CONVERTING OLD TELEPHONES
TO PLUG AND SOCKET

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 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 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 the Transmitter and Rectifier Element No. 205

Click here to view a fully converted Tele No. 746

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

Click here on how to wire up sockets

GPO Telephone 150

Telephone is wired to standard diagram (N250) which includes the Bellset 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 L1 and E 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 and remove strap
  7. Blue of line cord to T - short out the two terminals of the capacitor
  8. Green of 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 (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:-
  1. Disconnect the green/white wire/wires from the capacitor and connect them to the blue wire of the line cord
  2. Insert the resistor in place of the strap on terminals 6 & 7
  3. White of the line cord to terminal 6
  4. Red of the line cord to terminal 9

GPO Telephone 232

Telephone is wired to the standard diagram (N332) but modified as follows:-

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 and insert a 3.3k ohm resistor in it's place.
  3. Ensue that Bellset strap B7 - B8 is in place.
  4. Wire the cords as per the table below.

With Bellset No. 31 and using the original cloth cord between the telephone and Bellset.
Remove Bellset strap B7 - B8 and then connect cord as table below.

Telephone Cloth Line Cord colour Bellset 26 Bellset 31 Line Cord Colour
T3 White B5 B9 Red
T1 Red B7 B7 White
T2 Green B3 B1 Blue

Without Bellset and not using the original cloth cord between the telephone and Bellset.
1. Red of line cord to T3
2. White of line cord to T1
3. Blue of line cord to T2

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 T11 and T12
  4. Insert strap T11 - T12
  5. Strap T1, T2, T8 and T9 together
  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.

  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 T11 - T12
  4. Insert a 3.3K ohm resistor between T2 & T1
  5. Red of line cord to T9
  6. White of line cord to T1
  7. Blue of line cord to T12
  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 both wires, at the terminal screws, that go to the switch.  They should be connected to T12 and T2

Bell switch needed - switch will disconnect bell from ringing

  1. Remove both wires, at the terminal screws, that go to the switch.  They should be connected to T12 and T2
  2. Remove the strap T1 - T2
  3. Connect the wires to T1 and T2

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 cord to T9
  4. White of cord to T1
  5. Blue of cord to T11
  6. Green of 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:-
2 pins, On/Off button, Switch No. 23A.

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

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

Telephone is wired as follows. The standard diagrams are (N722N866/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)

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

1. Strap T1 - T2
2. Blue of line cord to T1
3. Red of line cord to T4
4. White of line cord to T5
5. Green of line cord to T9

GPO/BT TELEPHONE SPECIAL RANGE (TSR)

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: August 03, 2006

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