HP80 battery pack voltage reversed

[Harald]

If you are lucky it is only the PSU on the calculator PCB. As PANAMATIK said you could check the diodes and transistors. But it is possible more than that is damaged.

Is the calculator of sentimental value to you? If repairing diodes and/or transistors does not help, you could try replacing the main PCB and see if that helps. I might have a replacement PCB for you soon, but I am still waiting for the mail to turn up
You can also use a main PCB of an HP35, 45, (or 70) to check if the display drivers are still ok.