Good Day, egear00;
Try looking at this, since you sound like a TECH;
The Fan alarm is activated when the Fan does not run. This can happen in two different circumstances.
(1) When the Fan Fails:
When the Fan Fails due to a malfunction.
When the Power is turned off. The FAN +12 regulators turn off and the fans quit.
In this situation the fan alarm will be activated. Inside each fan is a hall element that keeps pin 1 of the connector pulled low when the fan rotates. If the fan stops, the pull up resistor connected back to the Fan 12+ pulls the alarm line high. This high is routed through two Diodes to the shutdown circuit of Q905, Q906. When the base of Q905 goes high, it's collector pulls the Power 1 (Power On) command line low which shuts off the relay driver Q903. The purpose of Q906 is to make the shutdown circuit appear like an SCR. If there's any pull up voltage being supplied, (in this case from the Power 1 line), the emitter to collector current of Q906 will keep Q905 in the On condition. This circuit will remain latched as long as the Power 1 line remain in the on condition.
An additional route for the Fan Alarm is out the PPS2 connector pin 9 to Q562. The Fan Alarm high will cause Q562 to turn on and pull the Fan Alarm pin 48 of the Microprocessor low. When pin 48 goes low, the Microprocessor will in turn activate the Temp LED and cause it to Blink. (See the Temp Alarm Circuit for more details).
(2) When the Power is turned off.
When the Power is turned off, the Fans do not run because the Fan +12V regulators I906 and I907 are turned off. This would generate a false Fan Alarm, so the Microprocessor outputs a High from pin 56 to Pin 7 of the PPS2 connector. This high turn on the Fan Alarm Enable transistor Q908. When Q908 is turned on, the collector is held low to prevent a Fan Alarm. This low keeps D916 and D918 cathodes held low
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Good Luck, big IRISH.