4-18 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.6.9 AGC Dump Circuit
The synthesizer latch from the Microcontroller PCB pin 1 P10 is used to
turn on V4 thus grounding the AGC line on the Audio and IF 1650kHz PCB
via R23. This discharges the reservoir capacitor in the AGC circuit so that
the receiver has maximum gain immediately after a frequency change. This
is particularly important when in the scan mode.
4.7 8525B Front Panels and Control Head (04-02579)
The front panels of 8525B series transceivers and extended control heads are
similar. Overlays on these panels contain sealed tactile membrane switches.
A 2-digit numeric display and LED’s on a board behind the panel show
through the appropriate part of the overlay when lit. Circuits on a further
board behind the panel interface with the microprocessor in the transceiver
to drive the displays and indicators and decode the operations of the front
panel switches. In the extended control head, circuits on the Microphone
Amplifier PCB compensate for the long lines which connect the head with
the transceiver.
4.7.1 Display PCB
The Display PCB (08-03944) for the transceiver and the Display PCB
(08-03945) for the extended control head differ in size and layout to fit their
individual panels. They are, however, electrically identical.
Common connections of the numeric displays and the LED indicators
(except the Tx indicator) are taken to a positive supply controlled by IC1.
Indicators and segments of the displays are lit by grounding the appropriate
connection via IC2 and IC3.
The conduction of phototransistor V1 increases in proportion to the intensity
of the light falling upon it. Increasing conduction of V1 reduces that of V2
and V3 thus raising the reference voltage to IC1. Consequently, increasing
ambient light raises the output voltage of IC1 thus increasing the display and
indicator brightness. The circuit sensitivity is adjusted by R1.
The Tx indicator LED is lit by grounding its cathode via a transistor on the
PA and Filter PCB.
Two 16-output open drain drivers, IC2 and IC3 ground individual lines in
response to data received on the I
2
C bus.
A remote 8-bit I/O port, IC4, accepts inputs from switches on the front
panel, and under control of the microprocessor, converts them to a suitable
format and applies them to the I
2
C bus.
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