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36
Protection Circuits
Figure 32
Protection circuits are incorporated in the unit to prevent the failure of a single
circuit or component from creating catastrophic damage.
The P7 and P8 boards are daughter boards on the P1 main power supply board.
The P7 board monitors the Vbk (195V), Vda (75V) and +17V supply voltages.
The voltages are fed through individual voltage dividers and the result is
compared to a 3.3V reference voltage by a comparator IC750. If any of the
voltages drop low, the comparator outputs a low triggering Q751. Q751 triggers
Q750 on, applying a high to pin 95 of the main CPU IC9351.
The circuitry on the P8 board monitors the plus and minus 13.5V lines used by
the audio circuit, the +5V line and another +13.5V line. If any of these supplies
were to drop, the comparator will output a low signal turning on Q776. Q776
conducts applying a high to pin 95 of the main CPU IC9351.
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 98 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