Scrambled Nintendo logo on start up, working cart reader(?) and game.

I have a classic gameboy which I am trying to restore and I cant find the solution elsewhere online. When I insert a game it displays a scrambled Nintendo logo. At first I thought it may be a bad cart reader so i took a working game and soldered it directly too the DMG mobo and still the same problem. There are no shorts in the cart reader and the game is still fully functional. As well no visible damage to caps or resistors. Any ideas?