GPO generator 26A

[ppppenguin]

I'm not often found in the vintage telephony section

I have found one of these generators lurking in a dark corner. I remember using it years ago for phone ring FX in amdram. I found a photo and some notes from Russell_w_b of this parish:

http://www.***********/photos/russell_w_b/3995908243/

Mine is in a black bakelite(?) case and there is a chunky capacitor too. What was the changeover contact on the back of the generator used for? It seems to change over as you start to wind the handle. What was the main application of these devices?

[Dave Moll]

Presumably, the chnageover contact would be to allow the magneto to be wired into a telephone circuit and switch out the speech circuity while sending the ringing voltage.

Incidentally, despite its appearance, don't try sharpening pencils with it!

[russell_w_b]

Originally Posted by ppppenguin

'What was the changeover contact on the back of the generator used for?'

The contact on the back took the incoming pair to either the bell or the magneto of the instrument. Cranking the handle put the magneto across the line without ringing its own bell, and the handle in the 'release' position allowed incoming ringing to go to the bell.

Attached is a cct diagram of a 'Type 'F' field-telephone, but the concept is similar to other magneto instruments. I had one of the bakelite-cased ones (sold it, regrettably), and I guess it would be used with either a plain desk-set or a pendant telephone and external bell - the type of thing that may have had a pull-out dial under the desk.

[ppppenguin]

Any idea what the capcitor is for? It's 2uf, like a standard bell cap.

[russell_w_b]

The 2u cap in mine was strapped out-of-circuit. It may have formed part of a bell circuit with the bell-wiring taken via the magneto box, but this would be unusual given that the bellset would have a capacitor mounted within.

The only other reason I can think of right now is that it may have formed a d.c. block if the C/O switch was not used, and the genny was across the line permanently.

[BobGreen]

The 2u cap is there to block the 50V DC bias from the exchange line but allow the 100V+ AC ringing signal through to the bell.

[beamcurrent]

The tele F was a local battery magneto ringing field telephone, it was never meant to be connected to a central battery system. The 2u cap is there to block any dc voltage on the line.

In the off hook condition the Tele F puts a small voltage ~ 3 volts on the line and if the remote half of the pair had a poor battery it would still work (badly) from that 3 vols ish from the other phone. In the On hook condition the phone is High Z to save draining any battery in any other off hook phone.

It was not uncommon to have several field telephones wired all in parallel using the number of rings code so that the correct phone answered.

It could also be used with a branch local battery exchange using the ring-off procedure to end a call.

Jeffreys magneto generator is typical of the type and you could get an interesting electric shock if your mate (ex) wound away just as you were connecting the telephone pair to the terminals.

Brian S

[russell_w_b]

Originally Posted by BobGreen

The 2u cap is there to block the 50V DC bias from the exchange line but allow the 100V+ AC ringing signal through to the bell.

This configuration must've been used on magneto-signalling CB systems. But would lifting the handset on such a system not break the d.c. loop such that the loop current would be prevented from going through the magneto anyway?

Surely the bell capacitor would block the d.c. loop current? The only other time it might be beneficial to being in series with the magneto would be if the magneto was cranked, allowing 50V d.c. across it (it being thrown onto the line by the action of the C/O switch operated by the crank). But where was this type of magneto-plus-50V line loop current used?

Remember that in original magneto signalling systems, there was a local battery and no 50V d.c. loop.

[russell_w_b]

Originally Posted by beamcurrent

The tele F was a local battery magneto ringing field telephone, it was never meant to be connected to a central battery system. The 2u cap is there to block any dc voltage on the line.

In the off hook condition the Tele F puts a small voltage ~ 3 volts on the line

Not on mine, it doesn't. The 3V energises the microphone only, via a winding on the induction coil. How does this 3V get to the line? There's a circuit of a type 'F' in an earlier post of mine on this thread.

To avoid confusion I'd better say that I put up this sketch to illustrate the magneto switch-action, not typify all telephone systems that use a remote hand cranked generator!

[russell_w_b]

Originally Posted by beamcurrent

Jeffreys magneto generator is typical of the type and you could get an interesting electric shock if your mate (ex) wound away just as you were connecting the telephone pair to the terminals.

Indeed... A couple of turns of 'Sellotape' and a thin piece of wire attached to a door handle via the key-hole livened up the odd saturday-morning music lessons of my youth.

Don't think Mr Thomas has ever forgiven me...