I have a defective HP-41CV which is cosmetically in an excellent condition. But I have no idea why it is not working.
When I insert new batteries the calculator immediately shows the following display and doesn't react on any key press:

Any ideas what may be wrong with this HP-41CV?
Would this be a suitable unit for the HP-41CL upgrade?
The S/N is 2236xxxxxxx.

Try executing the BEEP function. If it beeps, the logic board is probably fine, the problem is most likely with the flex circuit or zebra strip. If it doesn't BEEP, the problem could still be with the flex circuit, zebra strip, or logic board.

If only the logic board has failed, then it is a great candidate for the 41CL upgrade.

If the flex circuit or zebra strip has failed, they would have to be repaired or replaced before the 41CL would work.

41s sometimes make a screen full of crazy characters if you wiggle an IL cable when it's in the process of transferring info (this probably includes interrupting any I/O, such as a memory chip or dedicated printer). Letting it set for a half a year with no power will sometimes fix it, but:
you NEED a donor and this looks like one.
besides,
the loco characters i'm talking about usually look more like japanese kangi.

As it does not work properly, it makes sense to open and inspect it anyway. I haven't seen many 41s where the logic board was defective, my first guess is some kind of contact problem. Check for broken screw posts and check the zebra strip. Probably, the fix will be easy, and then the question is obsolete :-) Good luck!