Dell :: Is There A Power Supply Gremlin?

I’ve seen 2 fairly new Dell laptops recently (a Latitude D520 and an Inspiron E1705) with the same symptoms. With a known good adapter attached to the unit, they do not charge at all,

but they run off the battery until the battery dies. I disassembled both and checked the connection between the power jack and the mobo and found them to be solid.

I connected a power source directly to the + solder point on the mobo and a ground and found they did not power up at all. If this were a desktop, I would change the power supply immediately, but being a laptop, I’m not sure what (if anything) can be done to fix the problem. Any ideas or suggestions would be very much appreciated.

What I've done:
1. Both units work normally with charged batteries, so I know I'm working with good laptops.
2. Removed, cleaned contacts and re-inserted RAM.
3. Cleaned accumulation from the internal lint trap, i.e., from the CPU cooling fins.
4. Removed, cleaned, re-applied thermal paste and reseated CPU'