HOW TO CHANGE THE TRANSMITTER 
TO A MODERN TYPE ON GPO TELEPHONES

Click here to find where to obtain your Transmitter No. 21A - Transmitter No. 21A page

Click here to see how to wire in a Rectifier-Element No. 205

Click here for a picture of a converted 700 type telephone

Fitting a GPO/PO/BT Transmitter No. 21A

  1. Obtain a Transmitter No. 21A - electronic type.
    For 200 type and 300 type ONLY use the red with grey button in middle type - marked DAE on the back and NOT the blue & black type - marked RIA on the back.  Both types will work on 700 type phones.

  2. On 200/300 types - remove telephone mouthpiece - use a nail,  pop-rivet, drawing pin (or similar pointed object), push into hole and at the same time turn the mouthpiece anticlockwise until it disengages.

    On 700 types the mouthpiece just unscrews and the transmitter can be replaced easily.  The wires can go any way round.
     

  3. Continue for 100, 200 and 300 telephones with Bakelite handsets.

  4. Remove the existing transmitter from the handset.

  5. Remove the centre pin which is exposed on removal of the transmitter - the plate is now loose.  Remove the two springs on the top of the transmitter housing and use the redundant screw in place of the pin to hold the plate in place.

  6. Connect two wires, one to MR and the other to M in the handset.  These are marked next the terminals in the Bakelite - hard to see if in the wrong light - check out the line drawing to the right (these terminals normally have the white and red wires of the handset on them).  MR is the metal plate.

  7. With reference to the picture to the right, take the electronic transmitter apart, by prising off the front cover and remove the black unit  (item 2).  Discard the housing (item 1) and keep the front plate (item 3).

  8. Connect the black unit (item 2) to the two wires that were previously fitted in 6.

  9. Place a small piece of foam inside the transmitter housing of the handset and then place the transmitter black unit (item 2) followed by the transmitter front cover (item 3) on top of the foam.  Settle them back into the handset.

  10. With the hole in the mouthpiece pointing at the handset handle, locate the mouthpiece into position, push down and then turn in an clockwise direction until it stops.

 

 

 

 

 

 

 


 

 

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Fitting a Non-GPO transmitter

The above transmitters are becoming more difficult to obtain.  Evidently a US electronic transmitter, the EMS-94, will fit in Bakelite handsets and does work.

 

Fitting a Rectifier-Element No. 205

This device is actually two diodes connected in across the receiver and back to back with each other.  The Rectifier Element No. 205 is two diodes specially packaged for the GPO.  The GPO device looks like a red or black blob with two leads from it.  They were never originally fitted to 200 or 300 type telephones.

The rectifier-element was fitted to stop acoustic shock.  There are instances where a loud noise is heard and this can cause headaches and is not very pleasant.

The device fits across the receiver and as all phones have different terminations then the relevant diagram must be referred to.

All 700 types phones have the receiver connected to terminals T1 and T2 and cord wires are red and green.  In nearly every case a 740, 741 & 746 will have the Rectifier-Element fitted as standard, whilst 706, 710 & 711 may not have one.  If the rectifier is missing then just wire the device across these terminals.

In 200 and 300 type telephone the rectifiers should be fitted across the relevant receiver terminals inside the telephone - these can be found by following the wires from the handset connections R and MR (normally - green and red wires).

The device is not polarity conscious and can be fitted any way round.

Click here on how to make a Rectifier-Element No. 205.

A Fully converted 706

The picture below shows a Telephone No. 706 fully converted.  Note that this is the printed circuit board version and therefore no dial wires will be terminated on the screw terminals.  On this type, the dial wires are directly soldered to the circuit board, except the brown wire which is terminated on terminal 3.

 

 

A fully converted 746

The picture below shows a Telephone No. 746 converted to PST.  The handset cord is shown on the left for clarity, although it should really be on the right.  The red blob shaped component to the left is the Rectifier-Element No. 205 and was a standard fitting on all 746 type telephones and most later 706 telephones.  The cream oblong object in the centre of the picture is the resistor, in place for a conversion (these can be different sizes and colour).

 
 
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Last revised: April 10, 2023

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