I'm working on a Fisher Digital Vortex Mixer that has an intermittent issue in which the AC motor is energized, but no movement occurs. By rough estimate, this will occur on one of every 8 attempts.
The circuit consists of a PIC16F73 that serves to activate a MOC3021 optocoupler that passes an AC signal to a Q4010N5 Triac. I have no access to circuit diagrams from the manufacturer, so this is my best analysis of the circuit:
As you can see, there does not appear to be any stubbing present, despite this system having an inductive load. It is certainly possible that I missed the stubbing circuit during my analysis, but having thoroughly examined the circuit over several days, I am relatively confident that it is not present.
The signal coming from the PIC microcontroller is a clean digital signal that seems to drive the optocoupler without issue.
The signals going through the circuit at points 1, 2, and 3 (Circled in orange) are visible below.
The first row (point 1) depicts the No Button Press, the Good, and the Bad waveforms, in that order. The second (point 2) and third (point 3) rows only depict Good, then Bad waveforms.
Early in the analysis, someone else working on this circuit had replaced the Triac and Optocoupler as they were the likely suspects. There has been no change in behaviour after the replacements.
Despite my best efforts to figure out the cause of the waveforms I've witnessed in this circuit, I have not been able to find it. If you can offer any advice on how you would proceed, or if you require any additional information I haven't provided, please let me know.
Edited to remove extraneous pleasantries.