Hi I have a Technics PR-305 Digital Piano, that I had since the fall of 2001. I went to play on it on it today Some of the notes in the chords weren`t sounding.

Attempted to clean? Not a good idea obviously...Take it apart like you did before and look at the wiring and imagine that you can divine the thing to reveal some logical explanation. If you used a contact cleaner or spray to clean, that could be death to that piano my friend.

I would suggest taking it into the nearest factory authorized repair center. Go to the Technics web site to find one in your area.

it can be repaired or replaced if the boards are still available
Take it to an authorized service to have it repaired and failing that some shops that repair micro wave ovens have the technical ability to repair items of this nature
may be a good radio technician or radio repair shop could also do the repair as it is basically an electronic ( wireless/radio) that is controlled by keys

Yes, can be repaired. Sounds like sound output power chip has gone bad.

Also, in addition to more info I need; look on p 91 of the online owner's manual linked below. Pay attention to Sounds and Effects, the first section therein. Follow the directions closely, esp., read pp. 26, 83, and 86 which are referenced for detailed help w/ sound issues. TECHNICS SX PR305 Manual Page 91

I don't see why Technics couldn't replace or fix it.

The Speakers can be replaced with any brand. The thing you need to know is the OHM rating for the speaker. OHM is also called impedance. Unless the sticker has fallen off the back of the speaker that information will be on that sticker. OHM rating MUST match the old speaker. For instance 4 OHM, 8 OHM, 16 OHM are common ratings. All different ratings but VERY important to get this correct otherwise you could damage the electronics of your instrument.

it will be wise to replace the speakers.

It sounds like you are in need of a new piano. These are common issues with keyboards. The age is a big factor. It would cost just as much in repairs as it would to buy a new keyboard.