I got a late call on a Tac50, randomly E stopping customer said, rode the car up one floor, leveled in and just died floor level. No power on COP, doors, PI out, I heard the rope gripper drop, and then it fired back up, went up to the machine room, hooked up the laptop, and the only fault was a door fault, probably because it lost power, I saw the car do the same thing a few more times, I checked the power supplies for voltage and ripple, they all looked good, but the whole controller seems to lose power, and boot back up, and the laptop disconnects and loses connection because of the power, and doesn’t log any events, don’t know if PS2 is dipping or has ripple randomly, but that seems to be the PS that runs what is going on, going back tomorrow, just trying to get other ideas what to check, plan on cleaning the cards tomorrow and reseeding

Don't take the incoming power for granted..

I find in cases like this using my fluke 115 on min/max set to minimum voltage put into a specific point in a circuit will help you figure out if a power supply lost power when this happens. I recently acquired a fluke 376fc and it has been even more helpful with logging features in the meter that can be viewed on your smart phone in a graph showing voltages etc over whatever time frame in whatever interval you want. So you could see if a power supply looses power completely, for a split second if it just dips a little. I also have a set of double ended banana plug test leads with interchangeable types of clips to put on the end to be able to leave my meters clipped on a circuit easily. If you've never seen them fluke and knock off also make a test lead end that Is a hook that slips over a wire, then the base when twisted drives a tiny needle through the insulation so you can test almost any wire anywhere without having to be at a terminal or stripping it out.

Is this an older tac 50 (t4) or is this a tac 50-04?

VM1, volt monitor, that was the cause of the intermittent problem

Good find, thanks for letting us know the solution

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.