TRANSMITTER INSET No. 21
BT Maintenance Information No. 030 (June 1984)
G.E.C. Ltd. - (manufacturers code GEN)
Rathdown Industries Ltd - (manufacturers code RIA)
The A.P. Besson and GEC models each contain an electret microphone capsule with line powered, Integrated Circuit amplifier. Rathdown Industries models contain a moving coil microphone insert with line powered, transistor amplifier, the only difference between the two types of Rathdown Industries Microphones is that the red bodied model contains an extra capacitor intended for Radio Frequency Interference suppression.
The Rathdown Industries model with the RFI capacitor will be separately available from April 1984 onwards as the `Microphone Inset 21B and will have a grey coloured body.
FIELD OF USE
Transmission problems can arise however if telephones with Microphone Inset 21 are mixed with carbon microphone instruments on the arrangements shown below:-
In these arrangements the telephones are connected in parallel and transmission current must divide equally to energise both instruments microphones/transmitters. The line powered amplifier in the Microphone Inset 21 presents a higher impedance to line than a carbon microphone and requires approximately 4 Volts to 'switch on'. When a carbon microphone telephone is used in parallel with a Microphone Inset 21 telephone, most of the current will pass through the carbon microphone causing distortion or transmission cut off from the Microphone Inset 21. The problem may be overcome by connecting two 4.7 Volt Zener diodes (e.g. CV 7409) anode to anode in series with any carbon microphones on the system. This will enable current to divide more equally, energising the transmission circuits of both instruments.
All types of Microphone Inset 21 may be affected by radio frequency interference (RFI), this may be overcome by changing for a different type of Microphone Inset 21 or as a last resort, by replacing with a Transmitter Inset 16.
Certain types of press button telephone rely on a D.C. path to energise the keypad and provide a transmission path, these are telephones marked:-
3/756 GNA 79/1 or 756 GNA 79/4 or 756 GNA 80/4.
Whenever a Microphone Inset 21 is fitted in one of these instruments a 1,000 ohm resistance e.g. a `resistor 91 EF,1 K' should be fitted to telephone terminals T3 and T10 (i.e. across the Mic.) to provide a D.C. path.
Bessons or GEC type Microphone Insets may cause `High Side Tone' when fitted in Herald or Ensign terminals, Microphone Insets 21B or Transmitter Insets 16 may be used in these instruments instead.
Transmitter Inset No. 21
A.P. BESSON GEC RATHDOWN INDUSTRIES
Last revised: June 30, 2019