Troubleshooting laptop power circuit with a multimeter?

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sdmobiletech

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Hello all. Long time visitor, first time poster. Sorry if this has been covered elsewhere. I did find similar threads (i.e. http://www.tomshardware.com/forum/57027-35-power-jack-multimeter-test ) but they were either closed already or they didn't answer my question specifically...enough). Sorry, too, for such a long question.

Okay, so I'm working on a clients DELL Vostro 1520 laptop. The unit doesn't power on, doesn't charge, doesn't do anything really. Just sits there pretending to be a laptop. I tried all the usual...with/without power connected, with/without battery inserted, re-seated the RAM, removed the hard drive, etc., etc. Nothing. No charging lights, no sign of life at all. I even happened to have a charged battery from another (working) machine of the same model # and still got nothing. So I take it apart to test the power jack. It is (surprisingly) not loose at all so I wasn't surprised when it reads good power, before and after the jack. Some pictures below (and yes, I'm holding the probes 1-handed...I'm that good haha :)):

https://lh5.googleusercontent.com/--lLtCvAk0tQ/UVsc0V9_VLI/AAAAAAAAACo/SQDWjYLGH9U/s685/bottom.jpg

https://lh5.googleusercontent.com/-GudGAkNKuaM/UVsc0yOBDDI/AAAAAAAAACw/jER4Poc9uFU/s685/top.jpg

Next I checked for power at the battery terminals and (of course) get nothing...all ZEROS. So obviously power is getting lost/shorted somewhere between the 2 points, right? I now tell the customer it's time for a new motherboard but he says "NO WAY!". Can't say I blame him really...he's already bought a brand new DELL battery AND power adapter (at the advice of DELL tech support, of course) trying to fix this on his own, and now he's almost 2 hours deep in a $20/hour job that's also gonna require a $100-$120 replacement motherboard. Just to get his 3-year-old, $200 (maybe) laptop working again. I agree, its just not worth it economically at this point.

Anyways my question is this: How would I go about testing the power circuit of a motherboard to isolate exactly the location where power is lost/shorted so that I could repair, replace, or bypass that location, component, etc.? I've already googled the hell out of DELL Vostro motherboard schematics and LA-4596P (mobo model #) power circuit diagram and several other iterations and combinations of basically the same thing but no luck. All I could find is a diagram of the whole motherboard plus a lot of people talking about how messed up it is that DELL went to so much trouble to make 3rd party chargers/batteries NOT work (and, in some cases, break) in their newer laptops. At this point it's more of a hobby/challenge for me...I like trying to get more life out of something most people would replace. Plus, nothing would make me happier than cheating DELL out of a few hundred dollars of their "planned obsolescence" money. =)

Any help or advice would be greatly appreciated, thanks in advance.
 
Hi :)

SEE MY SIG...

Although I appreciate how awkward customers can be in this position...its OBVIOUSLY the main mobo, good diagnosis by the way,,,and hes going to have to pay for a new one or drop lappy in bin...HIS choice....

I would NEVER bother getting my guys to repair a failed board, for one VERY good reason... it WILL fail again and YOU will probably have to pay for a NEW mobo for the customer....

Its just not worth it...

All the best Brett :)
 

sdmobiletech

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Hi Brett, thanks for the quick reply. I agree with your advice 100 percent and that's pretty much what I told him. Actually, after putting the laptop back together and returning it to him, I told him that any fix of the current motherboard will only delay its inevitable failure and that I'd try to find the part used somewhere (ebay/craigslist). I've worked in IT for over 10 years and have NEVER really tried to repair a failed motherboard (someone else's at least =)) however, I have done some pretty "unconventional" things in the past and had some good results. Working for a company this is never an issue...replace the mobo (or more likely, replace the laptop) however I'm somewhat new to the mobile tech scene. Plus like I said, its more out of curiosity that I ask.

OH yeah, also I remember reading somewhere yesterday on my google-quest that DELL power adapters transmit some sort of special signal over the inner pin that tells the system its a DELL/OEM charger attached and not a 3rd party replacement which (you probably know) is the reason for so many DELL owners getting the "plugged in but not charging" or "unrecognized adapter" error messages and I remember reading that in some cases, the adapter will send one of the motherboard components a command to go into some sort of "not charging" state that actually gets stored in EEPROM...I'm paraphrasing so I'll have to go through my million-page browser history from yesterday to find the page but it was something along those lines. Anyhow...thanks again.
 
Hi :)

You are correct about the sensing (centre pin) On Dells...

Also there are various laptops of differing makes that can get problems , that the only cure is to reprogram by Eprom...

We have that capability , but very RARELY use it as in most cases, it SHOULD be new motherboard time anyway..for reasons I stated above...plus of course I like to make money, so its usually a financial decision (Porsche petrol etc lol)

All the best Brett :)
 

sdmobiletech

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Wow, that's crazy. Seems almost criminal actually. For a company like DELL to intentionally make their laptops stop working just from using 3rd party power adapters....that would be like if Apple came out with a new iPhone but they changed the power plug design for no reason, making existing iPhone accessories obsolete.....oh wait. =)

On a side note, I heard that newer DELL laptops also store BIOS password data in EEPROM (not just system/admin password). Do you know if this is true? Yeah, I've done it before too but it's a pain in the ass, especially IBM EEPROM's. =_)
 


Hi :)

That last bit, yes its true, only answer it to blank/reprogram the chip....

All the best Brett :)
 
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