Vintage Synth Repair

Hope it is ok to ask for help here.

My keyboard had been sitting a while and a couple of keys weren't 
working.  I was able to get them going by striking the keys a number 
of times and all was well.

The keyboard was sitting for a while longer and a couple different 
keys stopped working, not the same keys.  Unfortunately, I cannot get 
the keys to work by that same method.

I would like to open it and see what is going on...dust, oxidation, 
rust, wear.....

I did check all the keys now and have found no pattern so I believe it 
is not an electronic issue.  Here is what is not working:
Octave 1 F# and A; Octave 2 C# and E; Octave 4 B; Octave 5 Eb

I haven't been able to find a service manual and there are MANY screws 
on the bottom to open this.  Any tricks or warnings? I have done 
computer/electronic repair before so not too worried.....On the other 
hand,  I don't want to destroy anything else.

Second question is if it is just the contacts, any economical 
suggestions besides replacing them?

Thanks very much........

Adele
in CT