Ok, here's what to check for a yellow boot screen. If the machine has a ram expansion in the trapdoor slot underneath, remove it. If this cures the problem, check the connections between the card and Amiga.
If that doesn't work, it's time to take the top off of the Amiga and check that the chips are seated, especially agnus (the square one) and the CPU (the big long one). Just press down on them with a little force, not too much though!
Try this and let us know how you get on.
