Where can I find a service manual for a Korg C303

https://www.korg.com/tmp/support/download/index.php/manual/0/90/1945/?country=us&page=manual&prodtype=0&prodid=90&fileid=1945

https://korg-pa-paradise.blogspot.com/2018/04/blog-post.html

https://www.manualslib.com/manual/1060488/Korg-Pa60.html

Email Korg customer service in your country for advice, or search the manual if you have it.

You can find it here-
KORG R3 SERVICE MANUAL Service Manual free download schematics eeprom...

Give your email, I'll send for free.

Oddly, I can't seem to find a thing; either from Korg or through the manual database servers that I've used in the past. You might barter a deal with someone on Ebay to make a copy for you -- someone who is selling the whole unit +manual. Other than that, it may take writing to Korg directly about this. Good luck! :)

Yamaha YDP223 :

YAMAHA YDP 223 223C PARTS Service Manual free download schematics eeprom...

Korg i3 :

KORG I2 I3 SM Service Manual free download schematics eeprom repair info...

Open it up and check the solder joints of the 9V DC input jack. Also, download the service manual here http://www.korg.com/SupportPage.aspx?productid=496

If you can't pinpoint the problem from the method given by Korg, then most likely the internal memory was erased somehow. This is the problem I have with my EM-1 and if anyone has a solution please share.

Seems processor board not starting. Here is user manual

KORG Pa800 2.0 User's Manual (E12)

Here is service manual

KORG PA800 SM Service Manual download, schematics, eeprom, repair info for electronics experts (elektrotanya.com)

It may help you.

Here is a FULL korg triton service manual:

http://elektrotanya.com/korg_triton_pro_prox_exb-scsi_moss_sm.pdf/download.html

Scroll down to "Get Manual" and click on it to download.

http://www.korg.com/SupportResults.aspx?productid=413 just click here :)