CONVERTING OLD TELEPHONES
TO PLUG AND SOCKET

Click here on where to obtain a telephone line cord (conversion kit)

Around 1979, Post Office Telephones introduced the Plug and Socket (PST) method of connection. This document explains how to convert many old British telephone instruments to PST working.  The following information assumes that a standard new plan BT line cord (light grey, flat plastic (ridge on one flat) with a flat plug one end and spades the other) is used.  Do not attempt to fit a new style plug to the old style round PVC or cloth telephone line cord - this usually fails - buy the correct cord or similar.

If your replacement cord has no grommet (only used on 700 type telephones) just slit the original grommet with a sharp knife, pull out the old cord, insert the new cord with a blob of glue and refit into phone.  The original 200 and 300 type telephone line cords were restrained by a cotton tie.  For these phones just insert the replacement line cord into the telephone and tie a knot in it.  This will, in most cases, stop the cord from being pulled out of the case.

There are references to strapping and straps in the text below.  This refers to the metal links that connect two terminals together. They can be removed be releasing the screws that hold them captive - always tighten the screws again afterwards as they have a tendency to fall out. In some instances these straps are replaced by wire.

There are also references below to T and B numbers.  T1 would mean terminal 1 in the telephone, whilst B2 would mean terminal 2 in the Bellset.  Any other letters will be self evident.

The cord colours used by BT are shown below and are used throughout this page.  If you are not using a standard BT cord then click here.

BT CORD COLOURS:- R = RED, W = WHITE, B = BLUE, G = GREEN

The conversions below assume that the original telephone line cord will be discarded and replaced with a modern line cord.

Converting an old GPO telephone does not make it approved and therefore it cannot be legally connected to any Telecom network in the UK.

Always fit a rectifier unit (Rectifier Element No. 205) across the earpiece of a converted telephone (stops acoustic shock) and because the original carbon granule transmitters are generally noisy or faint it is also advisable to change them to a modern electronic type (these are a direct replacement for 700 type transmitters and can be modified to fit in Bakelite handsets).

Click here for instructions on how to fit an electronic Transmitter and Rectifier Element No. 205

Click here to view a fully converted Tele No. 746 - also applicable to all 700 type telephones

Click here on where to obtain a telephone line cord (conversion kit)

Click here on how to wire up sockets

GPO Telephone 150 with Bellset No. 1 or Bellset 25

Telephone is wired to standard diagram (N250) which includes the Bellset but modified as follows:-

  1. Telephone cord - White - wired between T in phone and L2 in a Bellset No. 1
  2. Telephone cord - White - connect directly to Red of line cord in a Bellset No. 25.  Use a small nut and bolt and then cover with insulation tape
  3. Telephone cord - Red - wired between TR in phone and TR in Bellset
  4. Telephone cord - Green - wired between R in phone and R in Bellset
  5. White of new line cord to L1 in Bellset
  6. Remove strap between L1 and E and insert a 3.3k ohm resistor
  7. Remove the T - L2 metal strap in the Bellset No. 1.  There is a soldered strap in a Bellset No. 25, behind the terminal plate, in this instance leave the strap in place
  8. Red of new line cord to L2 in a Bellset No. 1
  9. Blue of new line cord to T in Bellset
  10. Short out the two terminals of the capacitor with a piece of wire
  11. Green of new line cord - insulate with tape (not used)

To disconnect bell (if needed)
Remove strap between L1 and E or leave out the resistor

GPO Telephone 162

Telephone is wired to standard diagram (N262) which includes the Bellset No. 1 or 25 but modified as follows:-

  1. Telephone cord - White - between 3 in phone and T in Bellset
  2. Telephone cord - Red - between 1 in phone and TR in Bellset
  3. Telephone cord - Green - between 2 in phone and R in Bellset
  4. White of line cord to L1
  5. Remove strap between E and L1 and insert a 3.3k ohm resistor
  6. Red of line cord to L2 - move the white wire of the telephone cord from T to L2
  7. Remove strap T - L2, which is under the board.
  8. Blue of line cord to T - short out the two terminals of the capacitor.
  9. Green of line cord - insulate with tape (this wire is not used)

To disconnect bell (if needed)
 Remove strap between L1 and E or leave out the resistor

GPO Telephone 221

Telephone is wired to standard diagram (N321) which includes the Bellset but modified as follows:-

Telephone with numbered terminals:-

  1. Disconnect the green/white wire from the capacitor.
  2. Connect the green/white to the blue wire of the line cord and insulate.
  3. Remove the strap between terminals 6 and 7.
  4. Insert a 3.3k resistor between terminals 6 & 7.
  5. White of the line cord to terminal 6.
  6. Red of the line cord to terminal 9.
Telephone with lettered terminals:-
  1. Disconnect and sleeve the Green wire that is on TR (it goes to terminal 2 of the Induction Coil and is not needed).
  2. Move the Orange wire from the Condenser that is on L2 - onto TR.
  3. Wire the Condenser tags together (square metal box).
  4. Remove the strap between L1 and E (this may be soldered under the terminal strip).
  5. Insert a 3k3 resistor between L1 and E.
  6. Connect the white line cord wire to L1.
  7. Connect the red line cord wire to L2.
  8. Connect the blue line cord wire to TR.

GPO Telephone 232

Telephone is wired to the standard diagram (N332) but modified as follows:-
(B = Bellset terminal & T = Telephone Terminal).

With Bellset No. 26 and using the original cloth cord between the telephone and Bellset.

  1. Remove Bellset strap B4 - B5.
  2. Remove Bellset strap B2 - B3.
  3. Insert a 3.3k ohm resistor between B2 and B3.
  4. Ensue that Bellset strap B7 - B8 is in place.
  5. Wire the cords as per the table below.
  6. Green of the line cord is not used - insulate with tape.

With Bellset No. 31 and using the original cloth cord between the telephone and Bellset.

  1. Remove Bellset strap B6 - B7.
  2. Connect 3.3k resistor between Bellset E and B9.
  3. Wire the cords as per the table below.
  4. Green of the line cord is not used - insulate with tape.

With Bellset No. 41 and using the original cloth cord between the telephone and Bellset.

  1. Remove Bellset strap B8 - B9.
  2. Remove Bellset strap B6 - B7.
  3. Remove Bellset strap B2 - B4.
  4. Connect 3.3k resistor between Bellset B5 and B8.
  5. Wire the cords as per the table below.
  6. Green of the line cord is not used - insulate with tape.
Telephone Old Cloth Line Cord
wire colours		 Bellset 26 Bellset 31 Bellset 41 New style Line Cord wire colours
T3 White B5 B4 B4 Red
T1 Red B7 B7 B5 White
T2 Green B3 B1 B9 Blue

Without Bellset and not using the original cloth cord between the telephone and Bellset (New style line cord directly connected phone).

  1. Red of line cord to T3.
  2. White of line cord to T1.
  3. Blue of line cord to T2.
  4. Green of line cord is not connected - insulate with tape.

GPO Telephone 246

Telephone is wired as follows.  The standard diagram for the Phone is N326 and the wiring Diagram for the phone and Bellset No. 37 is N4299.  (B = Bellset terminal & T = Telephone Terminal).

  1. Red of line cord to B6.
  2. White of line cord to B8.
  3. Blue of line cord to BE.
  4. B8 wired to T8.
  5. B1 wired to T9.
  6. If no dial is fitted then wire B6 to B4 and T7.
  7. B9 wired to T2.
  8. BE wired to T1.
  9. B3 wired to T6.
  10. B2 wired to T5.
  11. Remove the soldered wire link between B7 and B8 - it is under the board.
  12. Wire a 3.3k Resistor between B7 and B8 (leave out if bell not to be connected).
  13. Remove the white wire from Capacitor and insulate the end.

If dial is fitted:-

  1. Remove link between B4 and B6.
  2. Leave T7 wired to B4.
  3. Dial 1 and Dial 3 terminals to B5.
  4. Dial 2 to B1.
  5. Dial 4 to B4.
  6. Dial 5 to B6.

GPO Telephone 248 and Bellset No.39A

This method of wiring is only applicable for the base unit and no extension phones may be wired from the Bellset.  The Bellset switch should be left in the speak to exchange position.

Telephone and Bellset are wired to follows (no bells are fitted):-
  1. T3 to B19 and B20.
  2. T9 to B21.
  3. T10 to B17.
  4. T11 to B16.
  5. T1 to B15, B14 and B13.
  6. Blue of the line cord to T2.
  7. White of the line cord to B1.
  8. Red of the line cord to B3 and B4.
  9. Green of the line cord - insulate - not used.

GPO Telephone 248 and Bellset No.44

Telephone and Bellset are wired to follows (no bells are fitted):-
  1. T3 to B34 and B35.
  2. T9 to B31.
  3. T10 to B26.
  4. T11 to B25.
  5. T1 to B19, B20 and B21.
  6. Blue of the line cord to T6.
  7. White of the line cord to B1 and B2.
  8. Red of the line cord to B3 and B4.
  9. Green of the line cord - insulate - not used.

GPO Telephone 306

Telephone is wired as follows. The standard diagram is N406.

  1. Remove strap T10 - T11
  2. Remove any wire or straps connected to T2, T3, T7, T12 or T13
  3. Do not fit a 3.3K ohm resistor as this telephone does not have a magneto bell
  4. Strap T8 - T9
  5. Red of line cord to T8
  6. White of line cord to T1
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T7
  9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 308

Telephone is wired as follows. The standard diagram is N408.

  1. Remove strap T10 - T11
  2. Strap T11 - T12
  3. Install a 3.3K ohm resistor between T1 and T2
  4. Strap T8 - T9
  5. Red of line cord to T8
  6. White of line cord to T1
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T7
  9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 310

Telephone is wired as follows. The standard diagram is N410.

  1. Remove strap T10 - T11
  2. Strap T11 - T12
  3. Install a 3.3K ohm resistor between T1 and T2
  4. Strap T1 - T9
  5. Red of line cord to T8
  6. White of line cord to T1
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T7
  9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 311

Telephone is wired as follows. The standard diagram is N411.

  1. Remove strap T10 - T11
  2. Strap T11 - T12
  3. Install a 3.3K ohm resistor between T1 and T2
  4. Strap T1 - T7
  5. Red of line cord to T8
  6. White of line cord to T1
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T3
  9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 312

Telephone is wired as follows. The standard diagram is N312.

1. Remove strap T10 - T11
2. Remove strap T12 - T13
3. Insert a 3.3K ohm resistor between T11 & T12
4. Insert straps T1 - T2 - T3 - T9
5. Red of line cord to T8
6. White of line cord to T1
7. Blue of line cord to T11
8. Green of the line cord is spare and can be parked on T7
9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 314

Telephone is wired as follows. The standard diagram is N414.

  1. Remove strap T10 - T11
  2. Remove strap T12 - T13
  3. Install a 3.3K ohm resistor between T11 and T12
  4. Insert strap T7 - T2
  5. Red of line cord to T8
  6. White of line cord to T7
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T9
  9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 321

Telephone is wired as follows. The standard diagram is N421.  If this telephone is wired with a switch, thermistor and diode then remove these items and follow the instructions below.

  1. Remove strap T10 - T11
  2. Remove strap T12 - T13
  3. Install a 3.3K ohm resistor between T12 and T13
  4. Insert strap T11 - T12
  5. Strap T1 - T2, T8 and T2
  6. Red of line cord to T3
  7. White of line cord to T1
  8. Blue of line cord to T11
  9. Green of the line cord is spare and can be parked on T9
  10. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 326

Telephone is wired as follows. The standard diagram is N426.
This telephone has no ringer that will work on an exchange line, so your phone will not ring.

  1. Remove strap T10 - T11
  2. Remove any wire or straps connected to T2 or T12
  3. Do not fit a 3.3K ohm resistor as this telephone does not have a magneto bell
  4. Strap T8 - T9
  5. Red of line cord to T8
  6. White of line cord to T1
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T7
  9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 327

Telephone is wired as follows. The standard diagram is N427.

  1. Remove strap T10 - T11
  2. Remove any wire or straps connected to T2 or T12
  3. Do not fit a 3.3K ohm resistor as this telephone does not have a magneto bell
  4. Strap T8 - T9
  5. Red of line cord to T8
  6. White of line cord to T1
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T7
  9. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 328

Telephone is wired as follows.  The standard diagram is N428.

  1. Remove strap T10 - T11
  2. Strap T8 - T9
  3. Strap T1 - T2
  4. Insert a 3.3K ohm resistor between T11 & T12
  5. Red of line cord to T9
  6. White of line cord to T1
  7. Blue of line cord to T11
  8. Green of the line cord is spare and can be parked on T7

Bell On/Off switch

Bell switch not needed - phone will always ring

  1. Remove all switch wires from the telephone terminals and insulated with tape.  Wire telephone as above.

Bell switch needed - switch will disconnect bell from ringing

  1. Remove all switch wires from the telephone terminals
  2. Wire the phone as above, but omit the strap between T1 - T2
  3. Connect red wire of switch to T1
  4. Connect white wire of switch to T2
  5. Insulate any other coloured wire from the switch.

GPO Telephone 330, 332 & 333

Telephone is wired as follows.  The standard diagrams are N430, N432 & N433.

  1. Insert strap T8 - T9
  2. Insert strap T1 - T2
  3. Red of line cord to T9
  4. White of line cord to T1
  5. Blue of line cord to T11
  6. Green of line cord to T3
  7. Remove strap T10 - T11
  8. Remove strap T11 - T12
  9. Insert a 3.3K ohm resistor between T11 and T12
  10. Rectifier No. 205 (if needed) inserted between T6 & T5

GPO Telephone 706, 710, 741 & 746

Telephone is wired as follows. The standard diagrams are (N806, N810, N841 & N846).

The bell or ringing device should be 4000 ohms resistance.  This can be rectified by either fitting the correct ringer or by inserting a 3.3K resistor in series with the ringer.  The telephone should already be fitted with a Rectifier No. 205.  If one is not fitted then it is recommended that one is fitted.

  1. Remove strap T6 - T7
  2. Remove strap T4 - T5
  3. Insert strap T5 - T6
  4. Red wire of line cord to T8
  5. Blue wire of line cord to T6
  6. Insert straps T16 - T17 - T18 - T19
  7. White wire of line cord to T18
  8. Green wire of line cord to T15
  9. Insert a 3.3K resistor between T4 - T5
  10. Insert a Rectifier No. 205 (if needed) between T1 and T2

The original BT line cord is called a Cord Connecting No 4/502

Bell On/Off switch
To fit a bell on/off switch obtain and fit the following items:-
(746 style phones need 2 pins, On/Off button and Switch No. 23A, whilst 706 phones need Switch No. 5A and the appropriate coloured On/Off button).

Wire and install as follows:-

  1. Remove strap T16 - T17
  2. Connect Brown wire of switch to T16
  3. Connect Grey and Blue wires of the switch to T17

Telephone No. 706 switch installation

  1. Remove plastic plate from top of telephone case
  2. Remove one of the screws that fixes the switch to the bracket
  3. Slacken off the other screw.
  4. Slide the bracket round so that the plastic button can locate between the switch and the bracket
  5. Slide the switch back into position and insert the screws.
  6. Fit the switch to the phone by first ensuring that the fixing screw is unscrewed at least 3/8th inch
  7. Then offer the switch to the phone at an angle and engage the lug
  8. Drop the other end of the switch down and fix by means of the single screw

Telephone No. 710, 740 and 746 switch installation

  1. Remove plastic plate from top of telephone case
  2. Insert button so that slots in button line up with slots inside casing
  3. Insert one pin so that it goes through the button and rests on the two slots inside the case.
  4. Push the pin into the slots.
  5. Repeat steps 6 & 7 for the other pin.

GPO/PO Telephone 722, 766 & 786 (Trimphone)

Telephone is wired as follows. The standard diagrams are (N822, N866 & N886).

The bell or ringing device should be 4000 ohms resistance.  This is achieved by inserting a 3.3K resistor between T4 & T5.  It is sometimes preferable to not wire a Trimphone ringer up.  This is because it can sometimes upset the rest of your telephone ringers.  The Trimphone volume control has an Off position, which is obtainable by removing the case and slackening off the screw on top of the volume control switch itself.

  1. Remove strap T6 - T7
  2. Remove strap T4 - T5
  3. Insert strap T5 - T6
  4. Red wire of line cord to T8
  5. Blue wire of line cord to T6
  6. Insert straps T16 - T17 - T18 - T19
  7. White wire of line cord to T18
  8. Green wire of line cord to T15
  9. Insert a 3.3K resistor between T4 - T5

The original BT line cord is called a Cord Connecting No 4/504.

GPO/PO Telephone 776 (Compact) and Jubilee Compact

Telephone is wired to standard diagram (N876) but modified as follows:-

Telephone No. 776 and Jubilee without Bellset

  1. Remove strap T2 - T3
  2. Insert strap T1 - T2
  3. Blue of line cord to T1
  4. Red of line cord to T4
  5. White of line cord to T5
  6. Green of line cord to T6

Telephone No. 776 with Bell Unit No. 776 (using original cord between Telephone and Bellset)

  1. Remove strap T2 - T3
  2. Insert strap T1 - T2
  3. Insert strap B1- B2 - B3
  4. Remove strap B5 - B6
  5. Insert 3.3k resistor between B5 and B6
  6. Connect the cords as detailed in the table below (any colours in the original cord not mentioned below should be not used and insulated with tape.
Telephone Original cord Bellset New line cord
T4 - Red - B4 - Red
T1 - Blue - B5 - Blue
T5 - White - B1 - White
T7 - Green - B2 - Green not connected

Jubilee and Telephone SA4271 with Bellset Bell Unit SA4271(using original cord between Telephone and Bellset)

  1. Remove strap T2 - T3
  2. Insert strap T1 - T2
  3. Insert strap B5 - B6 - B7
  4. Remove strap B3 - B4
  5. Insert 3.3k resistor between B3 and B4
  6. Connect the cords as detailed in the table below (any colours in the original cord not mentioned below should be not used and insulated with tape.
Telephone Original cord Bellset New line cord
T4 - Red - B1 - Red
T1 - Blue - B3 - Blue
T5 - White - B7 - White
Green not connected

 

GPO/BT TELEPHONE SPECIAL RANGE (TSR)

TSR 1001
 (Mickey Mouse)
 
1. Remove black and grey wire links (if they exist)
2. Link terminals 1, 2 and 9
3. Connect one wire of the ringer to terminal 7
4. Connect other wire of the ringer to terminal 9
5. Connect 3.3k resistor to terminals 5 and 7
6. Connect the white wire of the line cord to terminal 5
7. Connect the red wire of the line cord to terminal 4
8. Connect the blue wire of the line cord to terminal 2
 
TSR 1002
 (Contempra)




	TSR terminals		Y X G L1 L2
	Cord Inst 4/502		- R W G  B 

NOTES:
1. Remove existing cord
2. Cut off grommet from new cord 4/502
3. Connect new cord 4/502 to terminals as shown above and secure cord
4. Fit a thermistor 1a-1 between terminals 1 & 4 (if no terminal 4 appears
   on PCB then the thermistor should be fitted between terminals G & Y).

TSR 1003
 (Astrofon)



	TSR terminals		9 8 7 6 5 4 3 2 1
	Cord Inst 4/502		      B   G R W

Notes:
1. Remove existing line cord and connect new Cord 4/502 to terminals as
   shown above.
2. Secure new cord and fit thermistor 1a-1 between terminals 2 & 5.

TSR 1004
 (Ericofon 700)



	TSR terminals		- 1 - 2 3 B R 4 5  W
				- 6 G 7 8 - - 9 10 -

Notes:
1. Remove existing cord.
2. Remove grommet from new cord 4/502 and connect as shown above.
3. Link W to the point below but to the right of 10.
4. Secure new cord.

TSR 1009
 (Eiger)



	TSR terminals		2 3 4
	Cord Inst 4/502		R W -

Notes:
1. Remove existing line cord and remove grommet.
2. Remove the ends of the spade connectors on new Cord 4/502.
3. Solder the stumps to the TSR terminals as shown above.
4. Secure new cord 4/502 by modification of existing grommet.
5. Solder a thermistor 1a-1 to terminals 3 & 4.

TSR 1000
 (Classic)




	TSR terminals		19 18 17 16 15 11 10 9 8 7 6 5 4
	Cord Inst 4/502		   W     G           R     B

Notes:
1. Remove existing line cord and remove grommet.
2. Remove grommet from new Cord 4/502 and connect to terminals as shown
   above.
3. Secure line cord internally by re-utilisation of existing grommet.
4. Fit a thermistor 1a-1 between terminals 17 & 18.

TSR 1007
 (Ericofon 600)



	TSR terminals		1         8
				2 3 4 5 6 7
	Cord Inst 4/502		  R   W

Notes:
1. Remove existing line cord.
2. Remove grommet from new cord 4/502 and connect to terminals as shown
above.
3. Secure new cord internally.



    

  

    
      

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Last Revision: March 15, 2008


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