Tv started screeching then screen went black volume and channels ok

This is almost certainly what you need:
http://www.coppelltvrepair.com/p/428/sanyo-dp42848-p42848-00-backlight-inverters-replacement-for-dark-screen-no-image-problem

This happened to me its probably too late for you but might help others. what i did to fix this is, I put the tv onto a radio channel (with no picture) and about 10 mins later i turned the volume down and realised the screen was still on as the volume bar showed up. so i guessed it the problem was to do with light so i went on the menu and went into features and turned the backlight onto minimum and the tv has been fine since. hope this helps!

Hello
If your TV displays a black box type shade on the screen means that your TV may be set to "Text" mode. To clear the screen, select "Caption Off mode".
To do this, press "MENU" button to display menu functions on the screen. Press "channel up or channel down" button to select the function "CAPTION". Press "Volume Up or Volume Down " button to select the mode you wish. There are five modes that you can select; Caption-Off; Caption-C1; Caption-C2; Caption-T1 and Caption-T2. As no other matereial is connected to your TV, you just select "CAPTION-OFF". When done press the Menu button again to exit menu display. Ok.

try hard reset unplug for hrs

When the screen goes completely black, shine a very bright flashlight closely at the screen. If you can see the image, then the most likely problem is that the backlight inverter has failed.

The inverter is a circuit board that works like a flourescent light ballast. If the inverter fails, there is no backlight, and the screen is black.

Since the invertor is a high-voltage component, I would also listen for any unusual sounds associated with the failure: e.g. buzzing, whistling, popping.

To rule out some sort of connection failure, see if you can recreate the failure mode when using the TVs internal setup menu. The setup menu does not rely on any external inputs....of course neither does the volume control or channel indicator on the screen.

Could be the power supply gone bad.
No easy solution I'm afraid.

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My Sony FD Trinitron TV did samething, started flashing, then eventually went completely black with volume. I got on internet searched found a solution, i tried it, it worked. unplug tv, remove all screws, remove back. towards bottom right theres a transmitter with a couple of knobs, we plugged tv in and moved knobs around and it worked. Also was extremally dusty, we used computer duster for a better connection. hope it helps you

the problem you are having is in the power supply you can go ahead and take your set to a repairman o look at your set good luck.