Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, the described symptom almost always point to an internal hardware issue or more particularly, possible suspects after a review of the Service Manual are:
All of the above would require that the unit be opened up for visual inspection, voltage/continuity/resistance checks, confirm faults and effect repairs.
If comfortably able and feel safe to do so, are you then considering a DIY (do-it-yourself) repair approach? If yes, do you have access to and familiar with a multi-meter and even a soldering iron?
Unfortunately, no external factor(s) can cause the described symptom. The Yamaha would have to be opened up even if just for exploratory checks and/or circuit, controls/switches cleaning using compressed air or better yet, plastic safe / non-staining DeoxIT DN5 or LPS 03116 Contact Cleaner or equivalent to clean but allow sufficient drying time.
I realize that the above information may perhaps be not what was expected. Unfortunately, there is no quick-n-easy fix and neither is there a workaround; sorry for having to be the bearer of bad news.