Panasonic TH-65PX600U Service Manual page 45

PA-Board_Loss of Main-Voltage Protection
The transistor Q5642 monitors the MAIN1.8V, MAIN9V, and MAIN5V lines. If any of
these supply lines develop a short circuit, transistor Q5642 goes into conduction
and applies a high to pin 209 of the MPU, IC1103, triggering an SOS condition.
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