8525B/8528 Technical Service Manual Technical Description 4-21
Publication No: 15-02036 Issue 6
Six incandescent lamps, H1 to H6, are mounted behind the LCD to provide
back-lighting so that the display can be read in low ambient light. V1 is
mounted in a position which allows ambient light to fall on the transistor.
Conduction of the transistor increases with the light, i.e. the collector
voltage, applied to the comparator IC1b, decreases. IC1b compares this
voltage with the fixed voltage at the junction of R3 and R4. In low ambient
light, V1 collector voltage is higher than the reference voltage, therefore
IC1b output is high. This turns on V2 to light the lamps. If the ambient light
increases to the point where the collector voltage of V1 no longer exceeds
the reference, the output of IC1b goes low, switching off V2 to extinguish
the lamps. The threshold level can be adjusted by R1.
Completion of Link D connects R30 and R31 in parallel with V2. This
provides an alternative path for the lamp current, therefore the LCD back-
lighting is only reduced to approximately half brightness in high ambient
light.
Common connections of the LED indicators H8 to H13 are taken to a
positive supply regulated by IC2. The indicators are lit by grounding the
appropriate connection via the 16-output driver IC3.
The Tx indicator LED is lit by grounding its cathode via a transistor on the
PA and Filter PCB.
4.8.2 Indicator and Display Dimming
The current sinking provided in IC3 for the indicator LEDs H8 to H13, can
be varied by operation of the Recall pad on the front panel. Any indicators
lit will be at full brightness when the transceiver is first switched on.
Pressing the Recall pad twice within one second reduces the current sinking
thus reducing the brightness of the indicators. Pressing the pad twice again
restores the original brightness.
The dimming operation is also applied to the LCD back-lighting via IC3 pin
4. Normally high impedance (tristate), this output goes low in the dimmed
condition. This is applied to the base of V2 to override the input from the
comparator IC1b. Thus if the LCD back-lighting H1 to H6 is lit, it will be
extinguished (or reduced to half brightness) when the indicators are
dimmed.
4.8.3 Links
A moveable jumper link closes either Link E for extended control, or Link F
for front panel control. Closing the appropriate link modifies the address of
the I/O port IC5 so that the microprocessor can recognise the type of control
in use.
Link 1 is used in programming the transceiver options (refer Operator’s
Handbook) and link 2 is for possible future use.
Link D is used in the display dimming facility (4.8.1).
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