'''<big>Bought a 464 or 6128 from eBay ? Not Broken or not working right ? Then read Read on..</big>'''
Firstly, make sure your you have a 5v '''REGULATED''regulated''''' power supply and that your monitor / tv are working ok. (You'd be surprised...)
Make sure your power supply provides a '''minimum''' of 2A continuous current. Some PSUs can be very "optimistic".
Now carefully remove the case.. Be bl00dy '''VERY''' careful with any keyboard membranes. You '''CANNOT''' buy these anymore.
Btw: eBay is one of the The best sources source for spare chips and other replacement components.that I have found