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Old 17th Oct 2018, 7:08 pm   #1
prppedro
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Default LM Dialog presenting faint (and squelchy) speech

Cheers, colleagues,

I'm new here, as one can tell by my message counter. I found this forum a valuable source of information, but now I'm confused about how to fix my old rotary phone.

To put it simple, it works like a charm. Dials out, rings, the receiver works fine. Except for the transmiiter. People on the other side says "your voice seems to be far away" or "Your voice is disappearing". On the first thought, I blamed the old 1982 original transmitter that came with my LM Ericsson Dialog (manufactured in Brazil, though), so I swapped it with another transmitter I found at our local version of eBay, the “Mercado Livre“.

Ehhh... Well, does it work? Sorta... From what I can tell, the speech still squeelch and people seems not to hear me crystal clear. I've also tried to put an audio source next to the phone and pick up another extension (it's a simple plan). And it's really bad.

Tapped both inserts into the table, no changes.

I'm considering to put in something like the 21A transmitter insert, like sugested by our colleague Russell Barnes. I even have found a schematic of two type 21 transmitter posted by him. The problem is that I need to do so with not too much knowledge in electronics. I can solder things alright, but it's a bit difficult to me to fing the right parts (not everything seems to be available back here).

Also, would it solve the problem, at all? How much blame can I really put on the transmitter for the faint voice output?

Thanks in advance,
-trp
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Old 19th Oct 2018, 2:46 pm   #2
russell_w_b
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Default Re: LM Dialog presenting faint (and squeelchy) speech

Quote:
Originally Posted by prppedro View Post

I'm considering to put in something like the 21A transmitter insert, like sugested by our colleague Russell Barnes. I even have found a schematic of two type 21 transmitter posted by him. The problem is that I need to do so with not too much knowledge in electronics. I can solder things alright, but it's a bit difficult to me to fing the right parts (not everything seems to be available back here).

Also, would it solve the problem, at all? How much blame can I really put on the transmitter for the faint voice output?
Pleased you found the drawings! I was looking for my copies but they're 'somewhere else...' The blue T21 is a dynamic version made by AKG. It is possible to remove the actual mic and electronics module from a T21 red case, but I haven't tried to shoehorn it into a Dialog handset.

Have you access to an identical dialog telephone that you know will work? I've only ever had low speech issues with a U.S.- made ITT 2500 telephone where the network impedances are different to those in the UK.

If you can compare to an existing telephone that is similar you could swap carbon microphone insets for comparison - that way you'll see if the fault lies with the mic or the induction coil speech network.

I know from 'Flickr' that you get 6.4V d.c. across the mic when the handset is off-rest. That sounds about right. I'll take some measurements on my Ericsson Dialog to compare (I'm on the UK network but still...). Can you measure the d.c. biasing current in milliamperes of the mic when the handset is off-rest?
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Old 19th Oct 2018, 8:54 pm   #3
russell_w_b
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Default Re: LM Dialog presenting faint (and squeelchy) speech

I've just had my 1968 Dialog out and it's working fine. 6.5V dropped across the mic, 18mA through it and 40mA line loop current off-hook. Not all loop current flows through the mic on this instrument as it's shunted by part of the sidetone network.

There's room for the working part of a T21 mic but it means a little re-modelling of the plastic microphone holder, as quite a large portion of it covers the handset cable.
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Old 19th Oct 2018, 8:59 pm   #4
ThePillenwerfer
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Default Re: LM Dialog presenting faint (and squeelchy) speech

If you're soldering is up to it there's a design for an easy-to-make electronic transmitter here: https://www.vintage-radio.net/forum/...d.php?t=145480
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Old 20th Oct 2018, 9:32 pm   #5
dagskarlsen
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Default Re: LM Dialog presenting faint (and squelchy) speech

Surprisingly enough, LM Dialog and LM Dialog is not necessary the same. Those used in Norway differs from the ones used in Sweden, the circuit is simpler. I do not understand the reason for the sound problems, but here are the Norwegian schematics: http://www.classicrotaryphones.com/f...?topic=16105.0

Last edited by dagskarlsen; 20th Oct 2018 at 9:33 pm. Reason: Spelling
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Old 20th Oct 2018, 9:53 pm   #6
russell_w_b
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Default Re: LM Dialog presenting faint (and squelchy) speech

Thanks for that Dags. Mine 'looks' like the one shown in the photo that accompanies that drawing, but was made for the British PAX market and although bears the same drawing number 360290, it has another capacitor in the sidetone network and a set of 'dial off-normal' contacts.
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Old 21st Oct 2018, 12:33 pm   #7
mike_newcomb
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Default Re: LM Dialog presenting faint (and squelchy) speech

Hi Pedro, (simple) thoughts:

a) does another Phone, known to be working 100%, work ok when using the same connections as the Dialog?

b) Purchasing another 'Transmitter ' on Mercado Livre, suggests it is used and therefore might itself be faulty.
Has it been proved to work 100%, eg working in conjunction with a good phone?

If yes to both, then it narrows down where the fault lies.

Regards - Mike
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