Hmmmm...... MOSFETS aren't "chips", they are transistors (Metal Oxide Semiconductor Field Effect Transistor). Don't try to troubleshoot with it powered on..... I can tell right off the bat you don't have the experience for that. Set your meter to diode check and test the legs of the big transistors that are screwed directly to the heatsink. If it beeps long, that output is shorted. Now, they must be replaced in pairs, and alot of times the emitter resistor is open, so you need to check those as well. Not to mention there may be a blown ground trace on the other side of the circuit, and the trigger sense line may be blown as well (usually a 2sb1243 or similar switching transistor). If what I just talked about is greek to you, I don't recomend you go any further with it. It can get complicated very fast if you aren't a trained electronic technician. I've been in the business for over 5 years, and including tech school, I've been working on 12 volt and 120 volt electronics for 7 years. And I will say I still don't know all there is to know (relatively). It's fun, but you need to understand alot of circuit theory.