Codan 8525B Series Technical & Service Manual page 59

8525B/8528 Technical Service Manual
The viewing angle is preset by the adjustment of R34. This sets the voltage
applied to the non-inverting input of IC4a, while the voltage at the junction
of R32 and D6 is applied to the inverting input. The output of IC4a is
proportional to the difference between the two voltages. The temperature co-
efficient of D6 applies temperature compensation tending to keep the set
viewing angle constant over a wide range of temperatures.
R34 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 through the hole in the bottom of the case.
Nine incandescent lamps, H1 to H6 and H15 to H17 are mounted on the
board. Six are behind the LCD and the remaining three in various areas of
the board. These provide back lighting so that the LCD and the panel
annotations can be read in low ambient light.
Common connections of the LED indicators H8 to H13 are taken to a
positive supply derived from the A rail and regulated by IC1. The indicators
are lit by grounding the appropriate connection via the 16-output driver IC2.
The Tx indicator LED, H7, is lit by grounding its cathode via a transistor on
the PA and Filter PCB.
The current sinking provided by IC2 for the indicator LEDs H8 to H13 on
the front panel can be varied by the 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 IC2
pin 4. Normally high impedance (tristate), this output goes low in the
dimmed condition. This is applied to the base of V1 to override the hold-on
voltage applied via R1. With the transistor switched off the current for H1 to
H6 is drawn through R3 and R4 thus reducing the brightness of the LCD
back lighting. The panel back lighting is not controlled and remains at full
brightness at all times.
A moveable jumper link closes either Link E for normal extended control, or
Link F for special applications front panel control. Closing the appropriate
link modifies the address of the I/O port, IC3, so that the microprocessor can
recognise the type of control in use.
Link 1 is used in programming the transceiver options (Refer to the
Operator's Handbook). Link 2 is for possible future use.
Publication No: 15-02036
Technical Description 4-23
Issue 6