Help with HP 15c

[edwincool2]

Hi everyone, I need help repairing an HP 15c that belongs to my dad.
Bit of a backstory... When I was in college I needed a scientific calculator and my dad introduced me to his trusty hp 15c, he had the manual lying around and it was certainly a pivotal moment learning RPN with that beautiful wire-bound manual. Being new to matrices I was in awe that such a perfect unusual device could perform matrix operations. I specially liked the landscape format.
Of course my next step was dropping a significant amount of cash on the flashy new hp 50g, but I digress.
Recently my dad closed up his auto shop, and in the moving out we came across the 15c. Naturally I just ordered a set of Energizer 357 batteries but the calculator won't turn on anymore.
I read a few posts which were helpful, like identifying it is a later model with the coil spring, and rotating it as the contact between the positive terminal to the PCB may be faulty, but still nothing.
There were a couple of small springs that go to the back plate, what is their function? Would it help to improve the connection to the back plate with aluminum foil?
I opened up the calculator but as far as I can see it looks to be in good state with no corrosion. I already ordered some Deoxit D100 just in case.
I will attach a picture of the open calculator, perhaps you could help me diagnose any problem. I noticed that I left a little aluminum foil on the negative terminal.
   
From what I read on some other posts on the forum by removing the PCB and therefore the heatstakes makes the calculator very unstable, so I was hoping to avoid that.
Any help is appreciated. Thanks!