my tv loses the picture every 20 mins est. The sound is still on but no picture.
I have changed all the leads at the back, scart etc.

martin, January 2017

Your TV has a warm-up fault - as the temperature increases a component fails and the picture disappears.

An engineer might trace the culprit using an aerosol freezer spray on a section by section basis - gradually on a smaller area each time until the specific bad component is located.

Electronic freezer spray might be had from Maplin or Amazon for about £10 - alternatively a TV doctor might repair your set.

It is sometimes the case that the component that appears to be the cause is not the cause. If A feeds a signal to B and cooling A makes things better it might be component B in error. The TV doctor will be aware of this possibility.

Good Luck..., January 2017

If its an LCD set, could be a backlight or backlight inverter issue. If you get the light on the screen just right, can you make out a faint picture?

There's not much more anyone can say because the only idea we have o fthe set you own is that it is a "TV" - If you had told us the make and model we could have done the same Google search you could have done for yourself, for - say - " 'make' 'model' loses picture" - or " 'make' 'model' no picture" - and see if anyone has experienced a similar problem.

Anon, January 2017