4-20 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
4.8 8528 Front Panel and Control Head
The front panels of 8528 series transceivers and extended control heads are
identical. Overlays on the panels contain sealed tactile membrane switches.
A liquid crystal display (LCD) and light-emitting diode (LED) indicators on
a board behind the panel show through the appropriate part of the overlay
when activated. Circuits on the board interface with the microprocessor in
the transceiver to drive the displays and indicators, and to decode the
operations of the front panel switches. In the extended control head, circuits
on a further board compensate for the long lines which connect the head
with the transceiver.
Surface mounted components are used on the Display PCB, LCD. For
removal and replacement of these components it is essential to use tools
specifically designed for the purpose. Repairs to the board should not be
attempted unless the appropriate tools are available.
4.8.1 Display PCB, LCD (04-02454)
The same Display PCB, LCD (08-03745) is used in the transceiver and the
Extended Control Head.
The LCD on the board has twenty digits and two enunciators which allow it
to display the channel number and transmit and receive frequencies of the
currently selected channel. In addition it can display the identification
number of a called or calling selective call station. The LCD is driven by
IC4. The display data is applied to the driver via the I
2
C bus. The drive
mode is 1:4 multiplexed with 1/3 bias.
The viewing angle is preset by adjustment of R12. This sets the voltage
applied to the non-inverting input of IC1a while the voltage at the junction
of R10 and D1 is applied to the inverting input. The output of IC1a is
proportional to the difference between the two voltages. The temperature co-
efficient of D1 applies temperature compensation tending to keep the set
viewing angle constant over a wide range of temperatures.
R12 is normally set during manufacture for viewing at right angles to the
face of the panel. It can be adjusted for viewing angles above or below if
required.
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