HP-50G Excessive Battery Drain / Fix?

[HrastProgrammer]

Recently I obtained an HP-50G (CNA00...) in perfect physical condition but not 100% perfect functional condition because it drains batteries very fast - brand new Duracells were drained from full to Low Bat in just a week! And that's with mostly sleeping on batteries and on USB when working (checked - USB power works).

Looks like the protective diode has been damaged somehow. I am curious to know if there is any other cause of such behavior? Is this a common fault with HP-50G? This is really a concern to me because I have a chance to buy a few brand new HP-50Gs for a great price locally. I plan to use them to run my HRAST BASIC but if this is a common fault then it doesn't have much sense ...

I will try to power it on rechargeable batteries but I would also like to fix it long-term. I don't have problems with SMD soldering as I've already done some SMD repair work on one of my Alesis Andromeda synthesizers:

Alesis Andromeda A6 ASIC Board Repair

Is there a HP-50G schematics available? I couldn't find any ...