Transmitter number 21a circuit diagram

[trevwgb]

Hi
As you know this transmitter is on a circuit board - I am trying to get it into a number 28 telephone ( a very small space ) whose origional transmitter had disintigrated! . I have removed it from the red and the black casing and have removed the terminals, but it's still too big. It has 3 resistors, a capacitor and something else ( I don;t know what this is ) which has on it "Ferranti ZN 470AE 8222". I think that I might be able to fit these into the space if they wern't on the board. Does anyone know where I can find the circuit diagram for this transmitter - or does anyone have any other ideas? Are there any smaller transmitters?
Thanks
Regards
Trev

[russell_w_b]

Originally Posted by trevwgb

Hi
As you know this transmitter is on a circuit board - I am trying to get it into a number 28 telephone ( a very small space ) whose origional transmitter had disintigrated! . I have removed it from the red and the black casing and have removed the terminals, but it's still too big. It has 3 resistors, a capacitor and something else ( I don;t know what this is ) which has on it "Ferranti ZN 470AE 8222". I think that I might be able to fit these into the space if they wern't on the board. Does anyone know where I can find the circuit diagram for this transmitter - or does anyone have any other ideas? Are there any smaller transmitters?
Thanks
Regards
Trev

I think it's a VOGAD IC (Voice Operated Gain-Adjusting Device). Have you considered drawing out the connections from the PCB on a sheet of paper? Start at, say, mic terminal 1 and draw out what it is connected to, then terminal 2, etc... Put a little mark on the PCB track with a felt-pen to show that you've drawn it out (it saves going down the same route twice).

[russell_w_b]

Some years ago when I had nothing better to do, I disembowelled both a red mic insert and a blue mic insert: 21A DAE 82/1 and 21/A (AKG) repectively. You may find the attached drawings of use.

Knowing what the IC does isn't necessary, as long as you can remove all the components satisfactorily from the PCB and remount them where convenient. The IC (as I recall) is a standard 14-pin chip, so it should fit in a standard IC holder.

HTH

[Nick G8GRA]

Russell - hi,

Interesting info on the Type 21A DAE red Carbon-Compatible mic insert. I just assumed it was a true carbon mic and didn’t know it had an electret insert and Ferranti chip to emulate carbon.

Do you, or any one else for that matter, know what the original specification for it was? Does any one know if there is a BT specification about still? What I am trying to find out is:

- Its impedance at 1kHz,
- Its output from speech (peak or average) when held normally in front of the mouth, and,
- What voltage or current it needs to drive it.

Thanks,

Nick
G8GRA/M0PYE

[russell_w_b]

Originally Posted by Nick G8GRA

Russell - hi,

Interesting info on the Type 21A DAE red Carbon-Compatible mic insert. I just assumed it was a true carbon mic and didn’t know it had an electret insert and Ferranti chip to emulate carbon.

Do you, or any one else for that matter, know what the original specification for it was? Does any one know if there is a BT specification about still? What I am trying to find out is:

- Its impedance at 1kHz,
- Its output from speech (peak or average) when held normally in front of the mouth, and,
- What voltage or current it needs to drive it.

Funny you should ask... I've just fitted one to my BPO 332 bakelite telephone and it works a treat! I can provide you with an impedance characteristic of a receiver or possibly a carbon mic at 1KHz, but not, as yet, an impedance for the 21A. I shall look, though...

I have only done subjective testing of the mic and nothing amounting to science, but I shall say that the speech is crystal-clear compared with a 'Mic, Inset, No:13', although I think the spit-cup mouthpiece on the bakelite 164 (or equivalent) handset tailors the response slightly.

Mine runs quite happily at 6V and draws 40mA - same as for the mic (Transmitter, Inset 13) it replaced. Now, the microphone capsule it was meant to replace is the 'Transmitter, Inset, No: 16'. I would consider the spec the same, as there don't seem to be any other mods to the telephones to which Inset 21A electrets were fitted. If there were, I'd expect those who know of them to speak up! I get the impression that the current / impedance spec hasn't changed from the original 'Transmitter, Inset, No:13', just performance and sensitivity have improved within the constraints of the original design. Do remember that 'Transmitter, Inset, No:13' (or inset No:10 - an earlier version) has been used on all instruments from the bakelite models of the 1930's and 40's through to the earlier type 706, and that they work equally well on a type 746 (or 8746), the instrument to which an electret type 21A was introduced in 1982.

We found at work that telephones (this was when we had a BT 'Herald' analogue system) fitted with carbon mics were more prone to demodulation of RF from the high-power HF transmitters than those instruments fitted with a 21A capsule.

If I find out more, I'll certainly let you know, but I have had no problems in the direct exchange of a type 21A for any earlier type of carbon insert.

[russell_w_b]

Originally Posted by russell_w_b

Mine runs quite happily at 6V and draws 40mA - same as for the mic (Transmitter, Inset 13) it replaced. Now, the microphone capsule it was meant to replace is the 'Transmitter, Inset, No: 16'...

I've just checked on our one remaining 741 telephone at work, fitted with a carbon 'Transmitter, Inset, No:16' (directly replaceable with an insert 21A) and I get 6.2V @ 40mA at the mic.

[Herald1360]

ZN470 see attached shortform.

Chris

[russell_w_b]

Originally Posted by Herald1360

ZN470 see attached shortform.

Chris

Excellent! There was an AKG dynamic version as well, coloured blue.