Wednesday, March 20, 2013

Repairing Laptop Power Bricks

Recently I've had a few power bricks die on me. In one of those cases, it was still under warranty but since Asus seem to be extricating themselves from the original 'Netbook' formula there doesn't seem to be too many spare parts to go around. I guess that's why they gave me the option of a refund/credit instead.

Anyhow, a few days ago I ended up with another semi-working power brick. The original owner of the netbook told me that the cat had chewed on the cable (you could actually see bite marks and sections where the cord had come apart) but after examining the device it was clear that there were other problems as well. Pressing at particular points on the power brick resulted in the power coming on/off (didn't help that the thing had a fairly large buffer which meant that the visual feedback (LED) wasn't necessarily instantaneous).

First task was getting it open. Easier said then done if you don't have a good toolkit.

This one used a 'Torx bit' (under the sticker) in combination with a snap-lock type system and possibly some glue? Prying it wasn't easy and seemed to result in more plastic deformation rather progress in opening it. However, using a combination of a sharp knife and a hammer seemed to work perfectly and with almost no perceptible damage to the case.

At this point, I should probably digress. If you've ever worked with anything with anything more then battery voltage you'll understand why getting 'zapped' isn't fun or healthy (for those who have never experienced getting 'zapped' it feels like 'buzzing', it's something you know is wrong for you and something that you know you want to avoid). Wear rubber gloves (kitchen gloves well do) and take other precautions when ever possible. Don't do it if you lack knowledge, skill, or are generally careless!

Since pressing on the capacitor caused the device to work/fail I thought that either it had failed or there was a dry solder problem. I removed/de-soldered it (use a layer of solder/flux on top of the existing solder in combination with braid if you're having trouble. Makes the job a lot easier and helps to minimise damage that is often caused by a combination of heat guns and pliers or extremely high levels of heat from a soldering iron.). Didn't help that I didn't have adequate equipment to properly test (I guessed based on some rather crude tests that it was working but I wasn't 100% sure without more suitable equipment) but after removing all solder from the immediate area and re-doing the job that the problem seemed to disappear and my guess that it was due to a dry soldering issue was confirmed. 

Then came the issue of replacing the cable between the brick and the laptop. Problem is that the tip isn't widely available. In fact, the closest you can get is this which juts out slightly from the side of the netbook.

I had two possible choices. Use that tip on a new cable or use the cable from the old brick. In my case, neither was ideal so I spliced the old tip into a cable from another brick that had another problem (broken tip)(if you want to ensure a clean job always try to follow the priciples of the original design. If it has two wires at it's core with insulation only on one wire follow that convention and so on. This will ensure that there are no strange kinks in your wire when it is finished.), used heatshrink to give a more professional look to the job, and put it back together (glue it if you want or else just screw it back together).

Using the slightly longer tip option is also possible. However, I like to make sure that things are more ideal from an engineering perspective (reduces strain on the tip as well as the laptop jack). I noticed that cap on the end of Bic ballpoint pens seemed to be roughly the same diameter as what I needed. Moreover, since it already had a 'ventilation hole', and was the right colour (black) things were even easier. I used a sharp knife that was placed in heat (too hot and it will melt the plastic, not enough and it will make the job more difficult. Aim should be for warm blade or else use no heat at all if your (super sharp blade is best. A Dremel/rotary tool may generate too much friction/heat and result in distorted plastic rather than a clean cut even at lower speeds) equipment is up to it) to cut it down to the correct length to slide it down over the longer tip, and then it was a case of sanding down until for a cleaner finish/more flush fit.

Another option is creating an adapter using a suitable jack/socket and plug though this isn't ideal.

Used these (ferrite cores/beads/chokes) to great effect recently on some sensitive devices (reduce EMI/RFI). Good thing is depending on version you use/purchase you may be able to remove and re-use them elsewhere.