My olevia 427-s11 want turn on changed power supply and main board still won't turn on

The main power supply regulator section circuit fault. 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. Pull up older posts. http://electronicshelponline.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.
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Fault with the main power supply regulator 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://electronicshelponline.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.

Two things, assuming you installed a working power supply and main board, check to make sure the kill switch is in the "On" position. If my memory serves me correctly, this set does have one. Are you sure you have a working outlet? power cord? When you say the set won't turn on , are you referring to the entire set or just the screen?

Most likely you have a fault in the power supply or main board. I would start with checking the power supply capacitors.

you can try shopjimmy.com, or ebay. You will want to search for the part # DPS-162CP. Verify that this part# matches your old one

It's not a firmware update, you do have a problem with the main board giving the power supply an ON command. Please have it looked at by a qualified technician.

Most likely the CCFL inverter has been the problem all along....

Silly suggestion, but did you hit the "Source" button again a few times?

According to the manual, the "Source" button changes between the following long list of options:
Tuner, Composite, S-Video, Component, HDMI, VGA, and VGA component.
So you'll have to hit it 6 more times to come back to what you were using.

Olevia went out of business in 2008, so their website is defunct. Here is a link to what I believe is your manual on another website.

http://www.retrevo.com/support/Olevia-427V-TVs-manual/id/563ci863/t/2/

the remote control code for DirecTV with Olevia 242T or 237T (37' or 42') model is 11610. this code will power on/off and volume up/down the buttons on directv remote.
Or buy the Olevia RC-LRN remote control. go to olevia.com - accessories - remotes.

You can try the following manual for the AEP016-37 power board:

http://blog.coppelltvrepair.com/2013/08/olevia-237-t11-power-aep016-37-testing.html

Hope this helps!