ANDROID Screen goes blank [SOLVED]

Hi everybody,
I use Lenovo TAB2 A7-10F. Android ver. 4.4.2. iGO_Israel_9.6.29.636868.
Everything works fine except one: the screen goes blank while voice announces next maneuver. After that, screen returns to its normal state. I had to turn announcements off. Other sounds are on.
Any hints?