Model pn50a550s1fxza. What causes it to cycle on and off?

Check the power supply, the high voltage circuit, which near where the plug-in from the 120 volts. There is a capacitor rated 450 volts at 100 uf (microfarad). (Could be more than one capacitors). The high voltage at the capacitor should be around 390 volts, anything less and you will need to replace this capacitor (or capacitors). Replace with a 300 uf at 400 to 450 volts. Got mine off some old power supplies.

Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electronicshelponline.blogspot.com/ To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Fault to the main power supply regulator section. Either it will be a separate board, or the part of main board. Check and replace damaged component/s at its main power regulator [SMPS] section circuit. Contact any service technician. 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/

It will be best to replace the power supply regulator board as card basis rather than making "Surgery" to it; which usually will make loss for both money and time. Even though you can make it right, it will cost around an amount equal to the cost of a new board; including the cost of (spare parts + service charge) More to say, most of the spare parts to it will be difficult to get from open electronic spare part market. This is true to most of the flat panel TV power supply boards; irrespective of its brand and screen size.

Hopefully, it is just the video signal cable. Make sure it is firmly connected or try another cable. If that doesn't work, you will have to have the TV checked by a professional.

check the main.board n power board one of them is faulty needs replacement

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.

This sounds like a defective start relay overload and capacitor if your model has one. When a compressor short cycles it causes the symptoms that you described. Short cycling can be caused by the parts I listed and a defective condenser fan or dirty condenser coils can cause the defective start components. What is your model number? Sea Breeze

Your model # is PN50A550S1F and your main board # BN94-01658B. The only place I can find them is Ebay

Try a cleaning detergent/descaler. some liquids do cause problems. Softner is 1, it tends to gel in the pressure chambers in machines. This in turn causes many problems.