iMac works but no back light

Hi, I have an A1311 iMac that works but has no backlight. When you push the power on button the screen flashes up a whitish yellow colour for a second then goes black. The power on chime sounds and it loads up. If you push the power button for a couple of seconds the screen displays for 2 seconds and goes off. When it goes off you can see the image when shining a flash light into it.

I’ve replaced the backlight board but it’s still the same. Replaced the PRAM battery and done a reset, still the same. I checked all connections and con checked the v ribbon cable which is fine.

whilst replacing the backlight board I noticed a couple of components on the power supply board look dodgy (TVS131 & TVS132) see pic. What do these control and could they be the culprit?