If the processor reboots, the PWM signal floats and is pulled low by a resistor. If the application firmware crashes for some reason?or hangs up - the interrupt processing?continues as it is in the highest priority interrupt. I don't know of any realistic scenario where the PWM signal can get stuck high, other than if there is a short somewhere.?
I'm quite hesitant to suspect that there is anything wrong with the capacitors, or with the design (which has a lot of failsafes built in, both in hardware and in firmware). To put it plainly - if we don't have any particular evidence for what caused a failure and are devoid of any explanation, I think it is a stretch too far to say that it is therefore probably a defective component. A failure of a component operated well inside its ratings is not very common.?
Rick, I will repair the board with an axial lead zener of higher power rating and put a ceramic capacitor in place of C107. Then I will fit a large electrolytic somewhere on the main board. I will do a simulation of AC-coupling the PWM signal before deciding if I do that modification
