Friday, June 15, 2012

Fixing a Dell Vostro 1000 Power Supply: part 2

A couple of weeks ago, I posted about fixing my son's laptop that stopped charging the battery and running off of the A/C adapter.  I had isolated it to one component on the motherboard that I thought was a capacitor and took off to find a schematic or at least the part value.

Turns out the part was actually a ferrite chip.  That makes the measurements I made pretty worthless when I was assuming it was a bad cap.  Anyway, thanks to a remarkably quick response from ghn on, I was able to find  which had a schematic for the laptop.  From that I found that the part in question  (FL1) was a ferrite chip BLM41PG600SN1L.  I found it on at 0.54 for quantity 1.  Of course I bought 5 - they are cheap and worth having on hand for future designs and repairs.  Well, that and about another $50 worth of parts that I had on my wishlist for future projects.

I decided since it was not completely clear that the capacitor PC134 (.01uf 25v Ceramic in a 0402 package) wasn't a problem, so I replaced that also.

My hot air rework station came in handy for this.  I cleaned up the pads with some additional solder and flux to get it to flow, then removed the excess with desolder braid.  Soldering the ferrite chip (a 1806 package) was a piece of cake.  The capacitor - not so much.  The 0402 package is really small, making it ridiculously hard to hold it in place.  I really need to get some really good tweezers to work with parts that small, and maybe a syringe of good solder paste for reflow soldering.

After a bit of fumbling, I managed to get it tacked down, then used the hot air to reflow it.   I cleaned up the excess flux with some alcohol.

I connected the battery, and power switch board, then plugged in the power.  The charging light came on, which I counted as a success.

It took me an hour or so to carefully reassemble the laptop, which has worked perfectly ever since.

For details on the troubleshooting process, see my previous post

Friday, June 8, 2012

Project: Bus Pirate Case

I bought a Bus Pirate v4 with a recent PCB purchase from Seeedstudios Fusion PCB service.  It is an awesome open device for debugging, prototyping and hacking hardware that was created by Dangerous Prototypes.

The Bus Pirate is sold as a bare board, which puts it at risk of shorts from messy work surfaces with conductive things like paper clips and staples laying around.  I've decided I really like this device, so I figured I'd make a case to protect it.

It just so happens that I recently joined Xerocraft -- our new Tucson hackerspace.  We have a milling machine that I have been working with, I decided to use it to mill a case out of hardwood.

I bought a small chunk of maple at a hardwood supplier and took it to the hackerspace to mill it out.  I set the mill spindle to run at about 600 RPM (too fast burns the wood), and mounted a 1/2" end mill in a 1/2" R8 collet.

I cut an appropriately sized piece of the maple with a hand saw (which took some effort - this is some hard wood).

Next I manually milled a cavity of the appropriate size in the piece and cut a slot for the USB connection during Thursday open hours.  This left me with a roughly shaped part, however, because of the size of the end mill I used, I had rounded inside corners instead of squared off corners that would fit the board.

Milling the case

I tried a few things to get the corners squared off - an oscillating tool with a basic plunge blade was the first thing I tried.  It cut well, but was a bit too hard to control.  I found that the best tool was a sharp chisel and a bit of patience.

I finished shaping it with a palm sander, then used a fine sandpaper to prepare it for staining.  I used a bit of stain, then a couple coats of polyurethane to finish it.

Once it was dry and looking good, I drilled appropriately sized mounting holes for #4 machine screws all the way through the bottom of the case, then tapped the hardwood for the mounting screws.

I used four nylon #4 machine screws and nylon spacers to mount the board directly into the cavity.  The screws I used are longer than I needed, but nylon is easy to cut cleanly, so I screwed the board in all the way and marked where they extended through the other side.  I pulled the screws out one by one and cut them to size just a little shorter than the mark.

The final case is quite functional and I think a step above the usual project case in looks.  The dark stained maple has a nice contrast with the red circuit board.

I was originally thinking I would put a clear acrylic top on the case, but I've found it is not really necessary.  I may do that later, but for now, I'm very happy with the results.

I also printed a label with the pinout and pin assignments and stuck it on the bottom of the case for easy reference.

I think I'll make a similar case for my Open Workbench Logic Sniffer board next (another awesome Dangerous Prototypes project).  I'll try to remember to take some photos at each step of that project.

I'm also planning on expanding a bit more about how I am using both the Bus Pirate and logic analyzer in future posts.

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