There is a high pitched (approx 4K) sound induced into the system by the board. What could be the cause?

The speaker resonates with itself, reabsorbing the, little as it may be, and then amplifies it, spits it out, reabsorbs it... It goes on as long as no other input is connected.

Use the factory reset
Hold down highest white key whilst turning on
This should return to factory settings - pitch should now be back at A440
If you need to adjust then see page 19 of the user manual where it explains fine tuning.

You are proabably getting regeneration. This is nothing more than feedback as these acoustic guitars are good microphones. Make sure the guitar CANNOT "hear" the speakers of the PA system. Crank the high frequency EQ for that channel all the way down. This is common when soundment decide they ALSO need to boost the high frequencies of the PA up when they shouldn't. Regeneration is a feedback that is below that to fully sustain, but will cause a ringing "tail" on notes.

Look for something related to the whammy bar causing unstable tuning. The only other thing is string QUALITY and gauge. Use the recommended strings for the guitar.

Some of the Yamaha pianos have several pianos emulated and selecting a "Grand" will sound different than if you select an "Electric" or "***** Tonk" setting.

There is NO internal adjustment for this. other than the voice selection. Choose either the Grand1 or Grand2. Do not use reverb as this can "ring".

This sounds like a ground loop. Make sure ALL (including the computer) interconnected devices are powered from the same receptacle. Make sure there are no lamp dimmers around as these can cause hum...

This would indicate a problem with the scanning matrix for the keys. I don't have my schematics handy right now, to analyze which part might be bad...

FIRST though, check the settings for the keyboard splits on the utility page that has the splits. I have had this get clobbered and caused an unwanted transposed voice to sound..

If you can find other combinations that cause the problem that would aid in pinpointing the exact cause.

This could be a bad disconnect diode or short or break on the keyboard contact board or even a cable between the upper and lower contact boards. (The keyboard contacts are actually split into two groups high and low).

Please be careful in opening the unit. There are lots of screws and some in wells in the back. There are different threads and lengths, Make sure they go back correctely. Be VERY careful of the ribbon cables and their orientation. ALSO the power switch leads are very short... I often remove the switch rather than try to take the leads off.

I can try to find my schematics but that will take a while. If you are not comfortable working on electronics, this is a problem for the shop.

Check your power supply to the unit. Low voltage COULD cause this.

Also make sure a setting is not "looping" the audio back into itself.

Change the audio amplifier ic are fried

Check for ground loops.

You MIGHT be able to cut the noise by setting the EQ's to clip the highs.