Laptop Motherboard Repair

[precision001]

Re: Laptop Motherboard Repair

Its what we do for TVs. I don't know how well it will work since its only a ~19v source. Maybe with a smaller bulb.



Laptop or desktop? On desktop boards its usually a dead chipset, like on the emachines.

Desktop. It seems to be more common on a desktop for the fans to spin on a dead board. I'm trying to find out why. As for the laptop, I'll try to isolate that short when I get some time. It started out as the battery not charging on an HP DV9k- and the DC adapter would stop working after a few minutes - but it would be intermittent. The problem only happened when a battery was inserted, so I replaced the Maxim chip responsible for charging the battery. After that, it looks as if I have a plain short. I tried putting the old maxim chip back in - same problem. Maybe some mosfets that drive the charging chip?

[NxB]

Re: Laptop Motherboard Repair

We usually give up on those shorts. Its too much time to try to find them.

[xaviero1412]

Re: Laptop Motherboard Repair

i have that problem often.. fan spin, but no POST.
usually 1st is change with other processor. toshiba n ibm had this issue.

[Pyr0Beast]

Re: Laptop Motherboard Repair

Check 5V & 3.3V power (Standby 3.3/5 (linear), and Run 3.3/5 (Switching). On older laptops (P1/P2) there is 12V as well. If absent standby linear supply, check for short, check IC supply. Replace IC if it has supply but does not give anything out.
If standby present, check for Run 3.3V and 5V. Check/replace mosfets. If there is a short 'pulse' on 3.3V and 5V might be overcurrent protection or weak (worn out) mosfet. Replace fets, they are cheap. Some will not output 5V if 3.3V is faulty and vice-versa.
Check capacitors if they are shorted. Check other SMPS's. Clean all! connectors, especially around docking port connector. Check PCMCIA and similar connectors for bent/shorted pins. Usually there is also a texas instruments regulator for PCMCIA cards. It often goes to short, pulling down 3.3V/5V SMPS. Remove all extension cards, modems, BT adapters, wlan cards. They often go into short doing all sorts of weird things.
If 3.3V/5V does not come up, remove everything incl. RAM&CPU, check if it does. If not VRM problem.
If no led lights up -> missing standby (presuming one is for adaptor power like on HP laptops). Might be broken button, DC jack.
If leds light, 3.3V present, 5V present. Check cpu voltage, should be around 1.0-1.5V. Check memory voltage (1.8V DDR2, 2.5V DDR1, 3.3V SDR etc.).

Often found IC's are MAX1999(Compaq, ACER) and TPS51120(HP). They DO fail. However, not very often. MAX is easy to replace (but expensive as hell), TPS is pain in the ass to remove from the mainboard, not so hard to solder back, quite cheap.

[Pyr0Beast]

Re: Laptop Motherboard Repair

Whoop. 10 Minute Edit time limit expired ... mod should sort this out.

//
Check 5V & 3.3V power (Standby 3.3/5 (linear), and Run 3.3/5 (Switching)). On older laptops (P1/P2) there is 12V as well. If absent standby linear supply, check for short, check IC supply. Replace IC if it has supply but does not give anything out.
If standby present, check for Run 3.3V and 5V. Check/replace mosfets. If there is a short 'pulse' on 3.3V and 5V might be overcurrent protection or weak (worn out) mosfet. Replace fets, they are cheap. Some will not output 5V if 3.3V is faulty and vice-versa.
Check capacitors if they are shorted. Check other SMPS's. Clean all! connectors, especially around docking port connector. Check PCMCIA and similar connectors for bent/shorted pins. Usually there is also a texas instruments regulator for PCMCIA cards. It often goes to short, pulling down 3.3V/5V SMPS. Remove all extension cards, modems, BT adapters, wlan cards. They often go into short doing all sorts of weird things.
If 3.3V/5V does not come up, remove everything incl. RAM&CPU, check if it does. If not -> VRM problem.
If no led lights up -> missing standby (presuming one is for adaptor power like on HP laptops). Might be broken button, DC jack.
If leds light, 3.3V present, 5V present. Check cpu voltage, should be around 1.0-1.5V. Check memory voltage (1.8V DDR2, 2.5V DDR1, 3.3V SDR etc.).

Often found IC's are MAX1999(Compaq, ACER) and TPS51120(HP). They DO fail. However, not very often. MAX is easy to replace (but expensive as hell), TPS (QFN package) is pain in the ass to remove from the mainboard, not so hard to solder back, quite cheap.
Mosfets, pick the same package, usually they are 30V (drain-source), 20V (gate-source), you may go with lower Rds(on) with no problem. Sometimes it is good to be careful with gate capacitance, especially with weak drivers. Usually, not a problem.
Search for datasheet. If lower-switching mosfet has free-wheeling diode, replace with the same type. Or replace with normal type (No FW diode) but add external diode. It may be omitted, but I do not recommend.

(Ok, this looks a bit messy

Sometimes fets on input burn up (no adapter voltage (approx. 15V-21V). P-channel usually (used like diode so no power is fed back to the adapter on battery). Switching fets are most likely N-channel.

Things are quite simple actually.

[precision001]

Re: Laptop Motherboard Repair

I am working on a motherboard, with two ic mosfets which got very hot. I removed them, but I could not find replacements. They were P-channel mosfets with the same pinout as a 4407, so I tried putting two 4407s in place, and they just hissed at me and got hot. (Note: beyond checking the pinout (s-s-s-g/ d-d-d-d) and verifying that they were both P-channel, I had not used any other technical information in considering substitute mosfets. Not sure if this was a bad idea)
I had once observed a motherboard which had two mosfets "jumped" - from source to drain. There was a tantalum cap separating the two. Presumably, those mosfets where bad. It seemed like a good idea. I removed the 4407s and connected where source would be to where drain would be - with no success.
Is jumping mosfets like this a reliable way of working around a failed pair of hard to find mosfets? If so, why have my attempts been unsuccessful?

... More information:
The original mosfets were "tpc8107" - manufactured by toshiba. They are P-Channel Mosfets. The board is from an Asus notebook, G1S.
With my DMM, when connecting source to drain caused no short with 19V detected. During on experiment, I connect the source to drain, the other way around - and no voltage was detected. However, when I connected S-D both ways around at the same time, a short was created. I probably shouldn't have tried this, but am curious about the behavior of this circuit and want to gain some more insight.

[precision001]

Re: Laptop Motherboard Repair

Also, getting back to shorted motherboards for a second, I have repaired many of these successfully using these steps. For HP/Dell laptops with plain shorts(that short the adapter when plugged in) - I have observed in most cases that the failure was NOT caused by bad mosfets, but by bad capacitors. I have found that adding a light bulb or a fan in series is a good way to observe the path of electricity to get a general idea of where the short is. I prefer a fan because I can hear it spin while keeping my eyes on what I am doing. In some cases, increasing the voltage while using a load of some kind to prevent a complete short has caused more damage to the bad capacitor. The capacitor starts to look like it is cracking. I then remove the bad cap, test it with a DMM - it's usually shorted. Replace then I'm ok. I am sure that the problem that I am experiencing now is mosfet-related, and my experience with this kind of troubleshooting is somewhat limited. I have trouble finding suitable substitutions, but I have stacks of dead laptop motherboards to pull from.

[Pyr0Beast]

Re: Laptop Motherboard Repair

Emm .. shorthing things is not the solution unless you really know what are you doing.

Some mainboards use P-channel (or N) mosfets to turn power on and off to devices, like bluetooth adapter for instance. (This can sometimes be bridged)

Hot mosfets would indicate overload or weak driver. Perhaps even mosfets are bad.

If those two mosfets are for the adapter input (close to the DC jack), they might be used as a replacement for a diode to prevent backfeed from battery into the adapter. (this may too sometimes be bridged)

In other cases I would recommend not to do that as you will probably fry any logic that is left there.

I would look for a shorted mosfet(s) further 'downstream' on one of the switching converters.

[precision001]

Re: Laptop Motherboard Repair

So just because the two mosfets closest to the DC jack were hot doesn't necessarily mean they were bad? How would I check the mosfets downstream to determine if they are bad or not? Maybe I should start at square one by putting those mosfets back? But I don't understand - how was that other circuit working that I saw with the two mosfets jumped source to drain? They were closest to the DC jack too, and in that case, it worked.

[Pyr0Beast]

Re: Laptop Motherboard Repair

Sometimes mosfets go open and conduct thru their body diode -> means plenty of heat.

That case it worked because nothing was burned further down (or was repaired).

Check with multimeter in 'diode' mode. -> look for inductors. If on any multimeter reads close to 0 -> upper and lower mosfets are burned.

[precision001]

Re: Laptop Motherboard Repair

So I am checking inductors in diode mode? But aren't inductors just coils meant to oppose the change of current? That would mean that they have little resistance, as marked on top such as r45 (I think .45 ohms ??). I thought that I was supposed to get a low reading on an inductor. So, how would checking the continuity of a coil/inductor tell me if a mosfet is bad? I am probably misunderstanding what you mean.

[stj]

Re: Laptop Motherboard Repair

if you find a shorted rail - in anything,
a low-ohms meter will find the exact component real fast without lifting stuff.

years ago everybody built there own low-ohms devices - often using a speaker that changed tone.
very handy kit.

[Pyr0Beast]

Re: Laptop Motherboard Repair

No, you are not checking the inductors.

Inductors are needed for switching mode converter. You need upper and lower mosfet (or diode) to do the switching meaning, where is inductor, there must be a switcher behind or infront of it.

Current thru the mosfets goes thru the inductor and into output capacitors. It would be the same to check output capacitors, to determine a short, but finding inductors is much easier
Measure between inductor and ground.

[precision001]

Re: Laptop Motherboard Repair

if you find a shorted rail - in anything,
a low-ohms meter will find the exact component real fast without lifting stuff.

years ago everybody built there own low-ohms devices - often using a speaker that changed tone.
very handy kit.

I bought one. I bought a leakseeker 82b. I can't find anything with it yet, I need more practice.

[precision001]

Re: Laptop Motherboard Repair

No, you are not checking the inductors.

Inductors are needed for switching mode converter. You need upper and lower mosfet (or diode) to do the switching meaning, where is inductor, there must be a switcher behind or infront of it.

Current thru the mosfets goes thru the inductor and into output capacitors. It would be the same to check output capacitors, to determine a short, but finding inductors is much easier
Measure between inductor and ground.

I've started measuring between inductors and ground. It always shows some kind of reading - which I guess it a good thing. This looks like a good standard check to do on any motherboard. As far as that Asus motherboard, I think substituting the 4407s may have caused more damage. I'm more interesting in gaining a better understanding of the cause of the failure than actually fixing that unit. I was working with a DV6000 motherboard yesterday that had 19v shorted - most caps showed shorted. I lifted about 20 caps before I found the bad one, but even after that it only powered on for a second then shut back off. I tried reflowing what I believe was a BGA mounted I/O chip, but after doing so the plain short strangely re-appeared - suggesting that 19V lines can be brought down even by bad bga chipsets. Motherboard repair is very interesting and I want to learn as much as possible and get better at it. What could I have done differently with that Asus board, and the DV6000, that may have changed the outcome of the repair attempts?

[Pyr0Beast]

Re: Laptop Motherboard Repair

Yes, any type of reading is a good thing. Except dead short. You do have to know that certain components do have their resistance in the mOhm range, may that not mislead you. I probe with a standard multimeter (unimeter?) in the diode range. If it shows from 000-003 it is defenitely a short. Anything more, like 020 is acceptable. It proves really useful to trace circuits, like which caps are connected together and to which line.

I have not had any mainboards in repair which had main rail shorted to ground. However I bought a few (7) dead ones (cheap-3€ a piece), fixed one of them to replace the other one in the laptop, both had water damage. But on one the bga chip was a toast (heating way more than it was supposed to. Desktops are quite similar with that, SB usually dies), the other one (I bought) had working internal logic so it was good for repair. Replaced the mosfets on 3.3V line and replaced TPS51120 chip. Had to rebuild some parts of the regulating circuit since certain pads were etched away. It took me a while to figure that out since copper was the same colour as PCB and I was removing what was left behind water with acid, so that automatically means, one swipe and copper oxides are gone as well. Hydrochloric acid is good because it doesn't eat copper (noticably), only its oxides, so it is quite good to clean it. There was no 19V on the chip due to eaten via - missing, so no way it could work.

Other mainboards are 'beyond repair', so mostly used just for parts. I cut apart one and made myself a nifty DC-DC converter 12V (or more) to 5V and 3.3V out of it. Some were hit by lightning which is quite obvious, burned pads and traces around LAN socket.

You will gain knowledge fast. I was repairing PC mainboards before I switched to repairing laptops. That knowledge is quite universal (interchangeable), both have similar faults. But PC's are more 'obvious'. No voltage to certain parts or just burned.
The first thing I do is to check voltage(s) (with no components like memory, graphic card on the board (however, no CPU will mean no voltage on that output). I have just repaired one gigabyte mainboard - DDR ram fet was open. It was brand new (meaning it never worked as it should from a start) and it was sitting in my 'to be repaired one day' bin for a long time. I noticed that just by accident, as RAM led was not giving any light. I have stumbled across a similar fault before, but the board was beeping (a good sign to see it is alive (internally), this one didn't do anything. It even didn't beep with no memory like it should (RAM error since no voltage for it to work), tho the speaker worked since it did made the POST complete short beep.

BGA's can be tricky sometimes since you can't see underneath. I have seen a great deal of damage caused by 'dust' in the connector, especially metal shavings (can also be from metallic layer over casing's plastic to give it greater strength). I always give the docking port connector a good sweep across the contacts and where it is soldered. Pins are just so incredibly close.
And you have all sorts of voltages there. From 3.3 to 19V. Something will definitely go wrong.

(this is getting a bit long now

It is difficult for you because that is a completely different world. It took me a long time (years) just to see what the obvious fault was, like burned mosfets and similar. However, I did not have had _any_ help. Everything was self-learned.

If you don't see a problem instantly, don't rush and don't worry about it. Seep on it and you'll remember a different approach the next morning.
I will draw a simple schematic tomorrow (today?) that will clear a few things up.

[Wizard]

Re: Laptop Motherboard Repair

I had seen a notebook crater the N/P combo MOSFET in one IC (8 pin type) caused by bad switching regulator controller IC. Need to trace from their gate pins of this hot MOSFETs to the said IC and replace this also.

Cheers, Wizard

[Pyr0Beast]

Re: Laptop Motherboard Repair

Yes. Sometimes driver output of the IC fails. It is rare but it happens.
It may even not completely fail, just so that it slows down the charge of gate, causing excessive heating.

[precision001]

Re: Laptop Motherboard Repair

I am currently experimenting with a simple logic probe. I think that with a logic probe, I may be able to check certain parts of a motherboard while it is powered on to check for faults. The problem is I am not yet sure how to interpret what I am finding. I am hoping that eventually the logic probe will help me identify bad mosfets, bad low-signal transistors and other bad chipsets.
Some technicians in India have found it easier to remove chipsets that are non-essential to booting/smps, such as card controllers and audio chipsets. Apparently if one of these is shorted (and from what I've heard - they do go bad), it can bring down 3.3 or some other critical rail, crippling the board and preventing it from powering on.
I also use a mini-pci express POST card with a M-M usb cable. With this, I am specifically looking for the motherboard come out of reset and see if is giving out any codes. If it doesn't, the first thing I check is the reference crystal or the clock generator itself. My reasoning behind this is that motherboards in which I have removed the 14.318 have been unable to come out of reset. I have not yet found a board that actually had a bad crystal, but have removed different crystals from working boards to observe the effect on the board, so that I would recognize the signs of a bad crystal. With 14.318 out, boards would power on, but display no video. Removing a crystal near the southbridge often prevented a board from powering on at all.
I have seen many HP motherboards (dv6000 and DV9000 series) that power on for one second then off - never fully powering on. I have not yet successfully repaired any in this condition, because I haven't been able to determine the cause of the failure. I can't get post code reading, I can't use a logic probe, but I do have my new leakseeker. Perhaps this new device will offer some insight into the cause of these failures.

[precision001]

Re: Laptop Motherboard Repair

As far as BGA chipsets, I successfully reflow most. It's the ones that have been tampered with that I usually can't bring back. I have seen some terrible things done to BGAs. I have pulled out pennies, all kinds of metal and found burnt and blistered BGAs from home users with access to a heat gun and Youtube. I prefer to use soft, pliable metal (like aluminum) that can conform to the die cast without putting pressure on it. This seems to dissipate heat better - but I make sure to protect any caps with polyimide or kapton.
Completely removing a BGA seems so hard for me - I don't know why. I have not yet successfully removed a BGA chip, cleaned up the pad, and put the chip back on. Every attempt at doing so has failed. Either I can't get the chip to line up right, or something goes wrong. I am considering buying stencils for the most popular chipsets and reballing some of them - over and over again until I get good at it. They sell "dummy" kits online for training purposes but I would rather get a feel for real chipsets, and there's no shortage of bad BGAs out there.