Just received a Casiotone MT-46 from a friend who warned me it might not even turn on, but I'm determined to fix it! It turns on and when plugged into my speaker system, makes a low humming noise on certain settings. Humming changes pitch and volume slightly when any keys between C and F are pressed (C, C#, D, D#, E, F, all octaves). The built in speaker makes no noise whatsoever. I opened it up to see if anything looked obviously wrong, and aside from a little rust on a single wire, everything's attached and pretty clean. Any way to fix it, or is this thing a goner?