Posting this for a co worker so this information is 3rd person.

He has a Dover solid state duplex with top,bottom, int, and car boards

If either car is out of service for any reason (inspection, independant service, or broke down) the other car will not answer the bottom hall call. Everything else works normal and the running car will go to bottom landing from the car call.
If both cars are running all hall calls work normal.
The voltages are good. 5.2v on power card and both units have capacitors on pc to 24d with voltage steady at 30vdc.
Any help would be appreciated

Has he checked that call placed at terminal '1-U' on the running car will set call or not. Sounds like a cross-connect missing.

Doesn’t the early Dover use a “top and bottom” card for duplexing? Sounds like a bad card. You can try to (identify it first) pull it, clean the contact edge and reseat it and see if that clears the trouble.