HISENSE PDH4239EU

[WILLYAM1]

HI  tv works fine pic and sound all ok
after a few minutes pic started to be smaller in the highet and with more time a 1/3 of pic is black and also started to be smaller from left of the screen
when taking out the signal and put it back the pic is ok and again after mor time the problem started again
when problem acuered it seemes like a zoom
all boards are samsung exepet the main board
any idai? control or main problem?
willyam