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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment

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Old 10th Apr 2017, 1:23 pm   #1
vintageghetto
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Default Caller cannot hear me on 746 conversion.

Hi can anyone help with a 746 conversion? I have done a few with no problems but this one works other than the caller cannot hear me, it rings, dials out, has dial tone and I can hear the other person - but they cannot hear me. Wiring looks ok, any ideas?
Thanks, Sarah.
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Old 10th Apr 2017, 2:08 pm   #2
russell_w_b
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Default Re: Caller cannot hear me on 746 conversion.

Quote:
Originally Posted by vintageghetto View Post
'...it rings, dials out, has dial tone and I can hear the other person - but they cannot hear me. Wiring looks ok, any ideas?'
Can you hear side-tone in the receiver? Can you hear yourself blow into the mic?
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Old 10th Apr 2017, 4:20 pm   #3
vintageghetto
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Default Re: Caller cannot hear me on 746 conversion.

Will check this when I get home from work - I did check the wires to the microphone and they were all intact. I have read that it is possible to get a 8746 and these have different bell decibels so may not need the T1-T2 convertor?
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Old 10th Apr 2017, 5:04 pm   #4
TonyDuell
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Default Re: Caller cannot hear me on 746 conversion.

The difference between a 746 and an 8746 (apart from the line cord) is that the latter normally has the 4k bell, rather than the 1k bell of the 746. I say 'normally' as the diagram N8846 (for the Telephone 8746) does mention a version with a 1k bell.

If you have the 4k bell you do not fit the 3k3 resistor as part of the conversion.

The thing fitted between T1 and T2 is a pair of diodes in inverse parallel which end up connected across the receiver (earpiece transducer). The idea is to clamp the voltage across the receiver and thus prevent loud noises.

The transmitter (microphone) is connected between the blue and white wires of the handset cable, going to T3 and T10 on the PCB. Which transmitter do you have? The original ones were carbon microphones, there were several types, the housing was normally at least partly black. There was an electronic one, the Transmitter 21A, either using an electret microphone (in a red casing) or a moving coil microphone (green casing?).

If you have a carbon microphone then it's possible it has failed. If you disconnect it (when reconnecting the polarity of the wires doesn't matter), what resistance do you get between the terminal screws? Does it change if you tap the transmitter?
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