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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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8th Jun 2018, 7:00 pm | #1 |
Diode
Join Date: Jun 2018
Location: Chippenham, Wiltshire, UK.
Posts: 4
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332L Excessive Sidetone Advice
I have a converted 332L Mk2A telephone, originally manufactured in 1956. The handset has a replacement electret 21A and the receiver looks to be the original type 2P with a diaphragm 25. I have been using it for about 4 years, but to me the level of sidetone is distractingly high. In all other aspects the 'phone is faultless. I've included a photo of the general wiring configuration. I've compared this to information contained in this forum and on other websites and all looks to be in order to me.
I was wondering if there is a problem with the ASTIC No. 27 and whether anyone can offer any advice on some voltage / impedance measurements I can make to begin to check the anti-sidetone circuitry. Regards, Andy |
9th Jun 2018, 10:40 pm | #2 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: 332L Excessive Sidetone Advice
Can you try a carbon mic No: 13 in it for comparison? What is the d.c. voltage on the mic terminals when the handset is off-rest? Are there any comments from those to whom you speak on this telephone about sound levels?
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Regds, Russell W. B. G4YLI. |
15th Jun 2018, 7:33 pm | #3 |
Diode
Join Date: Jun 2018
Location: Chippenham, Wiltshire, UK.
Posts: 4
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Re: 332L Excessive Sidetone Advice
Apologies for the delay in replying. Unfortunately I'm new to vintage telephones and so I don't currently have any spare components to try. I've just taken the electret transmitter out of the handset and I have 6.28V d.c. across the mic terminals off-hook. I have a square black transmitter with a detachable red surround held in place with a little foam. From a little internet research this looks like a standard conversion with the original transmitter casing removed to allow it to fit in the 164 handset. Maybe the modern transmitter performance is just a little too good in comparison to the original carbon granule version and hence gives a high amount of sidetone. I haven't had any comments regarding sound levels.
Andy |
16th Jun 2018, 7:07 pm | #4 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: 332L Excessive Sidetone Advice
6.28V is a reasonable voltage to be dropped across the mic and sounds correct to me. The mic is a vast improvement on the Inset No:13 that was originally fitted. And even they varied from mic to mic.
If you disconnected the handset at terminals 4, 5 and 6 and removed the link between terminals 1 and 2 you could measure the ASTIC d.c. resistances with the handset off-rest. Between 5 and 6 is about 105 Ohms. Between 5 and 1 is about 35 Ohms. Between 13 and 6 is about 105 Ohms. Between 13 and 5 is about 60 Ohms. Between 1 and 6 is about 140 Ohms. The above are give-or-take maybe 5% but anything radically different may indicate a fault. I would have thought it unlikely the ASTIC go faulty, mind.
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Regds, Russell W. B. G4YLI. |
19th Jun 2018, 12:09 pm | #5 |
Diode
Join Date: Jun 2018
Location: Chippenham, Wiltshire, UK.
Posts: 4
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Re: 332L Excessive Sidetone Advice
Thanks for the typical resistance values. I've taken the measurements you suggest, see below:
Between 5 and 6 - 109.7 Ohms Between 5 and 1 - 37.6 Ohms Between 13 and 6 - 112.2 Ohms Between 13 and 5 - 61.5 Ohms Between 1 and 6 - 147.2 Ohms So the ASTIC and associated wiring looks fine to me. Maybe it's just that I'm so used to using mobiles without sidetone these days, that I've forgotten what it's like to use a proper telephone instrument! Many thanks for your help and advice. Andy |
19th Jun 2018, 8:18 pm | #6 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: 332L Excessive Sidetone Advice
Andy,
You're welcome. I have a 706, a 746, an 8782 and an Ericsson N1071 deployed in the house, and sidetone is good and clear. 706 and 746 have carbon inset mics and still a decent level of sidetone. If you're used to modern instruments with VOGADS etc..., then 'traditional' instruments may come as a bit of a surprise! Your measurements are pretty similar to the ones I took on my 332 for comparison.
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Regds, Russell W. B. G4YLI. |