Hi,
Here is a link to the service manual for a Yamaha PSRE-303 (also known as a YPT-300).
In it you will find information on how to dismantle and service your keyboard.
To download the manual from the website, '''wait''' until the word at the end of the phrase ''This file is downloadable free of charge:'' which is found below the document preview box changes from ''processing'' to ''Get Manual''. When it has click on the ''Get Manual'' words (which is actually a link) to commence the download.
Hopefully this is of some help.
http://elektrotanya.com/yamaha_psre303_sm.pdf/download.html