746 wiring diagram needs some explaining please

[johnmarshmanagain]

Hi All,

Referring diagram as below, I can see R,G and W wires coming in...

I think the R and G do the microphone/earpiece dc-powered loop, so if W receives ac and operates the ringing on coil 23, then which is the return wire for the ac : R perhaps ?

I can see 'MIC' at upper left - presumably microphone, but is 'TL' the earpiece ?

Also, what's that semicircle marked '1k S' between terminals T4 and T16 ? and which switch contact is associated with the spinning of the dial ?

If there's a website that explains all this for UK phones, then please by all means advise.

Suitable topic for a sticky perhaps ?

Obliged, Regards JM

http://www.samhallas.co.uk/repositor.../0000/N846.pdf

[Station X]

In unmodified form red and white are the line pair. Green is there to provide an easy means of connecting an extension bell.

Ringing path A leg, BT8, BT4, BT3, T17, T16, Bell, T4, T5, T6, T7, C1/C2, T9, T8, Red, BT1, BT5, B leg.

MIC is microphone originally a carbon type. TL is the earpiece a rocking armature type.

The semi-circle is the bell with a '~' to denote AC.

The pulsing dial springs are 4 and 5. All other dial springs are OFF-NORMAL which indicates they operate as soon as the dial is turned to wind it up. D1 and D2 for example short the earpiece during dialling.

[AndiiT]

Hi,
This link might be a good starting point.

Red and White are the exchange line connections and the green wire is strapped to the white for the bell return, the bell motor being the component between terminals T4 and T16.

You need to remember that the telephone instrument itself does two to four wire conversion to separate the incoming and outgoing speech, the transmitter (microphone) being in series with the line voltage via the dial contacts, T3, T10 and the induction coil and switch hooks.

A separate winding on the induction coil provides the receiver connected to T1 and T2.

That's a very simplified description, the link above will go in to more depth.

Regards

Andrew

[Dave Moll]

I'm not sure whether this is what you're asking, but (as indicated by the "A" and "B") the line connects between T8 ("B" leg) and T18 ("A" leg). The incoming wires are normally coloured red and white respectively.

If the block terminal external connection isn't present between T18 and T17, they need to be strapped internally.

If you wish to use the master socket capacitor for the ringer, remove the strap between T6 and T7 and connect the ringer wire (normally blue) to T6.

"MIC" and "TL" are indeed the microphone (transmitter) and earpiece (receiver) respectively. The item you describe as "1k S" is the bell. It is on its side so the symbol is an indication that the bell is AC,

[Station X]

Brings back memories of my "Youth's A" course at Bletchley Park in 1968.

[johnmarshmanagain]

Many thanks to all - that's great ! Regards John

[Oldcodger]

Originally Posted by Station X

Brings back memories of my "Youth's A" course at Bletchley Park in 1968.

And memories of Yoofs courses at Muirhouse ( Edinburgh) in the less problem times.
I remember that we used to use the link in the block terminal ( green-white)on the older installations to make it easier for following blokes to fit an extension bell.

[Filament]

Yes! I did mine in Coryton TC in Cardiff in '68. Happy days!

[GlenTyan]

Recalling my "B" course at Muirhouse the class was asked for insulation types of which all the common types were trotted out. I offered P.T.F.E and the class erupted into laughter including the lecturer. When the din had subsided I was asked to explain and I said Polytetrafluoroethylene which brought about more laughter. The class was brought to order and the lecturer said he'd never heard of it and we moved on. The next day an apology was offered including the possibility that PTFE would replace all the previous, would be 1967/8.

[dagskarlsen]

English is not my native language but I hope this this should be understandable.