TRANSMITTER, INSET



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Transmitter, Inset No. 1

C.B. Transmitter Inset used on Telephones No. 78, 86 (Mark 235 & 236) and 94.

 

 
Transmitter, Inset No. 2

This transmitter is a solid back type but is constructed in a cheaper fashion.  It is a capsule type.

It was used on C.B. Telephones No. 80 and 140.  Also used with Transmitters No. 11, 12 & 15.

 

 
Transmitter, Inset No. 3

Early capsule transmitter used in Telephone No. 28 Mark 234 and other Local Battery (L.B.) telephones.

Used by the Transmitters No. 2, 3 and 10 Mark 234.

The picture shows the rear of the transmitter.

Drawing - 62685 (Mark 235).

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Transmitter, Inset No. 5

C.B. Transmitter Inset used on the Transmitter No. 1 Mark 236.

 

 
Transmitter, Inset No. 9

Used on Electrophone Stage Circuits and the Transmitter No. 16 (Mark 234).

 

 
Transmitter, Inset No. 10

Carbon granule transmitter introduced in the late 1920's and used in all Bakelite handsets and Candlesticks (when fitted with the Transmitter No. 22).

The mouthpiece cover of the Mark 234 was fixed by 4 screws (these were made in the GPO experimental shop).

The Mark 235 had a graphite electrode and the cover was fixed in place by a concealed spring.

Introduced in 1929 and superseded by the Transmitter-Inset No. 13 in 1939.

Mark 235 may have the TMC number 4006A on it.

Drawing - 62686 (Mark 235).

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Transmitter, Inset No. 11

Used on Handset No. 186 (Laryngaphone),

 

 
Transmitter, Inset No. 13

Carbon granule transmitter used in all Bakelite handsets and some early 700 type telephones.

There appears to be two variants.  The Mark 1 was all metal construction (Siemens), whilst the Mark 2 had the back part moulded out of black Polystyrene (Ericsson).

Transmitter, Inset No. 13A was used in the Transmitter No. 23 and Bakelite handsets.  There is no cover on this model.

Transmitter, Inset No. 13AA was used in the Transmitter No. 23A.  There is no cover on this model.

Transmitter, Inset No. 13B are identifiable by a screw-nut, located on the spring plate, on the rear of the unit.

Transmitter, Inset No. 13C were of similar construction as the Transmitter, Inset No. 13B but were also supplied with a nut on a threaded pin (1/DPL/397) which was pushed into the centre hole at the rear.  This version was used on 700 type telephones.

This Transmitter-Inset superseded the Transmitter-Inset No. 10.
It was subsequently superseded by the Transmitter-Inset No. 16 for 700 type telephones.

Introduced in 1939.

Construction diagram - EX22499.

Drawing - 62808.

Specification - S133.

Ericsson Code No. N7752L (No. 13C).
STC Code No. 4025A.

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Transmitter, Inset No. 14

Part of Telephone No. 252.

 

 
Transmitter, Inset No. 15

Carbon granule transmitter used in Trimphone models and headsets.

The rubber grommet on the front is removed when used in Trimphones, but retained when used with Headsets No. 1 and 2.

The granules could be Morganite Carbon Ltd, Type TG 68DO.

Drawing - CD1389.

Specification - S508.

 


 
Transmitter, Inset No. 16

Carbon granule transmitter in general use from 1964.  

Transmitter Inset No. 16 was generally fitted to all 700 type telephones.

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Superseded by Transmitter Inset No. 21.

Ericsson part No. N7755A1.

Mark 1- Has three screws securing the two haves of the device.  The granules could be Morganite Carbon Ltd, Type TG 68DO.
Depth - 0.865in.

Mark 2 - Sealed unit.  The granules could be Morganite Carbon Ltd, Type TG 68DO.

Mark 3 - Sealed unit identified by the number 3 after the year of manufacturer and red sealing compound will be evident.  Mark 3 insets were not to be used in public kiosks. Not used after 1969.

Mark 4 - Sealed unit, internally modified.

The carbon granule grade was to Morganite Carbon Ltd, Type TG 68D0.

Drawing - 92220.

Specification - S814.

 


  
Transmitter, Inset No. 17

This was an experimental high resistance carbon microphone for use in telephones connected to an electronic exchange.

Tested between 1962 and 1963.

 

 
Transmitter, Inset No. 21A

This Transmitter, Inset superseded the Transmitter, Inset No. 16.  These transmitters were electronic and require line current to operate.

No. 21B was introduced at the end of 1982 and incorporated RFI suppression circuitry.

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Last revised: January 30, 2024

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