HP 41 not working

[jebem]

If you are sure about your 5vdc power source quality, then most probable cause for this is the very well known problem of broken posts that are very common on the full nut models.

I have a few of these machines abandoned in a drawer with this issue and to be honest i don't bother anymore to fix them.

No matter how good these 41 series can be on features count, that design choice of using press contacts combined with poor choice of plastics results in a major fault that is hard to fix unless you just go an replace a now costly front cover/display/keyboard assembly.
There was a very good reason why HP finally changed that design on the later half nut models after all.