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I have a Roland FP-4 that only plans out of one speaker. The same problem occurs when I plug in to an amp. I opened the keyboard and checked the dead speaker, and it functions properly, so the problem is not the speaker itself. Has anyone had experience with this issue? Suggested solutions? I thought it might be the on/off toggle switch for the internal speaker, but jiggling that did nothing.

Likely the volume Potentiometer knob is worn out.

If you open the chassis, check the potentiometer with a multimeter set to 10k or 20k ohm. If the up and down is smooth, then it's working, if on one of the channels it's off spec, then that's the problem.

After you desolder the potentiometer, if its casing is -pryable- without permanent destruction, you can do that, and wipe the carbon pads down with a 20% alcohol /water solution. dry it, put casing back on, just Hold it together, then check again with the multimeter, it should work smooth.

If it is the switch that you checked, just short it manually to the ON position and if you get sound, then you know that switch is busted.

The piano has two visible knobs: volume and balance. The volume knob works fine. Are you suggesting that the balance knob is the problem? Or is the pententiometer knob an internal knob of some sort? Thanks for your help.

I looked online and found part #2418: https://syntaur.com/Items.php?Item=2418. This is the potentiometer for balance. My instinct is to buy it and replace the switch without bothering with multimeter. What do you think?