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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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3rd Aug 2017, 11:36 am | #1 |
Pentode
Join Date: Mar 2012
Location: Rotherham, South Yorkshire, UK.
Posts: 152
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Faint audio on 741 wall telephone
So, per my previous thread where the last problem I had was having was NU tone when lifting the handset I realised I HAD stupidly strapped T3&T4 instead of T4&T5! D'oh!! Anyway that's resolved now.
The problem I am now having (seem to be solving one, finding another!) is that the audio is very faint through the earpiece. Before I try to source a replacement speaker, is there anything else that could be causing this? The telephone otherwise seems to work as it should. Thank you! |
3rd Aug 2017, 12:42 pm | #2 |
Octode
Join Date: Mar 2011
Location: Sheffield, South Yorkshire, UK.
Posts: 1,453
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Re: Faint audio on 741 wall telephone
It's certainly possible that the receiver is faulty but it's more likely to be a wiring fault.
This is the clearest diagram I'm aware of: http://telephonesuk.co.uk/documents/...STRUCTIONS.pdf Check yours against it carefully, including the connexions on the back of the dial. I still sometimes get the D1 to D5 in the wrong order so somebody else may have. |
3rd Aug 2017, 2:06 pm | #3 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,130
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Re: Faint audio on 741 wall telephone
It's also worth checking that the "dial off normal" contacts are opening with the dial at rest. This can be done by comparing resistance across T1/T2 and T3/T10 with and without the dial moved from its rest position.
I once had a dial where the cam that opens the contacts wasn't resting in the correct place, meaning that the dial was registering as off-normal at all times and shorting out the insets in the handset. There was sufficient resistance in this circuit to allow a small amount of current still to pass through the receiver to give a faint signal.
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Mending is better than Ending (cf Brave New World by Aldous Huxley) |
3rd Aug 2017, 2:22 pm | #4 |
Pentode
Join Date: Mar 2012
Location: Rotherham, South Yorkshire, UK.
Posts: 152
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Re: Faint audio on 741 wall telephone
Thanks Dave. If that is the problem, what would my options be then? That would begin to sound more complicated than I was bargaining on!
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3rd Aug 2017, 4:56 pm | #5 |
Pentode
Join Date: Aug 2016
Location: Dunstable, Bedfordshire, UK.
Posts: 140
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Re: Faint audio on 741 wall telephone
You would need to adjust the dial so that the cam that holds open the contacts is in the correct place when the dial is at rest. The third pic down show it in it's correct position when the dial is at rest, holding the contacts open. The cam is labeled switching lever.
http://www.britishtelephones.com/tel...%20b5125.htm#4 |
9th Aug 2017, 12:13 am | #6 |
Pentode
Join Date: Mar 2012
Location: Rotherham, South Yorkshire, UK.
Posts: 152
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Re: Faint audio on 741 wall telephone
Just checked - it looks like it was a quiet receiver. I swapped it with a receiver from a spare 746 and normal volume restored. Thank you all for your help.
Any ideas on where to buy a replacement receiver? eBay doesn't seem to have any? |
9th Aug 2017, 12:30 am | #7 |
Hexode
Join Date: Nov 2013
Location: Lancing, West Sussex, UK.
Posts: 286
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Re: Faint audio on 741 wall telephone
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9th Aug 2017, 1:12 am | #8 |
Octode
Join Date: Mar 2011
Location: Sheffield, South Yorkshire, UK.
Posts: 1,453
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Re: Faint audio on 741 wall telephone
I don't think I've got one in stock but even if I had it'd cost nearly as much in postage as one of these: http://telephonelines.net/index.php?...ls3a2d71arb5r5
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9th Aug 2017, 4:56 am | #9 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,225
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Re: Faint audio on 741 wall telephone
The normal problem with these receivers (4T) is a dry joint. Not an electrical connection, but the mechanical joint between the armature and the pin that connects to the diaphragm. If you can open it up, it's easy to resolder it.
The problem is opening it up. You can carefully file the rim that's formed over to hold it together, take off all the terminal nuts and spacers, and take the cover off. Then just re-solder the joint. The cover will be held on when the receiver is put back in the handset. |