Laptop Motherboard Repair

[Pesti]

Re: Laptop Motherboard Repair

Well here's a good one on the HP dv series with ffc power cables, replace them, even if they look good, The originals are crap and I personally had two DV6000 that I went thru the ringer with trying to diagnose them until I read an obscure thread on a music forum of all places, One guy said he had a friend that worked for an HP supplier, and that there was a lot of "Rukus" about a ****load of crappy ffc cables......anyway on a whim I took down one of the offending laptops and switched around the two ffc cables from the power board, since they were both the same pin count and size, and VOILA!!!~
What a bunch of crap, believe me I thought I checked the power circuit pretty thoroughly and I really thought I would have caught this...That laptop has sat on that shelf for almost a year........

[Pesti]

Re: Laptop Motherboard Repair

Found this little Item on testing MOSFETS If anyone's still interested

http://www.electronicrepairguide.com...t-testing.html :>)

[Flounders98]

Re: Laptop Motherboard Repair

I have a DV6500 with no power on from battery or a/c. I do have blue ring around a/c adapter. The mosfet on the top of the board near the battery connector marked pq30 is a 4856a. I measured this ic in diode mode and got 450 and 0. I have 18.5v on this ic but not on the bottom one near the power in connector. That ic is marked pq3 6679gm it reads 430 and 93. I think i will replace both, but i cant find a replacement for the 4856a. Does anyone have any suggestions on a possible replacement for the 4856a?

[chevelle_nut]

Re: Laptop Motherboard Repair

I have a DV6500 with no power on from battery or a/c. I do have blue ring around a/c adapter. The mosfet on the top of the board near the battery connector marked pq30 is a 4856a. I measured this ic in diode mode and got 450 and 0. I have 18.5v on this ic but not on the bottom one near the power in connector. That ic is marked pq3 6679gm it reads 430 and 93. I think i will replace both, but i cant find a replacement for the 4856a. Does anyone have any suggestions on a possible replacement for the 4856a?

Can you post a picture of that area of the board? My schematics aren't matching up. I just went through this and found a bad zener as well as the 2 mosfets were needed.

[Flounders98]

Re: Laptop Motherboard Repair

chevelle_nut

I wont have access to my digital camera until Monday, but this board is totally different from the dv6000 and 9000 i have seen before. Can u elaborate more on your repair? Do you have a schematic available? Can you recommend a replacement for the 4856a?

Thanks

[mandacat]

Re: Laptop Motherboard Repair

Seems like a great subject / thread - anyone plan on continuing it ??

[ozer44]

Re: Laptop Motherboard Repair

Hi, i have a hp compaq nx 7400 but my laptop not charging and not power on.. i have a another problem TPS51120 very hot... What can i do?

thx

[Pyr0Beast]

Re: Laptop Motherboard Repair

Replace mosfets on 3.3V and 5V line and replace TPS51120

[Agent24]

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.

Do you have any recommendations for a design of one of these?

[severach]

Re: Laptop Motherboard Repair

Leak Seeker

I have one but haven't figured out how to use it. I have much better luck applying bench power to a shorted circuit and see which component heats up.

[Setarkos]

Re: Laptop Motherboard Repair

Hi im thinking about buying leekseaker for laptop pcb board

but im afraid its gonna do wild with all the coils.

Could anyone who got that device could tell me is it a good buy for laptop pcb diagnostics.

Becouise im afraid it will be a waste of money.

[Pyr0Beast]

Re: Laptop Motherboard Repair

Could be useful. Not sure where it could beat a standard multimeter.

[severach]

Re: Laptop Motherboard Repair

A common problem with laptop boards is a shorted ceramic or tantalum capacitor shorting out a single supply or the entire system supply. If 20 ceramic capacitors are in parallel and one of them is shorted, all 20 will show as shorted. A DVM will show you the list of 20, which is important to know, but it can't pinpoint the bad one. The Leak Seeker and the bench supply can. I just haven't spent the time with the Leak Seeker instructions to figure how to use because the bench supply finds them so easily and so fast with no downside. Even if a major chip is shorted the bench supply finds it easily, just as the Leak Seeker would.

The Leak Seeker measures the resistance as you place the probe along a shorted path. To a regular meter the shorted path has zero resistance. Like an ESR meter, the Leak Seeker is equipped to measure very small resistances so the path resistance is measurable. The Leak Seeker knows the resistance of the entire path and it knows the resistance from where the roving probe is placed. The probe point completes a voltage divider circuit. The closer you get to the shorted the component, lower the resistance of the lower divider and the more noise LS makes. Measuring resistance down near 0.001 ohms is not trivial and requires some setup. I haven't been forced into learning how.

[Agent24]

Re: Laptop Motherboard Repair

Isn't your "smoke out the shorted part" approach a little dangerous - aren't you at risk of burning PCB tracks this way?

[Pyr0Beast]

Re: Laptop Motherboard Repair

Eh. I just hook up a 1.2V nicd battery there and see where it smokes.

[severach]

Re: Laptop Motherboard Repair

Isn't your "smoke out the shorted part" approach a little dangerous - aren't you at risk of burning PCB tracks this way?

The point of the bench power supply is to add just enough power to get the part to heat up. If a trace were to heat up I'd know that the shorted part is far away. So far all the caps have been connected with really big traces--probably power planes that I couldn't possibly heat up.

[infringer]

Re: Laptop Motherboard Repair

So its safe to say some of the common tools we will use are:

Desoldering Braid / Desoldering Iron
Soldering Iron / Station
Hot Air Rework Station/ Ceramic Heater / Toaster Oven / Heatgun last resort
ESR meter
Digital Multimeter
O-Scope if you can afford one

I don't mind using redneck tools if need be I've done repairs with a heatgun and have been successful.


So a nice list of tools required for laptop repair would be of the order and here is a good start.

Benchtop adjustable power supply 3.3v 5v 12v should be the common voltages used.

[mattbrad2]

Re: Laptop Motherboard Repair

severach, can you walk me through your process of using a bench supply to test these shorts? Say you have a ceramic cap shorted on the opposite end of the dc jack, close to the charging circuit. Where are you applying voltages and what voltages are you applying? On a 19v (or 10-14v) rail, what voltage would you start at and how are you measuring "heat"?

[flex1]

Re: Laptop Motherboard Repair

Hi,

Can anyone give a brief explination of how to use a bench supply to "smoke" out the shorted componant ? i know its probably not good practice but i have a couple of boards that i have extensively tested with my meter but have been unable to find the shorted part, i figure i have nothing left to loose if i damage the boards even more so might as well give it a go, do i connect the bench supply to the DC Jack input ? and what kind of voltages / amps would i be looking to use on a board that requires the standard 19v

Thanks Ste

[Agent24]

Re: Laptop Motherboard Repair

Having never done this myself, I may be wrong!! Wait for someone else to confirm!

But, I would think, don't apply to the DC input jack unless your short is across it. Otherwise you may blow all sorts of things that aren't shorted with too high voltage or such.

Probably what you want to do is find the part\branch of the circuit where you have found the short, verify there is in fact not supposed to be a short, and apply power there, and see what happens.

I expect you'll need a supply with current limiting. Start off with a low current and work up, keep looking for signs of overheating components.

If you go too high you could burn out a board trace, then you'd really be screwed.


And remember, components like inductors can look like shorts to a DC ohmmeter, when they're not really!