Saturday, June 2, 2012

Fixing a Dell Vostro 1000 Power Supply

My son has a Dell Vostro 1000 laptop that suddenly stopped charging the battery a couple of weeks ago.  It would run okay from the battery until the battery ran down, but would not charge the battery or run from the power cord.

With the battery removed and the power plugged in, pressing the power button would start the fan and blink the charging light amber, but the laptop would not power up.

I tried a couple of power supplies (I've got several Dell laptops in various states of repair).  They all produced the same result, and worked fine with the other laptops, so I ruled out a bad power brick and put it aside.  My son managed to finish the school year by charging the battery in our Inspirion 1501 and running off the battery or using the Inspirion.

I finally got around to working on it today.

First I disassembled it using the very good instructions in the Vostro 1000 Service manual at:  Most of this was not new to me, having replaced the LCD display last year.  As a backup for when I put it back together, I setup my webcam to record the whole process as I disassembled it.  Too bad I didn't realize the reason the picture was fuzzy as I was setting it up was that I needed to focus the camera.  I thought it was just my cheap EBay afterthought purchase webcam.  It actually works pretty good when you focus it.  Oh well, next time.  I think I might set the webcam up so I can record what I am doing on my workbench more frequently.  If can rig it to do time lapse or just to take pictures when I a button.

Anyway, I got it taken completely apart and then took a hi-res scan of the front and back of the motherboard.   I find that with the scans I can map front and back to layers and actually trace some of the circuits when needed.  It is also a good way to zoom in and look closely at the components, read markings, etc without using the magnifying glass and lots of light that my middle age eyes now require.

I started with the area of the board top and bottom around the power connector, looking for discoloration, and getting a feel for what the components are used for.  I missed the problem on the first pass and moved on to check the components in the vicinity of the battery connections for the charging circuitry.

On my second pass, I concentrated on the tantalum capacitors (all fairly small surface mount packages), and saw the problem right away.

For reference, The silver thing in the lower left corner with the marking starting with "A75" is the power connector.  Note the scorching in the upper right corner of the power capacitor right in the middle - looks like PC134 from the PCB markings.  That capacitor looks like it is probably my problem.  I think the hair that is right on top of it is probably not the cause of the problem, but it is a bit suspicious that it is right there.  I sure hope hair is not that conductive, or with my dog I'm going to have some trouble.

FYI: I found out later that this part was actually a ferrite chip, PC134 is the tiny 0402 package capacitor next to it.  Details of the parts and replacements can be found in part 2

I blew it out really good with compressed air can and took a look at it with a magnifying glass.  Through the magnifying glass I could see that the solder connection on the right side (in the picture above) was completely cracked and separated from the board.  I grabbed it with my tweezers and it came right off the board.  Guess I won't need the hot air rework station for this one.

I tried measuring the capacitor and got nothing, looks like it is shorted out - reading at around 1 ohm (well, until I applied too much pressure with my test probe and send it skittering to who knows where).  Now the problem is going to be finding the correct value of a replacement capacitor.  I don't understand why all my 1206 resistors are labeled with a value, but the capacitors the same size or bigger are rarely labeled at all.

I'm posting a message to Dell support and an electronics/laptop forum or two to see if anyone can help out.  I'm also going to look on Ebay to see if I can pickup another motherboard.  Even a broken one would be a source for parts (assuming it is not broken in the same way).  Even if I cannot get the parts I need from it, there are quite a few other parts that I can use in other designs - including some nice Maxim DC-DC converter controllers and battery charge controllers and of course all of the connectors.

I'll post more details when I replace the capacitor, clean all parts well and do some testing.

See details of the repair in part 2

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