4-28 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.9.9 Transmit Indicator
The forward power voltage developed by D1 is divided and applied to V4
base producing a current in V4 proportional to power output to illuminate
the Tx LED indicator.
4.10 PA Exciter Interface (04-02434)
4.10.1 Introduction
8525B and 8528 Series transceivers may be fitted with a PA and Filter PCB
forming an internal PA stage, or with a PA Exciter Interface PCB for use
with an external PA unit. Software appropriate to the board fitted is installed
in the controlling EPROM on the Microprocessor Controller PCB.
Transceivers fitted for use with an external PA unit are identified by a letter
H suffixed to their type numbers on the rear panel. The PA Exciter Interface
is designed for use with Codan Type 4402 or 4404 power amplifiers and is
described in this section. The PCB fitted for the internal PA is described in
Section 4.9.
4.10.2 Control Lines
Control lines from the transceiver are taken through the board for
connection to the external PA unit. The lines are filtered on the board to
prevent RF, picked up on the cable to the PA Unit, from interfering with the
operation of the transceiver.
4.10.3 Power On/Off
The DC supply for the transceiver is connected to the external PA unit.
Operation of the On/Off switch pad on the transceiver front panel grounds
the Power On line to the PA unit. This energises a relay in the PA unit,
supplying 12V to the transceiver. A fuse on the PCB protects the supply
before it is taken to the remainder of the transceiver to form the A rail. The
latching relay on the Motherboard operates in a similar manner as described
in section 4.1.4.
4.10.4 PTT Circuit
The D rail (10V in transmit) switches on V4 in the transmit mode to ground
the PTT line to the external PA unit.
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