I have a samsung ps42c450 turns on then goes on standby could it be the main power board

Hi Ashley....Chances are your LED backlights need replacing, if you see a flash then black screen some of the lamps are shorted and the current draw is too high, and the protection circuitry is shutting down the inverter. You can check to be sure the main is working by testing the backlight on pin on the plug that goes from the main to power board, you need to see 3.3 vdc on that pin when you power the set on and it needs to stay at that voltage or you have main board issues

Not sure what voltage your reading that is 5.3 but if its the standby voltage that's about normal for your TV, next thing to test is the PS-ON and see if its at about 0 when the set is off and goes up to around 5 if the set is turned on...that will give a lot better idea if its the mail or power board that has issues.

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://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.
Before going further; try a hard reset first. If it does not help you, replace the main digital board. There are two methods to reset TVs. The first is simple to do; a Hard Reset. The other is somewhat skilled; is a Factory Reset (System Reset). To make it a hard reset, just unplug the TV from AC mains wall socket, and re-plug it back after 15 minutes or so. If you do this by overnight is better. This procedure will make it a hard reset. If you want to make it a system reset [Factory Reset]; you have to enter its service mode option, and select the factory reset option. If you wish to get some details; visit the site linked here.

probably the main board defective, see links below
press the helpful button
http://www.repairfaq.org/samnew/tvfaq/tvtmmqcs.htm

TV REPAIR for Beginners How to troubleshoot

Suddenlink Help Troubleshooting tips on basic HD video issues and problems

Samsung TV Won Turn On Has No Power No Standby Light Samsung LCD TV...

If it turns On just for a moment; at least, and the standby/power light at its front panel comes ON; and it goes OFF withing seconds; the main board to the set is faulty. If standby/power light at the front panel does not come On; the main power supply regulator board [inside] the TV is faulty.
If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/

Probably not main board but a short on the y- sustain board or a bad power supply

Replace the caps

B+ over voltage protection (OVP), unit goes to the standby mode then displays the 3 blink fault. This is also a problem in the power supply circuit, check T603 and R672.
Have you set the output voltage correctly?