Yup, I kept the battery pack in the calc compartment all the time. After several hours of trying to charge it, I unplugged the calc from the wall outlet and tried to turn it on but nothing happened. However, plugging it again to the AC outlet, the display lits up showing, again, some "Error" number. Will try tonight the trick with two AA's.
Can one get a replacement Ni-Cad battery pack for these machines? I'm thinking some cordless phone receivers use the same battery shape. Maybe these could be used in the HP-33C?
Thanks for your reply, Warren