Im looking for all the Roland JV-90 plastic push butons.

Read this please

https://www.justanswer.com/electronics/9xlyw-roland-digital-piano-kr-4500-piano-turns.html

and these

https://www.google.com/search?q=Roland+kr+4500+no+sound

But you will probably need a service centre for replacing components.

Software, download the Owners Manual and get into it.

Get the manual from web site and it will tell you how to replace the battery.

No availability. No need to try.
http://electronicshelponline.blogspot.com/

MIDI cable and MIDI ports

This problem will require servicing. The screen is bad, and has to be replaced.

I.m looking for all the ROLAND JV-90 plastic push buttons.

I would expect there is bad soldering or a circuit board crack. Inspect the soldering aound the leads, especially pots and heavy components. Finding all the retaining screws can be frustrating. Expect the screws for the handle to also attach to the amp chasis. They are often hiodden under plastic caps.

Most of the Roland buttons on this type equipment use a small switch that is actuated by a plastic button that is HINGED from a plastic panel behind the faceplate or a part of it. These plastic hinges get broken when too much force is applied to the button... it is USUALLY fatal and requires replacement of the whole button faceplate... and they are expensive... contact Roland parts to inquire.

Roland FP is a nice keyboard but has a known problem on cracking key weights. There is a counter weight under each key to push the key upward. There are two sizes of weight beams, longer one for white keys and shorter one for black keys. Roland did not make the original weight beams strong enough to last. They will crack and eventually break. When it happens, the key will stay down. Fortunately, they can be easily replaced.

Roland FP-8 has 4 black screws at the bottom on each side end. Remove the screws, flip the ends up and expose the interior. Near the hinge of the problematic key, carefully ply open the hinge tabs and remove the key from the hinge. You will find a weight beam under the key. Remove and inspect the weight. There should be a metal wrapped in a plastic beam. If the metal part is missing, find the missing part and replace with a new beam. You can order weight beams from Roland. It costs about $10 each plus shipping. Put everything back in the reversed order.