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Old 29th Nov 2017, 9:00 pm   #2
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 2,715
Default Re: GPO 706/746 Ringer on/off Button

The original GPO wiring put multiple bells in series. The bell on/off switches short-circuited a bell in the off position. The BT wiring puts multiple bells in parallel, so the switch needs to open that bell circuit when the bell it turned off.

The switch has 3 wires as you noted, it is a changeover contact. With your multimeter identify the 2 wires that are connected when the switch is in the 'on' position. Either insulate the other wire with tape (that was an official thing to do!) or park it on a spare terminal. Terminals T11 to T15 are normally spares (not wired to anything else inside the telephone) but one is probably used to park the green wire of the line cord.

Remove the strap between termianls T16 and T17 (notice that one of the bell wires is connected to T16, leave that there) and connect the remaining 2 swtich wires to T16 and T17 (either way round). This puts the switch in series with the bell so you can open the bell circuit to turn the bell off. Leave the other strap from T17 (to T18) in place of course.
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