4-2 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.1.2 Power and Control Lines
By controlling relay K1 on the Motherboard, the microprocessor on the
Microprocessor Controller PCB generates two switched +10V power and
control lines designated as the ‘C’ and ‘D’ rails. These rails are defined as
follows:
• ‘C’ rail—+10 volts on receive; 0 volts on transmit.
• ‘D’ rail—0 Volts on receive; +10 Volts on transmit
All other control lines come directly from the Microprocessor Controller
PCB and are described in section 4.5.
4.1.3 Overvoltage and Reverse Voltage Protection
(refer Dwg 04-02452)
Overvoltage protection is provided by V3, V2, R4, R5 and V1 on the PA
and Filter PCB. If the battery voltage rises above the 15V reference of V1,
plus V2 Vbe drop and the 0.6V diode drop of D6, transistor V2 turns on and
hence turns off V3 to de-energise the power-on relay K7. This occurs at
nominally 16V. Diode D6 provides reverse voltage protection.
4.1.4 Transceiver Power On/Off
When primary power is first applied to the transceiver, V3 on the PA and
Filter PCB (04-02452) is turned on (by R5 to ground) charging C56 through
D6 and K7 which momentarily pulses on. With primary power applied and
in the power off condition, K7 is de-energised and the dual-coil latching
relay K2 on the Motherboard (04-02453) is in the ‘off’ position.
Switching power on is a hardware operation; switching off is basically a
software operation.
When the power on-off pad on the front panel is pressed, a ground is applied
to the cathode of D4 on the Display PCB. This will forward bias V2 via R12
on the Motherboard. The emitter of V2 is connected to the positive supply
via K7, V3 and D6 on the PA PCB. When V2 on the Motherboard conducts,
latching relay K2 contacts close, operating the power-on relay K7. C56
ensures that there is sufficient current to toggle K2.
Operation of the on/off pad on a control head (when connected), switches on
V2 via the rear panel connector P502 pin 4. This has the same effect as
operation of the front panel on/off pad.
Once power ‘on’ has been established, the diodes connected to the power
on/off switch-pad are reverse biased, and no longer affect the operation of
those lines.
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