My Yamaha MM6 won't turn on

I suspect you'll need to replace the screen, but to be sure you should contact Yamaha customer service and go over the problem with them.
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The battery is only to retain patch memory and would have nothing to do with the instrument starting up. Your problem will be in the power supply. I'd suggest taking it to the repair shop.

only few technician could do some repair with the screen card or the connections fron screen to cards very complicated usually the screen goes off totally only few could do it luckily by chance.
you need to change the screen i suppose

Probably the transpose button on your keyboard has become short. You need to clean the contact by a cleaning agent such as WT-40 or similar one. If not, you need to change the button.

My Yamaha MM6 Synth is showing blank (white) screen when switched on. Keyboard sounds and banks are working. Everything looks nice but not showing any details on screen. Any help.....

It kinda depends on if a hammer is broke or key is broke. Or if a foreign object is stuck in there. Typically not hard if you can remove screws and remember how they go back in. TypicAlly the bottom panel comes off keyboards ( start by removing those screws ) or the top case will fold up ( still bottom of keyboard screws ) then multiple screws to realease keyboard assembly. May have to disconnect some wires/ ribbon cables so have a sharpie ready to label to ensure correct reassembly. Then it's all about if they have separate keys or a keytree or sometimes just removing ba key and reseating fixes sticking problems. Get yourself a can of blow off or contact cleaner just in case. Not sure who locally to you sell parts but you can always call manufacturer and ask who does their distribution for parts.

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.

Usually that is a programed feature, but you can try the support center at the manufactures website.

I had the same problem. This is how I fixed it:
1- Lay the MM6 upside down, remove the screws, open the back panel.
2- On your right side (the Volume PC board section), you will see component stand up (2 of them had 3 legs each, one of them is had more than 3 legs don't remember exactly) but 3 of them all had metal finish on their back. There are look like Power Transistor ( I do not have a schematic to be sure) but when these part get warm or hot (volume is only decrease when MM6 had been Turn-On for awhile), caused the volume to decreased.
3-Go to Radio Shack or other electronic stores, buy 3 HEAT SHRINK or you can make them your self (find- google to see what it look like, marterial...etc);
4- Use small screws and nuts, install these HEAT SHRINK into these 3 component then they will help these 3 components to stay cool all the time, therefore it will fixed the Volume decrease problem.
5- Yamaha should put HEAT SHRINK into these part when they designed.

If this s a common known problem, perhaps there is a recall on the instrument. Contact Yamaha directly to inquire.