I'm at a loss. If you already changed the K155 chip, and you already checked the tubes, it has to be a solder bridge or PCB fault somewhere. I confirm what Ty said, that there's no controller reboots or anything.
So, let's look at the display board, because that is simplest.
Please unplug the display board from the main board, and remove all the tubes. Check continuity from every tube pin to every other pin on the sockets. There should of course be no continuity anywhere.
If that all checks out, we can move onto the main board. Still with the display board unplugged, please remove the K155 chip, and with the unit powered up, please check the voltage between ground and pins 3,4,6 and 7. This is the input from the controller. Because the display is multiplexed, you won't be really able to see any pattern, but the important thing is that they change, not when they change. With this step we are trying to see if the controller is sending signals to the K155, and there is not a GPIO that is stuck high. (If there is, it will be a solder bridge between pin 4 and 5 - jes' sayin...).
If that checks out OK, power off again, and still with the K155 out of the socket, check the continuity between the K155 pins 1,2,8,9,10,11,13,14,15,16 (all the other pins except 5 and 12, which are power). Again, there should be no continuity.
Here's a picture of the schematic of the K155 so you can follow what I am trying to do.