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Old 21st Jan 2020, 1:31 pm   #1
Telephone Guy
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Question 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.
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Old 21st Jan 2020, 2:59 pm   #2
G6Tanuki
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Default 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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Old 21st Jan 2020, 3:18 pm   #3
Julesomega
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Default 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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Old 21st Jan 2020, 4:11 pm   #4
Dave Moll
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Default 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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Old 21st Jan 2020, 6:27 pm   #5
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Default Re: Telephone amplifier

As a test you could short out the microphone and see if it makes a difference.
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Old 21st Jan 2020, 8:54 pm   #6
OscarFoxtrot
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Default 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
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Old 21st Jan 2020, 9:18 pm   #7
Dave Moll
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Default Re: Telephone amplifier

Quote:
Originally Posted by OscarFoxtrot View Post
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
I'm confused by this statement, as there seem to be contradictory statements about the same handset number.
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Old 21st Jan 2020, 10:31 pm   #8
TonyDuell
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Default 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.
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Old 22nd Jan 2020, 12:33 am   #9
qazxsw123
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Default 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 )
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Old 22nd Jan 2020, 12:44 am   #10
qazxsw123
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Default 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
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Old 22nd Jan 2020, 2:38 pm   #11
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Default Re: Telephone amplifier

Well now ...
Firstly, thanks very much to everyone for their replies. Taking them one at a time:
  • G6Tanuki: The inductive pickup would, I'm sure, work, inasmuch as I've no reason to think it wouldn't; however, to be fair, I probably didn't quite explain the problem clearly. What I'm ideally trying to achieve is a situation where we can still use the phones ( all of them, ideally ) as we normally would, i.e. pick up the phone, hold it to your ear and talk to people, rather than have essentially a speakerphone arrangement ( unless I've misunderstood what you meant, in which case, my apologies ). Julesomega, by the same token - again, unless I've misunderstood - your suggestion either involves us changing the phone to a Converse 300 or attempting major surgery ( certainly beyond the realms of my capabilities ) to the existing phones.
  • Dave Moll: I'm aware of the handsets you mention. Just to complicate the situation, I know at least one of the phones has a 21A transmitter and at least one other has a carbon granule microphone. As far as I can tell using my ears as a measuring device - admittedly not exactly Spock-like in it's accuracy - the problem exists more or less equally on all three phones, give or take as little as makes no difference. Even notwithstanding the above, the other problem would be the likelihood of being able to pick up three fully functional amplified handsets that were the same colour as the phones.
  • Merlinmaxwell: inasmuch as it was an easy enough thing to do, I've tried shorting out the 21A microphone on that particular phone, and it doesn't seem to make any difference except ( obviously ) that when the mic is shorted, you don't hear any background sound, but the sound through the earphone doesn't get any louder or softer.
  • Oscarfoxtrot: again, I'm aware of the strap-on type of amplifier, and again, I'm sure it would solve the problem inasmuch as it would make the sound that comes out of the phone that much louder. However, as mentioned above, what I'm aiming for is a solution that allows us to use the phone exactly the same as we normally would, i.e. without any visible attachments etc.
I'm very aware at this point that it will look as though I'm just rejecting all ideas out of hand, and that's really not meant to be the case; nor am I ungrateful for any of those ideas.
  • Oscarfoxtrot, Dave Moll, and Tony Duell: I think I might have missed the boat a bit with the reference to diagram N1842 ... not sure what this relates to? - But in any event, the impression I get is that it might once again involve more electronic surgery than I had in mind, and / or might result in the phone looking physically different than it does now ( i.e. different handset, perhaps ? ). As I say, I might have totally got the wrong end of the stick, so apologies once again if that's the case.
  • qazxsw123: Amongst the numerous tests and adjustments I mentioned in the original post, I know ( or at least I was told ) that they manually turned the gain up at the exchange to the highest setting. That didn't make a blind bit of difference ( assuming they weren't just telling porkies in the first place ). With my fairly limited knowledge of electronics, I more or less follow your thinking; however, I'm not sure how I would ( easily ) add resistance to the line, but if I did, and if the equipment then did as you suggest ( notwithstanding the fact that the gain has already, so I've been told, been turned up full ), wouldn't that mean I'd have to add and then short-circuit the resistance every time I used the phone? { As will be blindingly apparent by now, what knowledge I do have is fairly rudimentary !! } I've downloaded the document to which you provided the link, and I'll do my best to interpret it, but I'm not sure I speak the language to quite the required degree!
To everyone who's responded, and anyone else who might be thinking of doing so, as I say, please don't think I'm ungrateful; far from it - I appreciate you taking the time. I'd hoped someone might know of some kind of fairly straightforward amplifier that went between the telephone and the wall socket ( as opposed to between the receiver ( handset ) and the telephone ) that would do what I need it to do without it being too much more complicated than that, but I'm guessing perhaps there's no such thing ...?
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Old 21st Feb 2020, 10:11 pm   #12
Oldcodger
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Default 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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