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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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21st Jan 2020, 1:31 pm | #1 |
Tetrode
Join Date: Jan 2020
Location: Canterbury, Kent, UK.
Posts: 71
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Telephone amplifier
Hello everyone,
We have three BT 782 telephones dating from circa early eighties. Since I absolutely can't be doing with modern phones ( which to me is anything manufactured since around the mid- to the end of the eighties ...! ), and absolutely not cordless phones (!), these are the only ones we use. Here's the problem. Because we're a fair distance from the exchange ( and I don't doubt that that is the reason, having had more checks done and adjustments made than I knew were possible ), the reception on the phones is relatively quiet; usable, just about, but a lot quieter than you'd like. By now, some of you might have figured out where this is going! Obviously, on modern phones, it's an easy thing to buy a telephone amplifier, plug the receiver cord into one end, plug the other end into the phone, and off you go. That doesn't work, of course, on any phone of this age ( at least, not unless you're going to go to the trouble of disconnecting the receiver cord, attaching the appropriate plug on one end, wiring the necessary terminals to a socket, and inserting the amplifier between the two ...! ) So the question is: does anyone know of any kind of telephone amplifier that does work with phones of this age, i.e., one that goes between the telephone and the wall socket? In case you wonder, yes, I've checked the internet and so far come up dry, hence this post. I'm fully prepared for the answer to be 'no' ... but it doesn't hurt to ask! Any suggestions or input will be gratefully received. Cheers. |
21st Jan 2020, 2:59 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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Re: Telephone amplifier
Could you not use one of the old-fashioned inductive-pickup-with-a-sucker things that used to be sold for recording phone calls, then feed the output into your amplifier rather than a tape-recorder?
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21st Jan 2020, 3:18 pm | #3 |
Nonode
Join Date: Dec 2012
Location: Stockport, Greater Manchester, UK.
Posts: 2,085
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Re: Telephone amplifier
Ours feels like a "real" phone, but it has a speaker and an amplifier (which can be switched to incoming or outgoing lines), with 3 switched settings plus a slider. BT Converse 300
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21st Jan 2020, 4:11 pm | #4 |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,118
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Re: Telephone amplifier
There are also handsets with built-in amplifiers - primarily designed for those with hearing difficulties. The volume is adjusted by a control on the side of the earpiece, and the amplifier is powered from the line voltage applied to the transmitter inset.
I did discover, however, that these don't work with an electret transmitter (type 21A), presumably caused by them having a higher resistance than a carbon granule inset. I do wonder, however, whether your problem is caused by a dodgy carbon transmitter inset. I know it sounds counter-intuitive, but this can reduce the current flowing through the receiver, resulting in low sound output. If you have, or can lay hold of, a transmitter 21A, it is worth trying - though (as I say) not with an amplified receiver.
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21st Jan 2020, 6:27 pm | #5 |
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Re: Telephone amplifier
As a test you could short out the microphone and see if it makes a difference.
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21st Jan 2020, 8:54 pm | #6 |
Heptode
Join Date: Apr 2011
Location: Edinburgh, UK.
Posts: 805
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Re: Telephone amplifier
You can buy strap on ones.
I used to work with a hearing impaired telephonist who used a strap on one at work. https://www.livingmadeeasy.org.uk/co...ifiers-3664-p/ The amplifier of the Handset No. 4 draws its operating current from the transmitter current whilst the Handset No. 5 requires a separate battery supply. Handset 14A may be re-arranged as shown in Diagram N1842, and two mercury cells used to provide the necessary power. The Handset No. 14A should be used as issued, that is, powered from the line, provided that the transmitter current with the amplified handset in circuit is not less than 26 ma. Also discussed here |
21st Jan 2020, 9:18 pm | #7 | |
Dekatron
Join Date: Feb 2005
Location: West Cumbria (CA13), UK
Posts: 6,118
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Re: Telephone amplifier
Quote:
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21st Jan 2020, 10:31 pm | #8 |
Dekatron
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,191
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Re: Telephone amplifier
It makes sense, although IMHO it's not clearly explained. As originally supplied ('issued') the Handset 14A draws its power from the line. For this to work properly the current through the transmitter must be at least 26mA
If you can't get enough current that way, you can connect the Handset 14A to a pair of mercury cells in series. Diagram N1842 shows how to wire it for either power input. |
22nd Jan 2020, 12:33 am | #9 |
Triode
Join Date: Nov 2016
Location: Frinton on Sea, Essex, UK.
Posts: 45
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Re: Telephone amplifier
the gain of the telephone equipment in the exchange can be set to "auto" this then works out the gain balance to use for optimum when the line is looped ( lifting handset).
try adding some resistance to the line, the voltage should increase as the exchange tries to maintain the line current, then s/cct the extra resistance the exchange should then maintain the same voltage thus increasing the line current and using the increased voltage to alter the gain balance setting in the exchange equipment. this was part of the original design for digital exchanges but caused problems with manual switchboards that answered incoming calls (gain balance set) and then switched the call to an extension further away (more resistance so lower current ) |
22nd Jan 2020, 12:44 am | #10 |
Triode
Join Date: Nov 2016
Location: Frinton on Sea, Essex, UK.
Posts: 45
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Re: Telephone amplifier
https://www.btplc.com/SINet/SINs/pdf/351v4p9.pdf
is the nearest doc I can find, suspect the gain balance working is within an internal document |
22nd Jan 2020, 2:38 pm | #11 |
Tetrode
Join Date: Jan 2020
Location: Canterbury, Kent, UK.
Posts: 71
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Re: Telephone amplifier
Well now ...
Firstly, thanks very much to everyone for their replies. Taking them one at a time:
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21st Feb 2020, 10:11 pm | #12 |
Nonode
Join Date: Oct 2014
Location: West Midlands, UK.
Posts: 2,181
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Re: Telephone amplifier
years ago ,I came across something similar on gain. Long story short- our sales guys had seen that it was possible to call forward on a certain small system, but did not understand that the process meant a a lot of attenuation . Partial cure was to ask BT to set gain on line to zero . But thinking as an ex transmission person, something like a two wire amp might fit the bill. Go and send are split in a hybrid ,and then combined in another hybrid. As long as loop gain is kept below 0db, then no howl occurs ( or that was the theory).
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