IIRC, “crowbar” of the power supply is what it was called. I’d bump the 5V to 5.2 or 3. Those are some old 1210’s or 1200’s. I’m not quite sure what you’re saying about “…not AC, 5 at 5.2…” Catching it failed, drop the Dispatcher 5V PS to about 4.6, then back up to 5.2 or 3. Or, once reset, bump it to 5.2. Continued failures could be the PS itself. In your picture, Car 1 is offline. Any reason? You’ve got the PCD and manual for those units?
Oh- you probably know, but everything do do with those US cars is a capital O as in obsolete. CCU/COU failure…good luck.

Job is in the process of MOD, car 1 has been cut out. What I mean "not AC" is no AC on the 5VDC supply. The dispactor 5VDC supply at time of frozen is at 5.2VDC. There is no PCD, manual or any terminal interface device just a few prints on job site. Never seen these controller till now.
Want to learn more to further troubleshoot issue. Any information is useful to me. Thank you

So just keep ‘em running as best you can during the MOD right?
Only other thing I can think is looking to see if the Dispatcher uses the same Power Supply as car 1 and substitute it with that, or picking up the 5V from the next nearest car and jump it over. The Dispatcher I think has a clock that scans left to right at the bottom left corner of the red plastic overlay panel. When the unit fails, is this clock scanning? Also, in the first photo, the image was blurry. I was unable to see the status of the other cars at the lower part of the red panel.
Available documentation for troubleshooting, fault diagnosis etc for those units will be tough to resource. Of course to do any of that you need the PCD designed for the MP1200 or 1210 as well. Early 80’s is a good guess for install year for that type of control.

One other thing; when the Dispatcher freezes up, and all cars are on auto and are “stuck” (and not running, doors closed) open the main line power of each car one at a time to see if one of the running cars is somehow the trouble.

Check that personally jumpers match the MJ jumper. You open the MJ jumper ( normally red) and see how the jumper wires are configured and make sure they match the elevator assignment of the personality board. Can can switch #1 car assignment on personally board but you must also match with proper MJ jumper. Otherwise check jumpers with PCD to assure they match actual physical locations. If you shut down #1 car then power for calls will be supplied by #2 car and so on. So if you have a bad board in the string you can find it by shutting down cars one at a time till problem goes away. If your modding a car the daisy chain has to be maintained and personally compensated. Sorry these are 1200s check PM

I haven’t heard from you I guess it’s fixed?

I’d be surprised if it were. Those 1200’s or 1210’s were a beast even for those with an ounce of U.S. knowledge. But they sure beat the old MDU relay control! Ha! Them were the days.

Yes my first 20 years were with US! So worked on all of it from relay logic till they stopped building controllers. 1200s we’re a short lived controller. Thank goodness