GPO phones - ringing intercom project

[Nickthedentist]

Interesting… I’m running one from an x-link via the supplied 2-wire adapter, and it rings. Presumably a different variant.

[Kilders]

So I have tried again to build the project and my lack of electronic knowledge and skill is definitely showing up (I might have blown up a capacitor today!).
So I am trying to stay as true as I can to Andrew’s diagram but have different transformers (dual 12v and dual 24v PCB transformers). So far I’m getting nothing from the phones at all and I’m not sure my relays are doing the job either. Although I am getting the transformers to read the correct voltage. Hoping someone can help me tweak what’s going wrong! Please see the attached pictures for Andrew’s diagram, mine and the actual prototype build.

I’ve tried to lean on AI (less than helpful), YouTube and text books but clearly missing something. Rest assured the transformer primaries and mains inlets are soldered and well insulated.

Thanks in advance for any advice!

[AndiiT]

Originally Posted by Nickthedentist

Interesting… I’m running one from an x-link via the supplied 2-wire adapter, and it rings. Presumably a different variant.

Interesting indeed, does your "two-wire" adapter contain a ringing capacitor, perhaps?
Or possibly your Viscount is a rare PBX variant, I'm going to dig all of mine out now and do some research

Andrew

[AndiiT]

Originally Posted by Kilders

.....So I am trying to stay as true as I can to Andrew’s diagram but have different transformers (dual 12v and dual 24v PCB transformers). So far I’m getting nothing from the phones at all and I’m not sure my relays are doing the job either. Although I am getting the transformers to read the correct voltage. Hoping someone can help me tweak what’s going wrong! Please see the attached pictures for Andrew’s diagram, mine and the actual prototype build.
Thanks in advance for any advice!.......

Hi, I've looked at your diagram and can see there is something incorrect on the way you have things wired up, sadly it's difficult for me to explain in text, so I may have to out, freehand, a diagram that is as close as it is to yours showing the correct way of wiring up, briefly from what I see your issue is with one of the connections of the AC ringing transformer and also where the bridge rectifier and relay rectifier diode is connected.

Watch this space and I shall go and put quill to parchment, it may be tomorrow before I complete the task so by that time some else may have been able to assist.



Andrew

[TonyDuell]

I have attached a reverse-engineered circuit diagram of a later model of Viscount. It appears that C29 (not fitted in my example) is a ringing capacitor so the telephone could be built for a simple 2 wire line.

As for the OP's intercom circuit, what voltage do you measure across C1? What do you measure between pin 2 of one telephone and pin 2 of the other?

Does RLA1 operate when both phones are off-hook?

Do RLA2 and RLA3 operate when at least one phone is on-hook and you press the recall button on a telephone?

[AndiiT]

Hi, I've re drawn (badly, but hopefully readable) the section of the circuit where you have wired incorrectly, hopefully this should work.
Let me know as I have also drawn another option, however realised that you would only be using 24 volt ringing in that one, which probably would be OK for your Viscounts though.

Andrew

[AndiiT]

Originally Posted by TonyDuell

....It appears that C29 (not fitted in my example) is a ringing capacitor so the telephone could be built for a simple 2 wire line.....

I'm going to have to go and take one of my Viscounts to pieces now
Thanks for the diagram though.

Andrew

[Kilders]

Thank you for all the comments! I’ll give it a try next week when I get more time on my hands! My viscounts definitely have 4 wires. I’m connecting to the original phones wires for the project. Although they are the tiniest strands of copper I’ve ever seen!

[AndiiT]

Originally Posted by Kilders

.......My viscounts definitely have 4 wires. I’m connecting to the original phones wires for the project. Although they are the tiniest strands of copper I’ve ever seen!

That type of wire is intended to be connected using IDC (Insulation displacement connectors) terminals which pierce the insulation to obtain a connection.

Regarding your issue with no speech, check for around 16 volts between terminals 2 and 5 of the phone "sockets" (in your case connections) positive on terminal 2 at phone 1 and negative at terminal 2 of phone 2, then check that you have connected the correct leads from your Viscounts to the intercom circuit terminals.

Have you checked your phones for speech on a known working phone line?

Another thought I had is the phasing of the windings on T2, if you find that you have considerably less than 48 volts AC across terminals 7 and 12, the windigs will be out of phase and you need to reverse on of them e.g. link terminals 11 and 7 and take the output from terminals 12 and 8.

[Nickthedentist]

Originally Posted by Kilders

Although they are the tiniest strands of copper I’ve ever seen!

Yes, it's called "tinsel wire" and is designed to be extra-flexible and not to break even if subject to years of daily bending. As Andi says, they're meant to be terminated by Insulation Displacement Connectors, for example the ubiquitous BT "431A" phone plug, where the gold contacts' prongs are forced through the coloured insulation into the conductors. Done properly, this results in an amazingly reliable connection.

Soldering or using screw terminals will always be a bit iffy, and is never done professionally/commercially.

[Nickthedentist]

Originally Posted by Kilders

My viscounts definitely have 4 wires.

They do indeed, but not all phones with 4 wires actually have all of them connected. Many just use the outer two (usually red and white).

[Dave Moll]

Originally Posted by Nickthedentist

usually red and white

... or yellow and black.

[dagskarlsen]

This is a smart way to learn, and it seems like you have learned a lot so far. If you could tell me witch 2 wires who make it ring in onhook position it would be nice to know. The second to find out, nothing else conected than a 9V battery, witch wires is needed to hear yourselves speaking in the phone? One wir will probably be common for thhose 2 tests.

https://www.vintage-radio.net/forum/attachment.php?attachmentid=321989&d=1764171909

[AndiiT]

Originally Posted by dagskarlsen

......If you could tell me witch 2 wires who make it ring in onhook position it would be nice to know........

https://www.vintage-radio.net/forum/attachment.php?attachmentid=321989&d=1764171909

From the phone line cord (not the socket)-

White is the A leg and common to the speech and bell circuit.
Red the B leg and for speech only.
Blue is the ringing/bell/tone caller wire, and
Green is recall.

Andrew

[Tim]

I did something like this at school, to provide comms from the school lighting box to backstage. I simply used a couple of 746 handsets, a couple of buzzers and a battery9 a six volt lantern battery as I recall). It was 40 years ago though, so I don't think there is any chance of me remembering what I did. The cable did need a few cores though ISTR, and the handsets may well have been connected in series( battery+-, mic, ear======ear-mic==battery-)

[Kilders]

So I tried again using andrews advice and managed to get voice but no ringing. I’m really busy at work so when I get the chance I’ll draw the diagram I used and trouble shoot again. I suspect the relays may not be working correctly because the continuity setting on the voltmeter wouldn’t give me the tone for the NC contacts but does for the +/- contacts.

[AndiiT]

Originally Posted by Kilders

.....but no ringing.....

Hi, check for the simple stuff first, make sure that the recall switches on the underside of your Viscount phones are set to Earth Recall and not Timed break.

Andrew

[Kilders]

Originally Posted by AndiiT

Hi, check for the simple stuff first, make sure that the recall switches on the underside of your Viscount phones are set to Earth Recall and not Timed break.

Andrew

They were definitely earth recall setting. I tried to attach it according to your advice and used this diagram. If it looks correct, I’ll try again at some point and also try and test the relays for function or get a different type.

[AndiiT]

Originally Posted by Kilders

.......try and test the relays for function or get a different type.....

Hi, The circuit looks fine now, try relays with lower voltage (within reason) operating coils, also check that the relays you are currently using aren't polarity sensitive, they shouldn't be however I have encountered some strange relays in my time

Regards

Andrew

[Kilders]

Thank you all for the help I finally got it working! (By using 5v coil relays). The kids loved playing with it in prototype mode so hopefully have it fully installed in the next week(work permitting). I’ll soon post a full parts list so those like me with no knowledge can adapt Andrew’s circuit with available parts. Also will allow more experienced folks to add suggestions and improvements