Actually have the 50in same model, TV power goes off after on for 30 minutes and takes 10 minutes to turn back on. Sleep timer is off.

I feel like I need more details here like what kind of tv you have (model number too) but you might wake it up by pressing enter on the remote.

Yes, cooling fan could be a problem. Just see whether it goes off during before shut off. If yes, the TV might need service. To know whether the fan goes off first or the TV, stick one side of any thin vibrating membrane such as a silver paper (like the one you see in cigarette packets) on the fan outlet, without blocking it, so that you can see whether it stops before the TV shuts down. Alternatively you can hand an inflated balloon which keeps flying off from the area of the fan and shall stop when the fan stops. I suggest this, in order to save your time looking at the rear all the time until the TV stops. Even then, one is likely to overlook whether the fan stops or TV stops first.

Programming the Sleep Timer:

1. Press and release the TV key.
2. Press and hold the SLEEP key for three seconds. The illuminated component name turns on.
3. Release the SLEEP key.
4. Using the number keys, enter the desired time in minutes (from 1-99 minutes). To set the timer to an umber under 10 minutes, first press 0 and then the desired number (e.g.,05 for five minutes). The illuminated component name blinks with each key press.
5. Once the second number is entered,the illuminated component name turns off.
   Tip Any key press other than the number keys is ignored. If you don't enter the sleep time within 10 seconds after pressing the SLEEP key, you must start over at step 1. The illuminated component name blinks four times and then turns off, indicating your attempt to program the Sleep Timer has been unsuccessful.
6. The Sleep Timer is now set.
7. Leave remote aimed at the TV. Once the Sleep Timer is set, you can continue to use the remote without affecting the Sleep Timer. However, because the timing mechanism for the Sleep Timer is built into the remote itself, the remote must be in TV Mode and pointed at the TV to activate the Sleep Timer.
   Note: If the ON·OFF key is pressed, the Sleep Timer is canceled.

When you press the Power/Sleep button the radio turns on and displays a number 90, or some other numbers. This is the play time that the radio will stay on then turns itself off. It could be that yours is configured with a sleep timer set to 01 minute, 10 or 15. You need to set the timer for a longer period. Thus we turn it on by pressing the Power/Sleep button, read the timer length then press again. Keep making short presses: The timer will cycle through all preset sleep timer durations. Select the length you want.

Hello
Before going to set the timer, you have to set the current time. Set it according to the manual with you. If the timer function work properly, there must be an uninerrupted power supply also. If the power goes off, the set time will also get resetted, and improper function will occur.
After setting the present time, set the "on" time and "Off" time. The TV will switch on at that time you set, and will switch off automatically after the end time. OK.

press display button & it shows the following
07
NOW 12:20 PM
SLEEP TIMER OFF/on
ON/OFF TIMER EVERYDAY
ON TIME 7:00 PM
OFF TIME 10:00 PM
• The current channel or AV input (Channel 07)
• The current time (12:20 pm)
• Sleep Timer status/minutes remaining (The Sleep Timer is off)
• On/Off Timer status (Set to turn on everyday at 7:00 PM, off at 10:00 PM)
• Each Press of the DISPLAY button changes the display mode:
Press the SLEEP TIMER button & set it to OFF

Try unpluging your T.V. for 5hours+, then turn it back on. Worked for It's called a "hard reset" by sony.

Otherwise
Do the following steps

1. Check the Sony remote for a button labeled "Sleep." This allows you to cycle through the sleep settings and determine how long you want the TV to remain on.
2. Step 2 Press the "Sleep" button. You'll see a number come up onscreen: usually 15 or 90. When you press it again the number changes, moving up and down in increments of 15. This is the number of minutes the TV will remain on before it goes to sleep. As you cycle through it, you should see a setting with the words "Sleep Off" appear onscreen. If you leave it there, the sleep setting should cancel, allowing you to turn the set off when you like. If you miss it, just keep pushing the "Sleep" button. It will continue to cycle through the options until "Sleep Off" appears once again.
   
3. Step 3 Access the timer function to shut off the timer by pressing the "Menu" button, then cycling through the onscreen options until you reach the "Applications" setting. Press it, and then cycle through to the "Clock/Timers" option.
   
4. Step 4 Highlight the "Timer 1" setting and press the "+" button. If it's showing a given time, you'll need to deactivate it. Use the up and down arrows and move through the possible options until you come to the "Off" setting. Leave it there; that should shut the timer off.
   
5. Step 5 Repeat the process with the "Timer 2" setting, a little further down the "Clock/Timers" menu. Set it to the "Off" position just as you did with the "Timer 1" setting.
   
6. Step 6 Press the "Menu" button when you are done to exit the menu and return to the screen. You should be able to view the Sony TV as normal with no danger of it turning off.

good luck

turn off the timer

TV turns off unexpectedly
• Sleep timer may have been activated.
• Electronic protection circuit may have been activated because of a power surge. Wait 30 seconds and then turn on again. If this
happens frequently, the voltage in your house may be abnormally high or low.
• Unplug the TV. Wait ten minutes. Then, plug the TV in again.

Setting the Sleep Timer The sleep timer allows your LCD TV to automatically turn OFF after a given amount of time. You can set the sleep timer using your remote control or through the onscreen display (OSD).
TO SET THE SLEEP TIMER USING THE REMOTE CONTROL
1. Press the SLEEP button on the remote control. The sleep timer appears at
the top of your screen.
2. Press the SLEEP button one or more times to select the time you want.
3. Press the EXIT button to hide the sleep timer display. Your sleep timer is now running in the background. The sleep timer will be hidden after 10 seconds.
Turning On the LCD TV.
1. Press the MAIN POWER switch in the back of the unit (see Back Connectors for location). The Status LED on the front turns orange
and your LCD TV is ready to turn on.
2. Press the POWER button on the front of your LCD TV or on your
remote control. The Status LED on the front turns green.