Suitcase FR-7054

[bulldog311]

Newbie here. I've had this piano for about 5 years. I haven't used it in two years. Toady when I fired it up it was dead. If I turn the volume all the way up and play it hard, I get a distorted signal.

I have cleaned all connectors with De-oxit.
I checked the 4 conductor cable from the preamp to amp with a multi-meter-OK
I checked the power supply and the voltages are good.
I connected the harp directly to a guitar amp and it plays perfectly.

What is the best route to take in troubleshooting this instrument?

Thank you in advance for any advice/suggestions.

John