I was bored and found a HP-27S

[franz.b]

Hi everyone, I try to resurrect this old thread.
I have a 27s that does not turn on anymore and I would like to try to fix it because it was my father's, but for now my efforts have been in vain.
I was able to open it, but the problem does not seem to be in the rubber strip, which, although in good condition, I replaced .. I obviously cleaned all contacts, even those of the screen, but the calculator does not turn on.
The pcb is very clean and the copper shining, but there is a lot of dirt on the keyboard. opening it is extremely difficult and destructive, is there any way to check if the keys produce sufficient electrical contact?
Here I found a lot of useful information but but I can't measure the resistance of the keyboard, I don't think my analog instrument is suitable, the indicator barely moves ... or is it the fault of the dirt on the contacts? I also don't understand if the protective film of the screen is also polarized, or if the polarized film is applied to the glass. The external part, superimposed on other LDCs screens does not produce any effect.. could it be that I simply cannot see that the calculator is switched on?

Thanks in advance