4-40 Technical Description 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
On completion of the tune operation a positive voltage is applied to the
Brake input (J1 pin 6). This switches on V1 which bypasses R1 to lower the
output of IC1 to approximately 1.3V. This is applied to the antenna motor
windings. At the same time V5 is switched on so that a small current
(approximately 120mA) is passed through one motor winding. This has a
braking effect so that the antenna tuning does not change with vibration.
Power for the comparator circuit is derived from the A rail by IC2. This
remains operative on completion of the tune operation so that any change in
the VSWR will still be detected. If a transceiver fitted with the option is
used with a non-tunable antenna an excessive VSWR will cause an
‘Untuned’ indication which cannot be corrected. The reflected power
voltage, via R10, is, therefore, taken to the disable line (P1 pin 5). When the
transceiver is used with a non-tunable antenna, pin 3 of the rear panel D
connector (J201) can be linked to pin 14 to disable the comparator so that no
‘Untuned’ indication is given.
When the transceiver Scan mode is selected, the SCAN input (J1 pin 9) is
taken low to switch on transistor V3. The collector current of the transistor
energises relay K1 in the antenna. This switches into circuit a wide-band
amplifier in the antenna which provides good reception over the entire range
of the antenna.
To retain the ability to use a transceiver fitted with the option with a
pretuned antenna, the lines from the Microprocessor Controller Board
carrying the BCD data for presetting the antenna are carried through the
option board to the rear panel D connector (J201).
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