Old G.P.O. Telephone

Posted on Feb 4, 2014

The induction coil forms part of the anti-sidetone circuit. It couples audio to the receiver, and also reduces the volume of the callers own voice in the earpiece, which would otherwise be loud compared to the incoming audio from the distant end. The original handset cord (top terminals) has a distinctly brown braid, while the line cord (bottom terminals) has a

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more purple color. It`s actually a length of old-style twin lighting flex, probably 1950s. Not an exact match, but somebody obviously thought it better than modern PVC! If you look at the schematic, you`ll see that the original cord was a 3-wire type, although on a regular single-party line the bell return was just strapped to one side of the line on the wall junction. Look back at the terminals on the underside of the chassis, and you can see the wire has been extended from the bottom left terminal to the one directly above it to get the bell to work. That`s where the third wire would have been connected originally (compare with the "official" straps on the terminals to the right). Yes, that`s what I meant when I said a 1/4-inch jack. And although the majority of single phones in homes were hardwired back in the days when this set was commonly used, those which were on a portable plan would have used just such a jack. Personally, I think the old GPO jacks were far superior to the modern modular BT plugs, but of course these days everything is done on the basis of how cheaply it can be made. Actually Paul, modifying a 332 to work on modern style phone jacks is quite easy [although technically illegal]. Remove strap 10-11, and hook the bell wire which is usually the blue one these days, to terminal number 11, leaving the line pair right where they are. That way, the capacitor in the phone jack replaces the 2 microfarad cap....

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