Jumat, 15 Januari 2010

Story: Finding a Short Circuit (contributed by Chris Wilkinson)

This laptop (HP zt1250) was used for 2.5 years. At 1.5 years it began running the fan constantly and would occasionally shutdown. At 2 years an external desk fan was required while burning CDs. At 2.5 years the laptop external power supply indicator light dimmed when the plug was inserted and was bright when the plug was removed. An electrical short had developed somewhere inside the laptop. The laptop would no longer run off external power.

The small charge left in one battery confirmed that the rest of the laptop was functioning.

Disassembly of the laptop revealed large accumulation of lint on the cooling fins of the heat pipe. See photograph. The cooling fan had slowly but surely suctioned lint and dust from the users bedroom, some of which was unable to pass between the cooling fins. Over time a huge blockage developed.

(Blowing air into the computer would push this clump backward towards the fan. This lint would likely stay clumped together and not leave the machine chassis.)

Fan Lint Picture
(Click photo to enlarge)

Removal of the lint did not remove the suspected electrical short. Further disassembly revealed a bad solder joint on the plus + pin of the power connector. Resoldering did not help. Further inspection did not reveal any abnormalities in the connector or surrounding printed circuit board. A shorted resistor, capacitor, or diode was now suspected but without schematics it was impossible to proceed.

The local fire department (where I volunteer) carries a thermal imager camera to see heat (infrared wavelengths) through smoke and walls. The camera was used to observe the circuit board while power was applied. The area near the power connector immediately began to show heat - thus confirming the connector or circuit card traces as the location of the electrical short. (A good thing I did not pry up any surface mounted components!).

Dissection of the connector did not reveal the short. (Dissection meaning nibbling with diagonal cutters.) I still measured zero ohms resistance from the plus side to the minus side. Removal of the plus connector pin and resoldering of a small wire in its place somehow cleared the short.

The laptop works fine but I am disappointed not to know the exact source of the problem. The circuit board is likely a multi-layer board. One can see the top and bottom outer layers of copper foil on fiberglass epoxy board, and see shadows of the inner layer (often a copper ground plane). Surface mount components attach to the surface, through-hole components pass from top to bottom through a hole drilled through. The inner layer ground plane (by design) should not get too close to this drilled hole. The "leg" of the +19VDC power connector passed through - one cannot observe what may have happened in that hole. (The short may re-appear.)

1 komentar:

Endang Kurniawan mengatakan...

Yup... I've found the same problem too... Thank's for sharing...