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If I do the math for the R5, R6, and R7, I get 7v on the top comparator and 6v on the bottom.
If the voltage at TP1 is too negative, CR1 turns off as designed. The problem is that CR1 is ON when TP1 is at 0 Volts. Which component has failed?
The problem statement ...
If the voltage at TP1 is too negative, CR1 turns off as designed. The problem is that CR1 is ON when TP1 is at 0 Volts. Which component has failed?
To turn the LED on with 0v input however (a failure mode) could happen if:
1. The output of the opto was shorted (more likely
2. The comparator was bad.
3. The op amp was bad.