Samsung HL67A750 has bad main board but is also arcing from all HD inputs.

If you have sound and no picture a lot of times that is caused by a bad tcon board

Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electro-medical.blogspot.com/
http://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

have a look round this site for good help..
http://electro-medical.blogspot.in/2015/11/lcd-tv-t-con-board-troubleshooting.html

Fault to its main board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. http://electronicshelponline.blogspot.com/ . In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.

IF TV IS WELL CONNECTED AND THERE IS NO SOUND ON ALL INPUTS REPLACE MAIN BOARD

most likely the power supply board has failed, call 1-800-samsung for nearest service center.

First check to see if you get it on all inputs (DVD, Cable, etc.). If you get no sound on all inputs, then do a hard reset by unplugging the TV from the wall overnight and plugging it back in and test again. If this does not work, then the main board needs to be replaced.

If you do not get it on all inputs, most likely it is just a loose connection. Check the connections going out of the source and into the TV for something that might have come loose. If everything is okay, test the bad input with something that you know has some on a different input on the TV. If this still does not work, replace the main board or simply do not use the bad input.

is there sound?Does the tv menue display? Likely main board. replace any swolen capacitors on power board first.

I think the Y-main board is bad. Replace the Y main board will fix it. This board is on the left hand side, if looking at the back of the TV.

when it blanks out, tuen the lights out in the room, and shine a flash light at the screen, see if you can see the picture, if you can, it is the inverter moard blamking out, thats the board that lights the back lights, if you cant, hit the input button on the remote, if the TV comes back with a picture, it is a problem with the main board, in paticular the componant input on the main board