SamyGO

Did you turn watchdog off?

eeprom reset does not help here, don`t try this, because it will switvh watchdog on and rs232 to uart = no hope to repair at home.

Only ExLink cable (sure if rs232 was previously set to debug). But here is still 50/50 success and hope you get your TV working.

Please answer:
Before to start, have you:
1. set rs232 to debug?
2. set watchdog off?

If first your answer is no, then it makes no sence to do anything else, just bring tv to Service. If yes, then make ExLink cable. We have chance to repair - here is still possible, usb hotplug works and is executing scripts from USB, but you have be quick.
How much time do you have from start to reboot?

Safety warning exists on the wiki page. As i remember i added this to all C and D series hack/root articles.

Second: you havent loose warranty and if it still exists, bring tv to Service. Sure it makes sence to dont tell althe story about rooting...

Btw, if you really gonq change mainboard, then why not C650 (or other C6 series but plasma? Dont know if c650 plasma exists.... we have success by upgrading lcd 550 B and C series, so think about )