Laptop Motherboard Component Overview

Checking Voltage and Short Circuits with Multimeter


Laptops aren’t resistors, which is the short way of saying you have to think about what you see when you try to diagnose a laptop with a multimeter. One of the more useful tests you can do is to measure the resistance between the positive pin of the power input (usually the center pin of the connector) and ground. The outer shell of the connector is ground, but it’s not usually possible to get both multimeter probes into the port without touching each other. Any exposed metal shielding on the outside of the laptop, such as the metal around USB ports, the video output, etc, should be connected to ground, which you can test separately. When you do find a good ground, the input resistance for a healthy laptop may be anywhere from a few hundred Ohms on up. Measuring on the 20K Ohm scale, this particular laptop read 6.48K Ohms. If you get a reading of just a few ohms or less, there’s a short circuit.


I’m testing the input resistance of a power regulator board. The connector on the bottom edge of the board married this daughter board to the laptop motherboard in the same plane. The connector to the left went directly to the battery bay connects. The input impedance of the board, reading 918 Ohms, was the same when the board was installed in the laptop, with the battery. Power regulation boards go for as little as $20 on eBay as pulls, and they can often be purchased new from Internet based liquidators on reasonable terms. The power board includes a couple of fuses that I’ll get to on another page.

Testing the output voltage of an AC adapter is pretty simple, providing it’s a standard barrel connect so you can get the positive (red) probe inside the barrel and use the ground probe on the outside, without taking any chances of touching the two probes together and shorting the output. The problem is, laptop AC adapters are switching power supplies, and they may require a load to start generating a voltage. It doesn’t need to be a perfect load, and the multimeter may be enough. But, if you observed that the status LED on the AC adapter was lit when it was plugged into the laptop, and now that you’ve borrowed a meter and are seeing zero voltage, it’s not lit, it’s because it’s not seeing enough load to fire up. Plug it back into the laptop for a moment, the LED will probably come on, and then stay on when you remove it from the laptop. The voltage should read a little higher than the voltage on the label.


The reading above is 19.8 Volts DC on a 19.0 Volt labeled AC adapter. If you’ve replaced the DC end on your AC adapter, be very careful when checking voltage. As mentioned above laptop AC adapters is that they are switching mode power supplies. While this gives them great advantages in low weight, low cost and flexibility on the input voltage, they may put out some audible high-frequency noise when not attached to a load, such as when the battery is charged and the laptop is turned off. As you can see in the picture to the left, the multi-meter probe is long enough to travel all the way up the inside of the barrel connector and possibly connect the ground. Unless you want to experiment with low voltage welding (AC adapter destruction) you don’t want to create a short.


Manufactured (molded) cable ends are much less likely to be open, but there’s no need to jam the multimeter probe all the way up the connector when checking the voltage. Another test you can do with your multimeter is to unplug the AC adapter from it’s power source, the 110 V to 240 V wall socket, and look at the impedance at both ends. From the AC input, measuring between the recessed pins where the AC power cord would be plugged into the brick if it were powered up, you should see hundreds of kilohms (K Ohms), which means you have to switch to the megaohm scale to get a reading. If you

get a beep on the continuity scale or a value less than an ohm on the 200 Ohm scale, it’s a short circuit, and it shouldn’t be plugged into live power. If you look at the impedance on the DC output side (this is still with no power), you should see a reading that keeps moving, as the capacitor charges up and the resistance increases.


If you start on a high scale, 200 K Ohms or more, the reading might start by dropping and then stabilize at some number of K OHms, but if you start on a lower scale, you should just see the capacitor charge up and the circuit look like an open (over scale or a “1” on many meters). Again, if there’s no resistance, it a short you can’t plug it in until you resolve it. A short on the DC connector end is likely in the connector. And remember that shorting the output, even for a moment, will often fry the brick, so be careful with your probes!

Testing for Backlight or Inverter Failure


I’ve been looking for an easy way to test for live laptop inverters for a couple years and I finally found a cheap, non-invasive method. The funny thing is I’d just ordered up a couple PC modding CCFL lamps with inverters to do a page about testing inverters with cheap replacement lamps. I’m not sure that would have worked given the impedance differences and the way inverters have to go through a timed sequence of voltage ramp up and down to strike and hold the plasma. Since the impedance drops when the tube lights and the plasma conducts, it’s quite a bit more complicated than simply providing an

RF power source. But as I was taking apart my old Toshiba screen today to expose the inverter leads for testing, it occurred to me to try the new Cen-Tech meter I picked up a couple weeks ago for $20. I’m showing the zero (well, 10Hz is well within 1% of zero on a 20KHz scale) reading with no power to the inverter.


Inverters put out fairly high voltage, in the 500 V to 700 V range, and a very low radio frequency, between 35 KHz and 60 KHz on data sheets I’ve looked up. That’s something you could easily pick up with a spectrum analyzer and a probe for either the electric or magnet field component, but the last spectrum analyzer I worked with cost around $30,000, so it’s a bit out of the reach of the home consumer. Now, the neat thing about the Cen-Tech meter is it comes with a Hz measurement. It’s limited to 20 KHz, after

which the display will simply show a “1” for over scale. Rather than equipping the meter with a special probe, I just held the two standard probes a fraction of an inch apart, and the son-of-a-gun picked up the cyclic field for the live screen almost an inch away from the inverter output, as shown to the left. The weak signal results in a lower than reality frequency reading, and as I moved the probes close to the inverter output, it simply went off scale. I’d try to be more technical, but the instructions that came with the meter were so vague about its capabilities and what it’s supposed to read that I’m just assuming here. The important thing is that the inverter test worked, and at no point am I touching the probes to exposed wires or terminals. This beats the heck out of a test I saw a guy recommending where he sets a multimeter to high voltage and shorts out the live inverter. He reported he could get a transient reading before the inverter shut itself down, and with luck, it would still work after rebooting a few times. The inverter is designed for an RF impedance, not to drive into a simple DC resistive load like a multimeter, and I wouldn’t be surprised if that voltage test has ruined more than a few inverters.



With my cyclic field test, I don’t see how it can do any damage, unless you’re careless with the probes and short out something in the screen. Then it occurred to me that the test could be truly non-invasive since laptop screens are so thin. To the left, I’m showing my newer Toshiba which I normally run plugged into a 19″ Samsung monitor so I can see what I’m typing. A little hunting around on the outside with the spread probes and the meter picked up the field to the left. A further small move and to the lower left, you see the “1” indicating the field is off scale, higher than the 20KHz maximum frequency the meter can handle. Below, just as a proof, I’m holding the probes in the same spot with the laptop live, but the image diverted to the Samsung. Hunted around forever, no reading. So, this is probably the best use I’ve gotten from that MSEE I earned in the RF/Radar concentration 16 years ago!



Of course, into every test procedure some rain must fall, and when I went out and tested some other random laptops, as well as a simple CCFL tube and inverter for modding, my $20 meter failed to register anything! So I borrowed a better meter from my neighbor, a Fluke 110 true RMS meter. The Fluke specs show it’s rated to 50KHz, which turned out to be critical in the inverter test application. As the measurment to the right shows, the Toshiba I’d originally tested has an inverter frequency of around 33 KHz. The reading varies a little with the exact positioning of the probes, the air gap, and the noise on the leads, but something in the sub 40 KHz was clear. That’s why my cheap meter that is spec’d to 20 KHz was able to pick up the field, even though it was over range, it was still within an octave. But higher frequencies are just filtered out or unmeasurable.



When I tested a Dell sub-notebook and my neighbors industrial rated laptop, I couldn’t pick up anything on my meter. Using his Fluke, you can see that the inverter frequency was picked up as nearly 68 KHz. By this point, I’d taken to using a piece of paper to make-up the air gap, rather than trying to hold the probes apart. So I’m guessing the Fluke, which sells for a little less than $100 in the aftermarket, will cover all laptop inverter testing applications. The model 110 has been replaced by the model 115 which goes for around $110 new. If I have some time I’ll design a little probe to function as an antenna, rather than just using the test probes, which involves a lot of fiddling around.

Of course, there are numerous things that can go wrong with laptop displays, so try to jump into testing the inverter before trying the obvious solutions. The first step, assuming you have power, etc, is to just plug in an external monitor and see if you can get that lit up. Newer laptops will usually autosense the presence of an analog monitor and shift the display without you having to use the function key (Fn-F5 on my Toshiba). If the external monitor works, you know that your video processor is good, that the laptop is booting into the OS, that the only problem is you can’t see the screen. Wiring harness problems are common with laptop LCD failures because the screen is a moving part. The cables may fail in the hinge, or the constant movement and lid flexing may cause the signal connector to work loose of the LCD screen. It’s also possible for the video connector to lift off the motherboard, especially if you’re a heavy typer:-) When the external screen works, the cabling all appears good, and a very faint image is apparent on the laptop screen, you know that the backlight isn’t lighting up. Not only is inverter failure more common than backlight (CCFL lamp) failure, but inverters are easier to replace. And now that it’s winter, keep in mind that temperature has an affect on the voltage required to get the backlight to strike, so if the laptop has been sitting in a freezing car for a while and the screen doesn’t light up, don’t rush to take it apart. Give it a couple hours to warm up, but don’t do anything whacky like sticking it in an oven or on a radiator!

Testing A CCFL LCD Backlight Tube


Warning: Laptop inverters put out high voltage, usually between 500V and 700V, so don’t try these tests unless you know your way around electricity. I’d also strongly advise against my sloppy approach of twisting wires and not even taping them.Since I recently did a page on testing a laptop inverter with a multimeter, I thought I’d go over some of the testing options for a backlight. Laptops employ CCFL tubes for backlights, due to their bright white light at relatively low power. Like all fluorescent lamps, CCFL tubes require a high voltage, high frequency input to strike a plasma and cause the tube coating to fluoresce. Someday not too far into the future, white LEDs should be available to do the backlight job with even less power and higher reliability. But in the meantime, a pairing an inverter with a CCFL is the way to light up a laptop screen.


The picture above shows a 4″ blue CCFL tube from a PC modding kit that cost around $3 by mail order. In fact, I think the shipping cost more than the whole kit, which included a dual inverter and the tube. Instead of using the PC connector provided with the kit, I cut the leads off and powered the inverter with 4 AA batteries in an 8 cell Radio Shack holder that cost $1.89. Since it was an 8 cell holder and I only wanted around 5V, I had to jumper the across the top, which I did by just sticking a piece of wire in the connectors. I also cut the output connector of the inverter since it was different from the connector laptop backlight I wanted to test. Then I remembered I hadn’t tested the original modding backlight first to prove the inverter was functioning, which explains the first twist together job.

The picture above shows the modding inverter hooked up to the LCD backlight. I’ve folded up the white flap that covers backlight to help keep the light in the LCD assembly. The inverter did fire up the plasma, but not completely, which left me wondering if the CCFL tube was bad after all. However, it turned out that the inverter output was essentially linear with the input, as my neighbor with a variable linear power supply was able to demonstrate. I came back and put 8 cells in my battery holder and it fired the whole tube up. Unfortunately, I got the other four batteries by taking them out of my camera, so a picture was out of the question:-) Next I decided to try to fire up the backlight with the original inverter from the laptop. This inverter looked pretty standard and I guessed it would be happy on 5V, I know some laptop inverters take a 12 V input but I figured the lower voltage wouldn’t hurt.



I’m simply touching postive lead of around 5V DC to the in-board side of the fuse, and you can see above that the backlight fired up at full brilliancy, even with the camera flash. I finally zoomed out so you can see that with the correct inverter, the whole backlight is lit up, and the light is in fact spread through the LCD substrate and visible at all the edges, as it should be. I had to take the metal back off the LCD to expose the backlight this way, which means that all the intermediate filters and layers wanted to fall out of the LCD if it was tilted. But it’s much easier to tell what’s going on from the back than the front, because in the “off” state, an LCD doesn’t transmit much light, all the little crystal cells remain twisted closed. If you move the whole screen to a shaded area and fire up the backlight, the screen will visibly light a little, a sort of dull grey that’s hard to capture with a digital camera. To the right you see the LCD with the backlight lit full on, shot in the dark. You’ll notice that there’s actually more light leaking out the back of the LCD than transmitting through the screen. The lighting gradient you see is an artifact of the camera angle, the screen was even lit with a dull grey grid. Keep in mind that an LCD displays white by turning on the red, green and blue subpixels, which mix the light together for white. But the fact that some light leaks through when the LCD doesn’t have any video input implies that for real black, the liquid crystals need to be twisted hard into an opaque state, their unpowered “off” mode allows some light to leak through.


Recovering Laptop Hard Drive Files With A USB Shell


The worst thing that can happen to most laptop users, aside from the loss of the whole laptop, is hard drive failure. The hard drive holds all of your data, files, and all your e- mails and contacts if you use Outlook, Eudora, or any other non-portal based e-mail. But I’m sure many more laptops have gone to the recycling facility with live hard drives than dead hard drives. If you have any files you value on your hard drive that aren’t backed up, you should invest $10 or $15 in a USB shell and attempt to recover the data. Hard drive data recovery is thought to be an arcane art, requiring expensive equipment and a high level of technical skill, but all of that only comes into play if the onboard electronics or the motor have failed. In that case, the drive platters are removed from the metal case in a special clean room, and the data is recovered by reading it off on a universal reader.


Most laptop owners are still very foggy as to where their data resides and consider the whole lower part of the laptop (everything except the screen) to be part and parcel with the hard drive. In reality, laptop hard drive are 2.5″ wide, about 4″ long and about a quarter inch thick. They weigh a couple of ounces, and can normally be accessed by removing a single screw from the laptop, as shown above.You should always unplug the laptop and remove the battery before attempting to do any repair work. I’ll admit I left the battery in here, because I knew it had been stone dead for some six months or more, since the AC adapter died. After removing the single screw, you can see the 2.5″ laptop hard drive installed in its cage. This hard drive is an IBM Travelstar, perhaps the most common hard drive used in laptops the past couple years. Because it’s an older laptop, there’s no shock mounting for the drive, little rubber washers that have become a popular way to partially shield the hard drive from the vibrations that can cause head crashes, in which case you can’t recovery the data with a million dollar lab. The series of pictures at the top of this page are for the older parallel ATA (PATA) drives, the newer SATA laptop hard drive is shown at the bottom of the page. The next step is to remove the whole cage from the laptop, which involves pulling back on the cage to free the drive’s IDE interface from the laptop connector. You can see to the right that the drive cage is held from lifting by two metal tabs, and that the screw that held the plastic lid on the drive bay went all the way through and secured the cage in the laptop. That’s all that held it together, one screw, and it’s a typical arrangement. It turns out that removing the old hard drive from the cage, once it’s out, is generally a bigger job than removing the cage from the laptop, because there are four screws involved and they are often overtightened and strip when you try to remove them. But it’s not necessary to take it apart any further if all you want to to recover your old files.



I’m holding the new USB 2.0 interface that came with the $14.95 Sabrent hard drive enclosure. The interface is really all you need to gain access to the old hard drive, if it’s healthy, and recover your data. The kit comes with software from Mac users as well as Windows based machines, but modern operating system versions don’t even require the software. They’ll just find the new USB hardware when it’s plugged in, recognize that it’s a hard drive, and allow you to recover your files as long as the file system types were compatible. I’m holding the interface card over the aluminum enclosure in which you could install the drive if you wanted to use it as a permanent external hard drive. But when I started taking the screws out of the cage, three out of four fought me and the fourth stripped, despite the fact I was using a high quality screw driver. It would be easy to bend and break the remaining tab off to remove the cage, but why bother, when the only point of the job is to recover some old files? So I plugged the interface on (to the right), then set the whole thing down on my table with the new laptop and plugged it into the USB 2.0 port. You can see that the little green LED on the drive is lit and active, if you have good eyes and a better imagination.



Immediately after plugging in the USB cable, Windows XP picked up on the drive, and asks what you want to do with it. Choose “View with Explorer” and you’ll gain access to all of the old folders, drag them onto your new laptop hard drive, and your data recover job is complete. Well, after you burn the recovered files on a DVD it will be complete, and you won’t face the worry again. If the LED doesn’t light up, you could be plugging the USB into an old port that doesn’t source the 500 mA required, or the interface could be bad out of the box, or the drive could really be dead. If you don’t hear the drive spin up, you can try picking it up gently, a few inches over the table, and try rocking in slowly to see if you can feel the centripetal force of the disk spinning. In the 2005/2006 time frame, laptops started changing over from the older IDE (PATA) hard drives to the newer SATA hard drive. The only difference, as far as the user is concerned, is that the SATA drives are faster and have a different connector. The drives are otherwise identical, and the SATA drives often cost less in the larger capacities as they are more common today. Since the SATA interface only requires a few wires (serial vs. parallel bus), ribbon cables aren’t required and a more flexible and robust connection is possible. The picture to the right shows an SATA drive installed in the laptop bay, and thanks to the rubberized shock mounting around the bay and on the cover, it simply sits tightly in place – no screws required. I only needed to remove one screw to take this drive out and put it into an SATA USB enclosure, and that was the screw on the drive bay lid.



Mounting the SATA drive on the circuit card for the external USB enclosure involves sliding the SATA edge connector into the circuit board connector and putting in a couple screws to hold it, if you’re going to make the enclosure its permanent home. But don’t make the mistake of thinking you’re going to be able to boot your laptop from an external SATA hard drive, I haven’t come across the laptop BIOS that can handle it yet. When the laptop BIOS gives you a “USB boot” option, it’s the option to boot from a memory stick. Sabrent makes an SATA hard drive shell In any case, if your laptop is a brick and you need to recover your data, pulling out the hard drive and putting it in an external USB case is usually the easiest approach, providing that the hard drive itself isn’t fried.


Advance Laptop repair equipment & tools








· BGA rework station,

· BGA ball making kit,

· 100 Mhz Duel Trace Digital Storage Oscilloscope,

· 400 Mhz Frequency Counter with PC interfaced Multimeter,

· 10 Mhz Function Generator,

· Duel Track Power Supply,

· Post Card Advanced PCI / Mini PCI / Express PCI,

· Universal ROM Programmer,

· Digital IC Tester, Analog / Digital R & D Breadboards,

· PTH De_Soldering Station,

· SMD Rework Station,

· Solder Bath,

· Micro Soldering Station,

· Microscope,

· Penscope.


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