Converting, and verifying proper conversion of British Post Office 700 series telephones to modern plug and socket working

Old British Post Office 700 series telephones are often to be found in working order even today. If you want to make use of one, unless it has already been done, you will need to convert it to the modern plug and socket system. This is not quite as simple as it sounds, and occasionally you come across one which has not been done correctly. However, it is a fairly simple job to check one out, or to do a conversion yourself.

NOTE: Completing a conversion of one of these telephones does NOT, technically speaking, make it approved for use.


The 700 series telephones were designed in 1959 as a replacement for the earlier 300 series. Gone (apart from on the very early model) was bakelite as a casing material, replaced with a more modern plastic. The colours available also increased. The 700 telephone circuit differs from the 300 in that there is a regulator, which reduces the volume on short lines. The bell circuit, however, remained low impedance, requiring that the bells be connected in series (from one phone to the next).

The new style plug and socket system, introduced in November 1981, changed the bell circuit to a high impedance one, enabling the telephone to be unplugged and reconnected during a call without problems, but also requiring that bells be wired in parallel. The old 700 series telephones were still around as the ready-converted 8700 series for a while afterwards, and the old ones could be converted. At the same time, a component added to the circuit was fitted to any telephone lacking it at the time of conversion.

The 700 series includes Trimphones, but I have never worked on one of these and so cannot comment specifically.

What to do

This all depends on whether you are converting a telephone yourself or merely checking and correcting if necessary, someone else's work. An 8700 series phone should not need any work as it will have had the correct bell and line cord from new, but if you are in doubt, you can check it in the same way. It is always possible that inexpert repair or tinkering might have left something incorrect.

For a new conversion, you will definitely need at least one part – a line cord designed to fit the phone and connect to the socket, because trying to fit a plug to the old coloured cord usually fails. However, you also sometimes need a rectifier, and you are likely to need either a new bell movement or a resistor to convert the bell.

The first thing to do is dismantle the telephone – I am assuming it has been disconnected from wherever it was hard wired, and is not plugged in currently.

First, the vast majority of these telephones had rotary dials, including all the common models (706, 710, 711, 740, 741, 746). In order to safeguard the dial fingerplate from being cracked in the handling of the cover, it is advisable to remove it. To do this, first remove the label cover by applying something sticky, like a foam self-adhesive pad, to it and pulling sharply. Remove the label, then undo the screw in the centre of the dial. Lift the fingerplate at the top and then pull it clear of the finger stop, and place it on one side, with the label cover.

Next, remove the handset from the hook. I have no idea how to remove the cover of the original 700 (the bakelite one), but on a 706, undo the two screws next to the switch hooks. Most other models, including the 708 and 746, have a single screw on the back (or bottom, in the case of wall telephones) of the case. This screw is captive and should not come completely out.

Once the screws are released, lift off the cover, from the back (bottom) first and then forwards (up).

With a wall phone, if it is still attached to the wall, loosen completely the screw at the bottom of the baseplate, then lift the baseplate at the bottom, and then up, to release the phone from the wall. If the bracket is to be moved, attend to that now, or at least before you refit the cover.

From this point on, any reference to the dial also refers to the dialpad fitted to the push button phones.

Remove any remnants of old line cord, then check the coils of the bell movement. They may be marked with their impedance, in which case add them together (they should both be the same), but if not, use a multimeter to measure the resistance of the bell coils. If you need a bell conversion resistor, it needs to be approximately the difference between 4000 and this figure (3k3 is usually used with the standard 1k bell), or you could replace the bell movement with a type 59D-1 or 79A. The mistake made in the conversion of my 746 was that it had been left with a 1k bell and no resistor. Check this point carefully. I would not expect to find bells other than the 1k or 4k versions fitted, but I am not going to say it is impossible.

If you are going to replace the bell movement, loosen the screw across the dial where it meets the hook support, lift the dial out and place it to one side. Remove the two screws holding the bell wires, and the two holding the bell movement, being careful not to lose the nylon washers. Lift and slightly twist the movement until it is free. You should be able to do this without undoing the gongs, but if you need to, they are not difficult to deal with.

Put the new movement in place and refit the screws. Connect the new movement to points T4 and T16 (this may be different if a ringer control is fitted). Remount the dial.

Now, to start or check the conversion, first check if a rectifier element (type 205, red in colour) is fitted to points T1 and T2. On some later models, it will be soldered to the PCB next to them, but on earlier models it will be held in place by the screws. If it is not there at all, one should be obtained and fitted. This component eliminates annoying clicks in the receiver which have been known to damage hearing.

Remove the shorting link between T6 and T7, and place it on T5 and T6. If a resistor is needed fit that between T4 and T5, otherwise fit a shorting link there. Fit shorting links also between T16 and T17, T17 and T18, and T18 and T19, that is, three right through.

Fit the new line cord: red wire to T8, blue to T6, white to T18 and green to T15. Slot the grommet into the space. On most rotary wall phones, no space is provided, so you may need to make other arrangements. Make sure all screws are tight before proceeding.

The reason there is no space for the line cord grommet on rotary wall phones is that the traditional way of wiring a wall phone, one which you may still attempt, is to use a piece of telephone extension cable, put through a hole in the baseplate, and connected as an extension socket would be. In this case, connect the blue/white wire to T8, the white/blue wire to T18, the orange/white wire to T6 and, for neatness, the white/orange wire to T15. Connect these wires to connectors 2, 5, 3 and 4 respectively in your junction box or Linebox wiring plate. On push-button wall phones, and the 1/706, a blanking piece is fitted – this can be removed and a line cord used, or you could use the traditional method.

If the phone is intended to be wall mounted, make sure that the bracket is now in place. Hang the phone on the top hooks of the bracket and refit the screw at the bottom of the baseplate.

Refit the cover. This can be fiddly, and the bezel may need to be rotated, or slid in the case of push button phones. Once it is on, tighten or refit any screws. Make sure the hooks still work freely – overtightening the screw can cause them to stick. Once this is done, replace the handset.

On a rotary phone, refit the fingerplate by sliding it under the finger stop and placing it, correctly aligned, on the centre spindle of the dial. Refit the screw, label if desired and label cover, cleaning off any adhesive first.