Inspiron 9300 Shorts Out AC Adapter At Power On

When I push the power button on the Inspiron 9300 the lights come on briefly but then the system shuts off (doesn't POST) and the AC adapter is left in shorted condition (green light goes out). I have tried: a different AC adapter, booting without the battery, reseating the memory modules, and I have even purchased a new motherboard, but no luck. Does anyone have any idea what could be causing this? The laptop belongs to my wheelchair bound brother and I'd really like to get it fixed for him (he can't afford to buy a new one). Thanks, Jminshal