k5-some keys not working

Thank you for responding so promptly to my dilemma about the memory
card.  I also have another issue.  The used k5 that i just purchased
has about 3-4 keys that do not get any sound whatsoever and about 1/3
of the keys lack uniformity of dynamics when you press the key down. 
The previous owner did not keep a cover on it.  Is accumulated dust a
possible cause for this issue? Also, in regards to the keys that do
not make any sound at all, does this sound like there is something
seriously wrong/is this piece of equipment even worth sinking some
money into taken into consideration that I already purchased it for $150?

kevin

ps. I love that there are so many people that are able to answer these
question.  What a wonderful resource!  Hopefully I can return the
favor down the road.