Trying to revive a HP67 (Dead on arrival)

[Hugin]

Hello,

I recently bought an HP67 from an older gentleman who seemingly used it until a few years ago. He then stored in in his desk for when he might need it again. And just to make sure the calculator would be ready when he needs it, he left the charged batteries inside.

Now, years later I got it and yes... its a damn mess. One of the battery contacts was completely rotten away (literally) and the long "connecting PCB" next to the battery tray was covery in corrosion as well as the charging port.

Today I spent nearly five hours cleaning, manufacturing a new battery terminal and then hooked it up to my lab supply (tried 3.5-4v).
According to the PSU (hooked up the the battery terminals) it draws 270mA when switched off, which drops to 150mA when switched on, which I consider to be rather weird.

So I guess the first thing to start with would be the power supply-components but I have no real clue, where they are. Also I didnt find any helpful repair flowchart.

Does anyone have an idea where to start from?
I have a lab PSU, a good multimeter, time and general electronic knowledge but not about how to fix calculators/vintage computers.

Any help would be appreciated, I realy hope that the HP67 might become my new EDC-calculator at university.

Thanks a lot already!
Oh and I live in Germany so my English might be slightly incorrect, but I am trying my best.