Carphone 1972

NON GPO Radio Telephone Control Head

In 1972, what became known as System 3 was launched in London,and in South Lancs in 1973

This system offered a higher degree of privacy to the previous two systems as now one subscriber could usually no longer hear the call of another.

An improved Radiophone service for London

Most details gleaned from POEEJ July 1972 and PYE 1975 Type LW15FM radiophone sales sheet

Air Interface Specification

37 Channels with a Spacing of 25 Khz FM deviation +- 5 Khz

In 1973 another 18 25 Khz channels were added to raise the number of london radiophones from 1750 to around 3000

Land Station tx 163.50 Mhz-164.40 Mhz rated 50w, mobile Station tx 159.00- 159.90 Mhz typical ?w

Each radio capable of tuning to 10 (of the 37) channels (1 calling for the whole of greater London,3 for traffic at each of the 4 Transmitter sites

Selective calling by means of a 5 digit code, signalled with 11 tones between 1060Hz and 2600Hz tones, on the calling channel transmitted from 3 of the 4 transmitter sites one after the other. The Central transmitter site ( located on an 11 storey telephone exchange 1.6km south of the thames near waterloo), does not transmit the calling channel.(164Mhz)

Radiophones alert the operator that they want to make a call with a short burst of 2400 Hz tone on a free traffic channel

At the end of a call a similar 2400 Hz tone is sent

When the Radiophone user finds a free channel after in response to a call on the calling channel, a short burst (150ms-400ms) of 2600 Hz tone is sent on a free traffic channel.

Pye System 3 Radiophone Type LW15FM 1975 control

Selective Calling

Good Pictures at http://www.qsl.net/gm8aob/pages_2/lw15fm.htm

Channel Lock Board

This board inhibits the transceiver if the Channel Selector switch is operated once a vacant voice channel has been selected and the CALL button operated,

Diodes D1 - 9 are connected to the positive lines of the nine voice channel oscillators in the associated receiver. Initially, with the Control channel selected and the handset on the rest TR1 base and emitter are both at 0V, as is IC 1 pin 12. TR2 is cut off, applying level l to IC 1 pin 9 and the latch output at IC 1 pins 3 and 4 is at 0, turning off TR3 and TR4.

Channel Lock Board

When an incoming call is received, lifting the handset applies +ve 10. 5v to pin 2 and the selection of a voice channel applies the same potential to one of the diodes D1 - 9. The diode conducts, TR1 base and emitter are therefore still both at the same potential and the level at IC 1 pin 12 remains unchanged.

The positive potential at pin 2 causes TR 2 to conduct, IC 1 pin 9 goes from 1 to O and the level at IC 1 pin 5 goes to 1. Since the level at IC l pin 1 is unchanged, however, the latch output at IC 1 pins 3 and 4 remains at 0.

Operation of the CALL button sets pin 15 to level 1.

If an attempt is made to select a different voice channel the 'break before make' action of the Channel Selector switch causes the conducting diode of D 1 - 9 to be momentarily non-conducting. TR1 base potential goes negative w. r. t. the emitter, the transistor conducts and the level at IC1 pin 12 goes to 1. Since ICl pin 13 is also at this level, IC l pin l goes to level 0 and the latch changes state, setting IC 1 pins 3 and 4 to 1. TR3 and TR4 are turned on and pins 12 and 13 held at level O to inhibit the transceiver .

The circuit is reset by replacing the handset on the rest.

Page updated

Google Sites

Report abuse