Play Station 2 Slim Cannot Read DVD

[Mechatrommer]

i tried to repair this but still unsuccessful... internet suggesting replacing laser unit which i did, twice! both laser still cannot read. i managed to twist adjustment screw on 1st laser unit purchased. and slightly able to read dvd games, but it was only for a moment and next power cycle it cannot read anymore. thinking laser unit expired, i bought the 2nd laser unit, same thing, so i suspect something else is broken but i'm unable to get clue from internet. nor i have no clue yet as to where to probe he problematic signal. any of you have done this? or have any educated clues? where can i probe laser eye pattern? the net only provides diagnosis of much older PS1 unit.. need quick help here. cheers.