Service manual

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viewing angle or contrast of the LCD display. The trimmed voltage is nominally set to -9.6V and may be
measured across TP1 and TP2 of the switchboard.
2.4.11 Printer
The monitor employs an Axiohm HPT4050 thermal printer module, fitted to the printer board, which contains
the interface electronics. The PIC provides stepper motor control signals M_OE, M_PHASE1 and
M_PHASE2 to IC3 on the printer board. This converts these to sequential drive signals MP1_1, MP1_2,
MP2_1 and MP2_2 for the stepper motor.
The PIC also sends a PRINTER_TRIGGER signal to initiate printer action; this triggers a timer IC4 which
generates the printer enable signal PRINTER_OE. To optimize print speed, print quality and the life of the
print head, a thermistor constantly measures the temperature of the head. As the system heats up, the print
head thermistor is used by the timer circuit to reduce the width of the PRINTER_OE pulse. The print head
voltage +VP is also employed to alter the width of the PRINTER_OE pulse. A typical print trigger pulse
width is 6 ms. An opto-transistor detects the presence of printer paper and a micro switch detects the status
of the printer door, open or closed. These two devices are combined to produce the PRINTER_READY
signal, which inhibits the printer action when clear and is sent back to the PIC as PRINTER_BUSY. Data for
printing is sent from the system processor to the printer board via the SPI bus.
Because the instantaneous power demand of the printer is quite high, a dedicated 6.1-volt supply +VP is
generated from VRAW by IC1 and extra reservoir capacity is provided by C16 and C17.