746 Redial Button Wiring Issue

[CurlyCord]

I'm having problems converting a 746 telephone.

I have connected a new line cord as standard and have put a resistor in between links 4-5 as the phone has a 2000 ohm ringer unit.

The problem I'm having is with the redial button. The wires from the button are connected as follows:

Grey & Brown to T8
Blue to T15

This was how the wires were connected when I removed the case, I'm not sure if this is correct?

On the left hand side of the cradle (looking from the back) is a micro switch with 4 wires coming from it grey, brown & blue (together) and another grey one. These wires were not connected to anything. At some point someone has disconnected them. I'm not sure what the micro switch is for or where the wires need to be connected.

Anyone out there know what terminals these wires need to be connected to?

Also I know the button is a 'redial' button but can a phone of this age store the last number dialed? Confused

[Station X]

700 series telephones never had a redial button. There is no electronics in the phone to store the number.

The button was commonly used to short-circuit the bell on extension telephones to render it inaudible (locking button), to obtain dial tone on a shared service (party) line, or to recall the operator from a PMBX extension (non-locking button).

There's not much you can do with the button on a modern plug and socket installation. I'd park the wires on a spare terminal.

[JohnBHanson]

Could use it as a microphone mute button.

[Station X]

As this diagram shows, in figure 3, a switch between T8 and T15 is indeed a PMBX recall button. The fact that the brown and slate wires both go to T8 is neither here nor there as T8 and T9 are connected together. Note that the diagrams show UNCONVERTED telephones.

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

The switch with four wires is probably an auxiliary switch hook switch. Is it operated from the switch hooks?

[andy1702]

Originally Posted by CurlyCord

On the left hand side of the cradle (looking from the back) is a micro switch with 4 wires coming from it grey, brown & blue (together) and another grey one. These wires were not connected to anything. At some point someone has disconnected them. I'm not sure what the micro switch is for or where the wires need to be connected.

Is that micro-switch not the hook switch? I've got a few like that.

Andy.

[Station X]

A recall button will be non-locking/non-latching.

[Station X]

Originally Posted by CurlyCord

I have connected a new line cord as standard and have put a resistor in between links 4-5 as the phone has a 2000 ohm ringer unit.

Are there not two 2000 ohm coils in series totalling 4000 ohms and making a resistor unnecessary? Please let's not reopen the discussion as to whether the resistor is necessary with a 1000 ohm bell.

[CurlyCord]

Hi guys,

I'm not sure what a hook switch is or what's it's used for.

The resistor has fixed the problem of the crackling noise.

Could the button be wired for a ringer on/off button?

[Nickthedentist]

Yes, I agree.

This probably says that it's a model 8746 on the base. These were fitted with 2x2000 bell motors and don't need a series resistor as their REN is 1 already.

N.

[Station X]

Originally Posted by CurlyCord

Hi guys,

I'm not sure what a hook switch is or what's it's used for.

The "buttons" on which the replaced handset rests are referred to as switch hooks, a hangover from the days when the receiver of a candle stick telephone would be hung on a hook. They operate switches within the phone to signal ON HOOK and OFF HOOK line conditions.

The resistor has fixed the problem of the crackling noise.

I cannot understand why this should be so. The (disputed) function of the resistor is to raise the Ringing Equivalence Number (REN) of the phone

Could the button be wired for a ringer on/off button?

No. It's non-locking.

[TonyDuell]

The switch on the side of the cradle provides extra contacts for the gravity switch or hook switch. The former is the UK name (it is operated by gravity when the handset is on the rest) the latter the US name (from the telephones where you hung the receiver on a hook when the telephone was not in use). It was used on some extension wiring plans in the GPO days to do things like interrupt the bell circuit and prevent 'tinkle'. You don't need it, you can actually remove the whole thing.

The button on top was, indeed for recall on a private exchange. As originally wired it would connect one side of the line (T8) to the blue wire of the original hard-wired line cord (via T15 which is an otherwise unused terminal) to local ground. I can't think of any use for it on modern wiring.

You could remove it along with the button on the case and fit a blanking plug to the hole. Or use it to mute the microphone (determine which wires from the microswitch are the normally open contacts and connect them to the blue and white wires from the handset cable (T3 and T10 IIRC). Then pressing the button will short out the microphone like the privacy button on a more modern telephone.

[andy1702]

Maybe the resistor stopped the crackling because the resistor replaced a fixed link which had dirty contacts. The act of fitting the resistor cleaned the terminals a bit. That's the only possibility I can think of.

If I were you, I'd try putting a clean piece of wire in place of the resistor and see if that also solves your crackle problem.

Let us know what happens.

Andy.

[OscarFoxtrot]

Originally Posted by TonyDuell

The switch on the side of the cradle provides extra contacts for the gravity switch or hook switch. ... It was used on some extension wiring plans in the GPO days to do things like interrupt the bell circuit and prevent 'tinkle'. You don't need it, you can actually remove the whole thing.

The button on top was, indeed for recall on a private exchange.

Extensions on private exchanges type PMBX 2 had an additional contact on the gravity switch to operate the signalling lamps.

http://www.britishtelephones.com/pmbx2.htm

People with such a PMBX might be grateful of the add-on gravity switch assembly as they're quite hard to find separately these days.

[TonyDuell]

IIRC those auxiliary gravity switches came in 1, 2, and 3 pole versions. They were used
for all sorts of things, Plan 4 Multiple (to open the bell loop when off hook), Plan 105
and 107 external (to disable the signalling button), in the Planphone (see N625) and so on. Due to my interest in not-just-normal telephones, I've fitted a few...

In general (and obviously) bits that were not needed for a normal single-telephone domestic installation are a lot harder to find than the telephones, and do attract some interest.