BT Maintenance Information No. 030 (June 1984)

This Maintenance Information paper gives general information on the various types of Microphone Inset No. 21 currently available, their applications and some of the problems that may occur under certain circumstances. There are Three manufacturers currently producing Microphone Insets No. 21, these may be identified by case design and manufacturers code as shown below.

          A.P. BESSON                                      GEC                           RATHDOWN INDUSTRIES

A.P. Besson Ltd. - (manufacturers code DAE)

G.E.C. Ltd. - (manufacturers code GEN)

Rathdown Industries Ltd  - (manufacturers code RIA)

BTE Consumer Products.

Microphone Insets 21 are designed for use in telephone instruments that normally use Transmitter Insets 16, they incorporate Non-Carbon microphone units for improved transmission and reliability. There are three manufacturers versions currently available, all three have been coded `Microphone Insets 21A'.

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.

Suitable for use in 700 type and other telephones that normally use transmitter insets 16.

Transmission problems can arise however if telephones with Microphone Inset 21 are mixed with carbon microphone instruments on the arrangements shown below:-

  • Simple plan arrangements where customers are in the habit of conducting three party conversations or answering a call at one extension and continuing it at. another.
  • Extension to Extension calls or Operator to Extension calls on small PMBXs with non-divided transmission feed (e.g. 2/PMBXs)
  • Intercom calls on Extension Plan 105/107 arrangements.

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.

There were problems with early production units made by A.P. Besson, these resulted in terminal posts coming loose from the circuit board and transmission cut-off on line reversal.  Quality control has been improved to overcome these problems.  GEC models were found to hum if used near mains powered equipment, these models (marked `S' on the front cover) have now been modified to overcome this.

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.

Microphone Insets 21 have screw terminals suitable for terminating 0.25 in. spade terminals.


Microphone Insets 21 incorporate line powered amplifier circuits that requires approximately 4 Volts to `switch on'.  They are designed to work satisfactorily from 100 mA down to 20 mA Line current and with possible degraded transmission between 20 mA and 10 mA.

Microphone Inset 21A (Item Code 436189)
Microphone Inset 21B (Item Code 877034)

Telephones with Microphone Insets 21 fitted may result in high resistance telephone loop tests, this is due to the Line powered amplifier circuitry of the microphone not switching on at low voltages.

Transmitter Inset No. 21
Each variant opened up

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Last revised: December 14, 2010