Clavinet E7 - Dead pickup/Amp?

[nonplus]

Hi, today I've got a E7 for relatively cheap.
It was not played by him for about 30 years, the owner said.
Nearly all hammer tips are broken, but that's not a Problem.

If I switch the pickup buttons to CA their is no sound at all. I think one short time I had sound, but I'm not sure. How can I check that the (lower) pickup works?
And the other thing is, that all Tone-Settings sound the same to me (the sound is really bad, but this is because of the sticky and broken tips).
I can turn all 4 tone-switches on or off, I can't hear any difference. Is this because of the non-working rhythm pickup?
I've expected no sound if all 4 tone-switches are off, am I right?

Thank you.

[nonplus]

And another question: Their is some "white" corrosion/oxidation on the brass (?) parts (the metal, ger: Messing). Is their a way to clean them?
Here is a picture: ftp://quake.sigabrt.de/pub/messing_oxi.jpg