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Old 24th Nov 2020, 7:08 pm   #1
phonelover555
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Default TMA 7214 Faint outgoing speech

I have just bought a phone with TMA 7214 marked on the bottom and have found that the outgoing speech can only be heard very faintly if I shout down the phone. I am not very familiar with these sorts of phones can anybody help me with it?
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Old 24th Nov 2020, 7:29 pm   #2
Nickthedentist
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Default Re: TMA 7214 Faint outgoing speech

Hello and welcome.

The code you quote tells us it's made by the Telephone Manufacturing Co in 1974, but not what model it is. Does it say "746" underneath as well?

Either way, you probably need a new "transmitter" (microphone) as the old ones often went faulty.

N.
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Old 24th Nov 2020, 7:44 pm   #3
phonelover555
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Default Re: TMA 7214 Faint outgoing speech

Hello,

Thanks for the information, no I don't believe it does. I've attached a photo of the bottom of the phone.
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Old 24th Nov 2020, 8:09 pm   #4
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Default Re: TMA 7214 Faint outgoing speech

It appears to be a PAX/PABX phone: https://www.vintage-radio.net/forum/...436#post280436
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Old 24th Nov 2020, 8:35 pm   #5
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Default Re: TMA 7214 Faint outgoing speech

Hello,

I've had a look at the transmitter, is this red part the thing that may need replacing.
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Old 24th Nov 2020, 9:16 pm   #6
phonelover555
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Default Re: TMA 7214 Faint outgoing speech

Another thing I have noticed is when on a call I can hear myself faintly with a slight time delay. This makes me think that the microphone is working okay and there is some other problem with the phone.
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Old 25th Nov 2020, 4:40 pm   #7
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Default Re: TMA 7214 Faint outgoing speech

That 'red thing' looks like one version of the Transmitter 21A. That's an electronic unit using an electret microphone and an IC and is normally very reliable.

Can you measure the DC voltage between the 2 terminals on that Transmitter (with the telephone plugged into the line and off-hook)

Can you also post a photo of the insides of the main part of the telephone (the fixing screw for the cover is between the 2 cables at the back, it should be captive, just undo it until it springs out slightly and the cover is free). I want to see how different it is from a normal Telephone 746.
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Old 25th Nov 2020, 4:59 pm   #8
phonelover555
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Default Re: TMA 7214 Faint outgoing speech

Hello,

The voltage across the transmitter terminals reads a constant 3.14V. I've attached some photos of the phone. Any help would be greatly appreciated.
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Old 25th Nov 2020, 5:12 pm   #9
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Default Re: TMA 7214 Faint outgoing speech

3.14V is a bit low for a Transmitter 21A to work properly I think. That may explain the faint speech.

The PCB in the telephone is quite different to that in a 746. The 'induction coil' (transformer) is there but the components for the voltage regulator are not. That would confirm this was a PABX telephone designed for short lines. Assuming the wiring is right, the terminals are arranged differently to a normal 746 too. Unless you can track down a circuit diagram, you may have to trace it out from the PCB -- diagram N846 (circuit of the Telephone 746) would help with the dial connections, handset wiring and induction coil windings (note that the pins on the that are not arranged in an obvious sequence)
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Old 25th Nov 2020, 5:33 pm   #10
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Default Re: TMA 7214 Faint outgoing speech

Hello,

The voltage across the transmitter is now reading around 15V. I don't understand why this has increased. There is a circuit diagram printed on the inside of the case.
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Old 25th Nov 2020, 5:37 pm   #11
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Default Re: TMA 7214 Faint outgoing speech

Clearer photo
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Old 25th Nov 2020, 5:41 pm   #12
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Default Re: TMA 7214 Faint outgoing speech

It appears there should be a circuit from one side of the line, through the impulse contacts of the dial (orange and pink wires), the transmitter, the 5-4 winding of the induction coil, one section of the gravity switch (hook switch, handset rest, whatever)--MD2, and back to the line.

If the voltage across the transmitter increased then either the line voltage across the terminals has increased (perhaps the telephone is drawing less current for some reason) or, more likely, one of the switches (dial impulse contacts, gravity switch) had dirty contacts causing a high resistance in the circuit. Before doing anything else, clean the contacts on the back of the dial by running a strip of clean paper through them when the are closed. Do not use anything abrasive on them.
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Old 25th Nov 2020, 5:52 pm   #13
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Default Re: TMA 7214 Faint outgoing speech

Quote:
Originally Posted by phonelover555 View Post
Clearer photo
It is a Plessey 7P a version made by their Portuguese subsidiary AEP: Automatica Electrica Portuguesa (Portuguese subsidiary of ATM Ltd) ATE: Automatic Telephone & Electric (ATM was renamed ATE in 1936). ATM: Automatic Telephone Manufacturing Company. Plessey then took over ATE Ltd in the 1960's . Very similar to 706 circuit used on PAXs in the UK.

Ian J - ex Plessey Comms Systems
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Old 25th Nov 2020, 6:09 pm   #14
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Default Re: TMA 7214 Faint outgoing speech

I've had a look at the back of the dial and am unsure how to clean the contacts.
The voltage across the transmitter is now reading just over 3V again.
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Old 26th Nov 2020, 4:13 pm   #15
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Default Re: TMA 7214 Faint outgoing speech

When the voltage across the transmitter is about 3V, what is the voltage across the line wires to the telephone. If that's also around 3V then dirty contacts are not the problem.

The dial has 3 sets of contacts. Just to the left of the yellow bit (cam follower) in your photo are the impluse contacts. You'll see where they have little 'pips' that touch. Carefully move the cam follower to just open the contacts, then put a small strip of paper between those 'pips', let the contacts close on the paper and pull it out. Repeat with clean paper each time until you no longer get black streakes on the paper.

The other 2 sets of contacts are to the right of the central spindle in your photo. They're called the 'off normal' contacts and close when the dial is turned away from the stop. Again you'll see the tiny 'pips' where they touch. So put a strip of clean paper between one set, turn the dial so they close, and pull the paper out while the dial is held 'off the stop'. Again, repeat until you get no streaks, then do the same to the other set.
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Old 26th Nov 2020, 5:00 pm   #16
phonelover555
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Default Re: TMA 7214 Faint outgoing speech

When the phone is off the hook the voltage between the red and white wires into the phone is 3.6V. When it is on the hook it is around 50V.
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Old 26th Nov 2020, 6:09 pm   #17
phonelover555
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Default Re: TMA 7214 Faint outgoing speech

Thank-you very much for the help. I have cleaned the contacts using the method described but it does not seem to have helped.
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Old 30th Nov 2020, 10:51 am   #18
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Default Re: TMA 7214 Faint outgoing speech

Hi, just wondering if anyone has any final ideas on how to get this phone working properly?
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Old 30th Nov 2020, 11:37 am   #19
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Default Re: TMA 7214 Faint outgoing speech

The fact that the voltage across the entire phone is so low suggests that the phone is drawing more current than the line expects. I assume other phones work fine on that line.

There would then seem to be 2 possibilites. Either there is a fault in the TMA7214 causing it to draw more current than it should, or as it was designed to work on a PABX system it was also designed to draw more current than normal phones (the PABX lines being very short after all).
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Old 30th Nov 2020, 11:49 am   #20
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Default Re: TMA 7214 Faint outgoing speech

When it comes to the dial, if you short the 2 normally closed contacts the possible error caused by the dial should be eliminated. All the other contacts should remain open in rest position. (Of course this is just for testing since you will not be able to dial)
The next may be the handset-cord may it be bad, and change resistance when it is moved?
My third idea is the plug in regulator, not sure but I believe this would be turned around and and plugged in to just bypass that function.

Regarding cleaning of contacts, I have been a bad boy and cleaned some when everything else failed by using a strip of sandpaper, that I got from a dentist.
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