That’s a TAC50-04 then, bad VM on a TAC50 card rack gives unexplained 56 faults because it drops the 15v supply to the drive/DSP. TAC50-04 VM cuts AC1 that feeds all the 24v supply so you lose power to the CPU. Usually when they’re failing they’ll trip out when you tap on them. Or if you suspect it but can’t recreate it, change some parameter and don’t write it, if you come back and it’s back to default you know the CPU lost power.
I use a Fluke 289 logging meter for super intermittent issues. It’s great for proving or disproving something and you can graph the log over time and even print it out. I’ve used it for logging temperature in the machine room too.
