Service manual

through the PA filter L1 to L4. L4 of this filter, with diodes D101 and

D102, forms a simple forward and reverse power detector to provide

power control and transmitter status indication on the front panel display.

The power control signal is fed to differential amplifier IC101 together

with the reference power signal from VR100 and VR101, to set the high

and low power levels. The output of this amplifier forms a regulated

supply with TR113 and TR114 to supply the PA drivers TR100 and

TR101. The reference input and supply for TR114 is fed from TR111

which is only turned on after the synthesiser is in lock.

Local Oscillator / Synthesizer. The main LO consists of TR105

connected as a Colpitts oscillator. Tuning is by L113, varactor VC102

and VC104. Band switching between receive and transmit is by PIN

diode D103. The output of the oscillator is buffered by a cascade pair

formed by TR104 and TR103 in receive and TR102 and TR104 in

transmit., the supply to TR102 and TR103 being switched between the 2

modes. The output is also buffered by TR118 to the input of the

synthesiser IC105. The reference frequency at 9.6 MHz is generated by

TR119 and controlled by XTAL101. C200, C115 and R157 form the

main loop filter with additional suppression of the reference frequency by

R179 and C204. The synthesiser is controlled from the control panel via

a 3 wire serial interface.

Modulator. Audio from the control panel is amplified by IC104b, the gain

being configured by R169, R173 and C169, to give the signal pre-

emphasis. The output of the stage is peak detected by D3 and detector

TR1167 to provide a gain control signal to TR117 to enable limiting of

high level input signals. Temperature compensation of the limiter is

provided by thermistor TH1 and IC104a forms a high pass filter to meet

the 14dB / octave roll-off above 6kHz. The final output is fed to the

modulation diode VC101 via gain control VR102 which is set to a

maximum of 5kHz deviation.

Power Regulation and Switching. The 12 volt supply is switched on

the front panel and then feeds the audio amplifiers and regulators. The

RF power module takes power before the switch to minimise voltage

drops. D104 and D105 provide reverse polarity protection. A regulated

8v supply is provided by IC103 and 5v from IC107. The supplies for

receive and transmit circuits are switched by TR106 and TR110

controlled form the synthesiser via TR107 to TR109. IC105 controls the

switching of PIN diode D103 ensuring that there is a suitably high

reverse bias across it in the OFF condition.

NMEA Reception. (RD68 only) Position and time information, from a

GPS unit, is received in the form of NMEA data. This is opto-coupled into

the radio via R225, D5 and IC4. The data is then routed through to the

control PCB via PLG3.