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Old 18th Jul 2014, 10:37 am   #41
craggus2000
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Default Re: GPO 746 wiring query 2-wire

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Originally Posted by Station X View Post
You could wire a Thermistor 1A/1 in series with the bell
Presumably this could go between T5/T6 or T6/T7 in place of a strap?
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Old 18th Jul 2014, 10:43 am   #42
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Default Re: GPO 746 wiring query 2-wire

If your phone is wired as per post #25, that will be OK.
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Old 18th Jul 2014, 11:55 am   #43
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Default Re: GPO 746 wiring query 2-wire

I think you only want the thermistor in series with the bell, not the receiver circuit, therefore I would put it either between T4 and T5 or between T16 and T17. Looking at the N846 wiring diagram, T5 is shorted to T6 when the handset is on its cradle, thus nullifying the thermistor.

I would have thought that a thermistor in series with the receiver (as it would be between T6 and T7) might reduce volume, given that it would be at its maximum resistance at the low current produced by the speech.
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Old 18th Jul 2014, 12:00 pm   #44
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Default Re: GPO 746 wiring query 2-wire

The link I posted in post #32 suggests 16 to 17.

4 to 5 can't be used as the resistor is wired there according to post #25.

As you say 5 to 6 is a no no as there are switch hooks contacts across these points.

Likewise 6 to 7 which would put the thermistor in series with the receiver.
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Old 18th Jul 2014, 4:02 pm   #45
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Default Re: GPO 746 wiring query 2-wire

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Originally Posted by ajs_derby View Post
Quick answer: Wire it basically as per N diagram 846; but connect a 3.3 kΩ resistor between T17 and T18.
The 3.3K resistor won't do much at all. It is a myth propagated by people who don't understand the concept of impedance. Although old ringers have a DC resistance of 1K the AC impedance is much higher. The resistor certainly won't change the REN from 4 to 1.
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Old 18th Jul 2014, 7:57 pm   #46
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Default Re: GPO 746 wiring query 2-wire

It was an official BT mod, but only for Trimphones which used an electronic ringer. For other phones the recommended mod was to fit a 4000 ohm bell.
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Old 18th Jul 2014, 8:56 pm   #47
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Default Re: GPO 746 wiring query 2-wire

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The 3.3K resistor won't do much at all. It is a myth propagated by people who don't understand the concept of impedance
Indeed... Fitting the resistor drops approximately half the ringing voltage across it! I make a type 59A bell-motor impedance something like 1598 +j1928, or 2504 ∟50.3° Z

When you add the 1u8 capacitor in series the impedance becomes 1598 -j1466, or 2169 ∟-42.5° Z. All this for 25Hz.

I've never had any problems not fitting a resistor.
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Old 24th Jul 2014, 10:40 pm   #48
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Default Re: GPO 746 wiring query 2-wire

Just thought I'd say I've not dropped off... Just the phone is now in pieces being stripped down, cleaned and polished...
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