I have a Yamaha MM6 - only the piano category works - the wheel jogs through the voices. No other category works and no functions work. No lights are on apart from the display.

SOURCE: displays code Scn

Either the keyboard is disconnected or the Keyscan on the keyboard itself is but. More than likely a loose connector.

SOURCE: Kurzweil digital piano unpredictable voices and volume

The contacts of various switches and keys are likely dirty. They are often conductive rubber and need cleaning with 99% isoprophly alcohol and q-tip. Clean both the conductive rubber pill and th circuit board traces they contact.

SOURCE: Digital Piano has sticking key

Typically LOTS of screws... concentrate on those around the edge. USE GREAT care and force nothing when removing the cover...and great care of cables between the cover and the base. Mark the cables so you remember the direction they attach.

When you get edge screws out, then gentle lift and see where it seems to be held...

Some of these synths have 50 to take out to get apart. MOST are in plastic bosses and keep track of the size and length as you can put too long a one in the wrong place and crack the boss.

I use a compartmented plastic tray to keep the screws in order.

SOURCE: Problem with Yamaha YDP223 Digital

First thing is to check the connector is fully seated... sometimes most of the contacts make but one might not. Beyond that, contacts in the pedal are next suspect.

SOURCE: My Yamaha mm6 doesn't work

There is probably a setting that is wrong. The left channel is the default for mono operation. Look in your user manual regarding "PAN" which controls the output going side to side. You MAY be "panned" clear to the right. When you plug into the left, it is the default so in spite of being panned right, they would send signal to the left IF nothing is plugged into right.