GPO 746 wiring query 2-wire

[craggus2000]

Re-done diagram now, so T14 and T15 and blue and green respectively...

Sorry...

[Nickthedentist]

Looks spot-on as far as I can tell at a quick glance. Try it!

Did you see post 20?

Nick

[craggus2000]

Hi Nick,
Yeah I did. I'm not sure what C1 value is (can't see it at the mo). Although I thought it was supposed to be 0.9uF? But also was thinking it might be better to use a modern equivalent?

Craig

[Nickthedentist]

Some phones had one 1.8uF, others had two 0.9uF in parallel, which is essentially the same thing. Yours will be the former.

No, the old cap will be fine. They were very good quality and don't seem to fail with age, even in phones that have been stored in awful conditions.

Make use of it!

Nick.

[craggus2000]

Originally Posted by Nickthedentist

No, the old cap will be fine. They were very good quality and don't seem to fail with age, even in phones that have been stored in awful conditions

OK! so assuming it's ok, Just linking T6 and T7 should solve it? (attached)

[Nickthedentist]

Yes, looks right. Try it and see. The nice thing about this kind of phone is that they're almost indestructable so you won't do any damage.

N.

[craggus2000]

Will give it a go tonight if i get a chance, and let you know...

Next step is to add a rotatone...

[craggus2000]

Ok, I re-wired the 746 last night to use the internal capacitor for ringing, as you said it was a 1.8uF. It works!

However, when replacing the phone on the hook (or just lifting and replacing at any time), a single strike of the bell is heard. Is this just a case of adjusting the bells or could there be something odd going on with the wiring?

[Nickthedentist]

Well done!

As for your single strike; that's absolutely normal. It's a good way of seeing if they're using a real phone in a film or TV programme. If it goes "ding" as the receiver is lifted or replaced, then it probably is a working phone as opposed to a sound-effects CD.

The only problem you're likely to get with this wiring arrangement is if you use multiple dial phones on the same line, when you might get "bell tinkle" i.e. a series of strikes on the other phones' bells corresponding to each dial pulse.

N.

[craggus2000]

Thanks!

Originally Posted by Nickthedentist

The only problem you're likely to get with this wiring arrangement is if you use multiple dial phones on the same line, when you might get "bell tinkle" i.e. a series of strikes on the other phones' bells corresponding to each dial pulse.

Is there any way to get around this with the phone wired as a 2-wire phone?

[Nickthedentist]

Not as far as I know, but some of the others are more expert than me, so see what they have to say.

[Station X]

You could wire a Thermistor 1A/1 in series with the bell:-

http://www.samhallas.co.uk/collection/bits_706_4.htm

Sorry, but I don't know what modern thermistor you can use, but someone on here will know.

[craggus2000]

Ahh, Presumably if I install a rotatone (pulse to tone converter) in the phone I wouldn't get tinkle?

[julie_m]

Originally Posted by craggus2000

However, when replacing the phone on the hook (or just lifting and replacing at any time), a single strike of the bell is heard. Is this just a case of adjusting the bells or could there be something odd going on with the wiring?

That is absolutely normal. The energy stored in the ringing capacitor has to go somewhere when the bell circuit is re-established; it takes the easy route through the bell coils and pulls the clapper over to one side or the other, resulting in a single "ting".

A GPO engineer probably would fob you off by saying that was to let you know the receiver was set down properly and so you wouldn't be charged any more for the call. And I bet more than one person has by happenstance had the stored charge exactly cancel the DC bias on the line, resulting in no "ting", and picked the receiver up and hung it up again!

Bell tinkle is pretty much inevitable with two-wire phones and loop-disconnect dialling. The only cures are: three-wire system, shunting out all the bells in parallel while any phone is off the hook; four-wire system, series-wired bell chain, interrupted while any phone is off-hook; or DTMF.

[craggus2000]

Originally Posted by Station X

You could wire a Thermistor 1A/1 in series with the bell

Ahh yes, I removed a pretty ropey looking 1A/1 from the phone when I first converted it, anyone know the modern equivalent?

[Station X]

Just because it looks ropey doesn't mean it's non-functional. Try it and see what happens.

[craggus2000]

Originally Posted by Station X

Just because it looks ropey doesn't mean it's non-functional. Try it and see what happens.

Need to see if I still have it though!

[Dave Moll]

Originally Posted by craggus2000

Ahh, Presumably if I install a rotatone (pulse to tone converter) in the phone I wouldn't get tinkle?

This would stop this 'phone causing bell-tinkle on other 'phones, as it is no longer sending loop-disconnect pulses onto the line. As far as I know, though, this won't stop other LD 'phones on the same same causing tinkle on this one.

[AndiiT]

Originally Posted by Station X

...You could wire a Thermistor 1A/1 in series with the bell.....

This would certainly reduce or eliminate bell tinkle although a side effect will be that the phone fitted with the thermistor will begin to ring slightly later, on an incoming call, than other phones (or calling devices) connected to the line.

I may have a spare 1A/1 thermistor kicking around somewhere just send me a PM if you can't find your original thermistor.

Andrew

[craggus2000]

Originally Posted by Dave Moll

This would stop this 'phone causing bell-tinkle on other 'phones, as it is no longer sending loop-disconnect pulses onto the line. As far as I know, though, this won't stop other LD 'phones on the same same causing tinkle on this one.

Originally Posted by AndiiT

This would certainly reduce or eliminate bell tinkle although a side effect will be that the phone fitted with the thermistor will begin to ring slightly later, on an incoming call, than other phones (or calling devices) connected to the line.

Presumably then if I install a Rotatone and a thermistor this should solve both problems? I think I could live with the phone ringing slightly later. I suppose also I could try adjusting the bells so they are just a smidge further away from the striker?

I may have a spare 1A/1 thermistor kicking around somewhere just send me a PM if you can't find your original thermistor.

Thanks, will take a look tomorrow first if i get a chance, would be nice to test it both ways.