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Old 30th Apr 2017, 4:56 pm   #1
Higgo84
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Default Bell RTT 56 wiring circuit and wiring problems

HI. I've acquired a Bell RTT 56B phone and I've been trying to sus the wires for weeks. Someone has replaced a circuit board within which looks very similar to the schematics from a GPO 746 phone wiring diagram, because the induction coil has the same ratings etc. However there is no regulator within the circuit, and I'm just trying to scratch my head working out how to rectify. Many of the wires have come off, and the are not colour coordinated as they've been retrofitted. Can anyone help? Also the microphone piece has had an aftermarket transmitter fitted.
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Old 30th Apr 2017, 6:16 pm   #2
Higgo84
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Default Re: Bell RTT 56 wiring circuit and wiring problems

Sorry, I typed the main thread on my phone and it wasn't quite clear. The original 1950s Bell circuit board/guts have been removed by someone (BELL RTT56 B AP209794 circuit) and replaced with something entirely different as per the pictures on the previous thread and more on this one. I've searched all over the web for a wiring diagram for the new circuit board, however I have a feeling it has been custom made DIY. The nearest schematics I can find is that from an BT 746 type rotary phone however it doesn't have all of the same components, such as the regulator MR1 / RU1 section of the GPO 746 wiring diagrams. The Induction coils have exactly the same rating which is why I think it has been designed like the 746 but without a regulator. Also the microphone and earpiece has been replaced with something more modern to suit the new board. as I mentioned, none of the wiring coincides with anything on any standard diagrams because whoever did this has used their own colours (apart from the Blue Red and white wires from the BT cable)

Any advice would be greatly appreciated, please be gentle as I am a novice with a soldering iron
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Old 30th Apr 2017, 8:29 pm   #3
ThePillenwerfer
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Default Re: Bell RTT 56 wiring circuit and wiring problems

That circuitboard does have a home-made look to it.

Don't worry about the regulator being missing. The 'phones prior to the 746 didn't have them and work fine.

I used bits from a 746 to rebuild an Indian candlestick. I attach the diagram showing how I did it but obviously your wire colours will be different.

Good luck with it.
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Old 30th Apr 2017, 9:38 pm   #4
Higgo84
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Default Re: Bell RTT 56 wiring circuit and wiring problems

Thank you. That looks very similar to my circuit board, except for the extra capacitor which I'm assuming was for the ringer which I can't see in your diagram. Hopefully I will work it out soon
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Old 30th Apr 2017, 10:21 pm   #5
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Default Re: Bell RTT 56 wiring circuit and wiring problems

Yes, that's right. Telephones don't need an internal ringing capacitor these days as it's in the Master Socket. It would have gone from the red (on my diagram) wire at the top to one end of the bell coils. The other end of the bell coils would go to the white wire at the bottom.
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Old 1st May 2017, 12:06 am   #6
Higgo84
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Default Re: Bell RTT 56 wiring circuit and wiring problems

l will get there in the end . The blue signel wire where it attaches on your diagram to the handset switch 2 and 3 and loops round to a wire showing 'B' when its closed , that section of the circuit is that for the ringer? Im guessing when the handset is lifted it switches off the bell circuit it connects the headset ? Finally pin 3 on the induction coil (7 ohms ) on my circuit board only has one hole on the circuit board to connected it anywhere, I can't see how I can connect to the dialler and the ear piece? On the dieller there is a link accross so I'm not sure if that has something to do with it? I only have 4 wires on my dialler.

Thank you again in advance .
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Old 1st May 2017, 1:45 am   #7
ThePillenwerfer
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Default Re: Bell RTT 56 wiring circuit and wiring problems

The top part of the gravity switch doesn't actually disconnect the bell. It does however stop it, and those in other 'phones on the same line, from tinkling as you dial. A diagram with the bell is below. You'll notice that the ringing capacitor is only connected at one end as, as previously said, it isn't needed.

I would imagine that you'd run a wire from the hole on your PCB that connects to Pin 3 of the Induction Coil to a terminal on the block shown in picture two of post one. From there you'd connect one of the earpiece wires and one to the dial.

Your dial has five terminals but only four wires. It's different to British ones and, as I've no experience with foreign 'phones, I don't know exactly how to connect it. There are people who use these telephones in the UK so it's certainly possible. Hopefully somebody who does know will read this soon.

It should be nice when it's done as it will have the old aesthetics but modern workings which are meant to work here.
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Old 1st May 2017, 1:57 pm   #8
Higgo84
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Default Re: Bell RTT 56 wiring circuit and wiring problems

Good News!!

I've wired up the circuit board now, everything accept the Dialler until I work out where that goes. It all works, including the bell so thank you again, I wouldn't have done it without your diagrams guys.

Now with the dialler, there are two sides to it, the left side in the photo makes contact when you turn the dialler, the right side pulses when you let go. It has 4 wires attached to it. The Black and Brown wires are permanently connected in the dialler on their contacts, When you turn and open the dialler the left side moves and makes contact with the link which joins the black and brown pair to to the red on the pulse side. So when the dialler is turned and opened the black, brown and red are all connected together until it reaches its home position. When you let go of the dialler, the right side taps and pulses to whatever number you've reached and taps the orange wire to red, and obviously the red is linked brown and the black. Hope that makes sense, any thoughts would be fantastic where this fits into the diagram?
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Old 1st May 2017, 2:52 pm   #9
TonyDuell
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Default Re: Bell RTT 56 wiring circuit and wiring problems

The impulse contacts (orange/red on your dial I think) should be normallyclosed, and open the right number of times (depending on the digit dialled). These need to go in series with the line. The off-normal contacts (the ones that close when the dial is away from the 'normal' (rest) position) would short out the rest of the telephone during dialing. UK dials have a second set of off-normal contacts which short circuit the receiver to certainly prevent annoying clicks.

The diagram I'd look at would be N806, that for the Telephone 706, since that can be configured without a regulator. Ignore the off-normal contact across the receiver, and connect the other contacts (3 wires total) up in the same way.

Personally I am not at all in favour of putting the 'wrong' electronics in a telephone. To me, a telephone is defined more by the circuit than by the casing, so doing this would make it worthless to me. But it's happened, so you might as well use it.
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Old 1st May 2017, 5:21 pm   #10
Higgo84
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Default Re: Bell RTT 56 wiring circuit and wiring problems

Hi Tony

Thanks for the info. I've the pulse inline now with the line feed and the brown wire to the microphone which shorts the mic when you turn the dialer, however there isnt another switch for the head set so when the dialler pulses, theres a load pulse noise on the headset. Not sure if I can wire it another way?

Thanks
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Old 1st May 2017, 6:38 pm   #11
TonyDuell
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Default Re: Bell RTT 56 wiring circuit and wiring problems

Try connecting the brown wire to the other side of the line to the side that has the impulse contacts, after the gravity switch if there's one in that line wire instead of just shorting out the transmitter. On Diagram N806, it's the wire indicated by Note F
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