That can’t be right. If you hook +5 to -12, besides the damage it would cause, they can’t possibly still read +5 and -12. They will read the same voltage. There is an old US bulletin to help with fluctuations on the 5v supply that says to run a jumper from TV-2 to the unused 5v pin on the power supply, J7 pin 3. And from TV-46 to HB1-4 for common.