhead cable or an extension on another site outside the curtilage with a one pair price wire rented from the GPO/PO/BT. Generator ringing was used in both directions - at the 'main' it was the generator in the BS20 and at the extension it was a generator separate to the telephone.

Ian J GPO Telephones in 1950/60's

[vic0239]

Thank you Ian, your explanation should now enable me to make full sense of the wiring diagrams. Many thanks for your help and for sharing your wealth of experience. Andy.

[vic0239]

I’m making some progress with this project. I am following the N4309 Issue B diagram on page 5 for an external extension and doing it in two stages. Getting the main side working first before moving on to the extension. I’ve mocked up a block terminal 20/12 so I can replicate the connections as per the diagram. Main telephone is a 1/232 with BS26.

I’m puzzled a little by one of the connections from the BS26. On the diagram the connection from terminal 8 (G) on the BS connects to terminal 3 on the BT which is strapped to terminal 4. This then becomes (B) which is connected to the extension terminal B on the BS20. I see BT terminal 9 is also marked (B).

In this configuration the BS26 doesn’t ring.

Terminal E on the BS has a connection marked BN, but there is no corresponding connection on the BT. Is this a misprint perhaps and BT terminal 4 should be marked (BN)? If I connect BT 4 to BS20 E then it all works as it should.

Hope this makes sense!

[Pellseinydd]

Originally Posted by vic0239

I’m making some progress with this project. I am following the N4309 Issue B diagram on page 5 for an external extension and doing it in two stages. Getting the main side working first before moving on to the extension. I’ve mocked up a block terminal 20/12 so I can replicate the connections as per the diagram. Main telephone is a 1/232 with BS26.

I’m puzzled a little by one of the connections from the BS26. On the diagram the connection from terminal 8 (G) on the BS connects to terminal 3 on the BT which is strapped to terminal 4. This then becomes (B) which is connected to the extension terminal B on the BS20. I see BT terminal 9 is also marked (B).

In this configuration the BS26 doesn’t ring.

Terminal E on the BS has a connection marked BN, but there is no corresponding connection on the BT. Is this a misprint perhaps and BT terminal 4 should be marked (BN)? If I connect BT 4 to BS20 E then it all works as it should.

Hope this makes sense!

Try the attached diagram!

Ian

[vic0239]

Many thanks again Ian. Your version of the diagram makes perfect sense. Onwards to the extension!

[Pellseinydd]

Not my version ! It is a genuine GPO one - not a later Post Office one like Sam Hallas' ones I keep the different editions going back to the early ones of 1917. Have fun putting the overhead wiring up to the external extension

[vic0239]

I’m still plodding along with this and have it mostly working as (I think) expected as I can carry out all the functions of the rotary switch most of the time. Sometimes the switch doesn’t appear to connect properly and I have to open/close the door which gives a click and off we go again! I haven’t done anything with the switch, so perhaps it just needs a clean and lubrication?

The other issue is bell tinkle. When dialling from the extension the BS26 on the main telephone tinkles in unison. Is this normal for this setup?

It’s been great fun playing around with this, but I’m so glad technology has moved on from this clunky setup!

[vic0239]

Ah! Might it have been the "Engaged" flag, is it gravity operated? I had the BS on its back, but now I've stood it up it is working better and the click I heard previously is the flag dropping back.

[vic0239]

Originally Posted by vic0239

The other issue is bell tinkle. When dialling from the extension the BS26 on the main telephone tinkles in unison. Is this normal for this setup?

For future reference, removing the straps between terminals A-A1 and B-B1 on the BS20 stopped the bell tinkle (Plan 7A).