HOW TO CHANGE THE TRANSMITTER
Fitting a GPO/PO/BT Transmitter No.
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.
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).
Last revised: April 10, 2023