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The Unregulated 15V and 1.8V lines are monitored for a voltage decrease.
Under normal conditions, the voltage drop derived from the voltage divider
circuits at the base of Q9714 or Q9716 is enough to turn the transistor on.
However should the voltage drop decrease, Q9716 or Q9714 respectively, turn
off, allowing the 5 volts at their collectors to forward bias transistor Q9715. This
effectively grounds pin 3 of the CPU triggering the shutdown condition.
The four ventilation fans are monitored to be sure they are operating properly. If
one of the fans opens or increases resistance, the resulting current change is
applied to pin 37 of the main CPU.
The Scan and Sustain voltages on the SC and SS boards are monitored in a
similar manner. The +15V Line on the SS board is fed to a voltage divider and
the result is compared to a reference voltage. The reference voltage is provided
by a zener diode. If the output of the comparator goes high, a transistor turns on,
effectively grounding the SOS line. The SC board uses a similar circuit.
Additionally the SS and SC boards contain LED indicators to alert the technician
when a problem exists. The LED should be lit during normal operation, a dark
LED indicates that a problem exists on that board.
Pin 63 of IC9709 monitors the 170 V, 15V, and 20V lines. If any of these supply
lines develop a short circuit, the inline transistor goes out of conduction and
applies a high to pin 63 of the Sub CPU, IC9709, triggering an SOS condition.
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