Only External Monitor, not Notebook Display, visible

I am trying to install Ubuntu 12.04 on my notebook, Acer Aspire 7715Z from USB key.

On boot up, system recognises Ubuntu and loads up installation screen. A few pages of white text roll down the screen and then the screen goes blank. I attached a 2nd screen and everything is visible. I choose to run programme from USB key and this goes ahead ok and OS is visible on 2nd Screen while notebook is blank.

I went into display settings and tried to make display visible on Notebook but nothing makes a difference. I switched off 2nd monitor and Mirror options and set display to maximum. When I hit Apply, 2nd monitor disappears and very faint shadow appears on notebook screen. I cannot brighten this from notebook keypad although volume control FN option buttons work ok.

I think I used to have Ubuntu on the same notebook years ago but not sure what version. MS win 8.1 now unusable.

I experimented with all options on settings menu but no change and don't have an idea of what to do now.

I want display only to appear on notebook, not second display.

Thanks