No one can tell you the exact component to replace because they are not there to do the trouble shooting - I'm afraid that is you.
To trouble shoot a STB (or any electronics) you need a knowledge of electronics, know how a STB works generically, and have a circuit diagram. It is unlikely that yo will be able to get a circuit for the box because they are bought in by WallMart/Asda and, being budget boxes, are not designed to be repaired but replaced as a unit.
t, April 2007