electronic wizkid needed!

[peteec]

hi all,great forum,looking for some advice here:
i have a old laptop that now fails to charge,the machine & windows says its charging when plugged in (charging led flashes on laptop and windows reports battery is charging)but no voltage actually gets suppiled to the battery to charge it,anyway... as i was checking some points on the mainboard i touched a resistor with the positive lead of my multimeter (negitive lead was earthed to chassis)and as i touched a resistor it sprung into life and started suppiling power to charge the batery,does this mean i have some sort of earthing problem one side of the board? does anyone have any ideas what might be going on! many thanks in advance for an help offered!

 

[Theatronics]

Laptop PCBs are subject to a lot of twisting and bending.
All the talk about 'Titanium cases' isn't to make the case harder to crush or protect it from dropping.  The goal is to make the case as stiff as possible so the motherboard doesn't twist.

Sony had a big problem with the PCB mounted memory because the sockets were so long and the board flexed enough to fatigue the solder joints.  A retouch with a soldering Iron fixed most of the problems.

You might also have a busted trace. A good pair of magnifying 'jewelers glasses' would be real useful in trying to spot the problem. 

A broken track will usually rip right at the edge of the solder mask.  Look for a crack at the edge of the solder.  If you see it you will need a steady hand and a needle OR a pointed exacto knife.  Scrape off a little bit of the solder mask over the broken trace and then reflow the solder so it covers the exposed copper.  This will make a stronger joint that should last.  Just soldering to the edge of the ripped trace isn't enough.

-Mike

 

[MP1]

How much charge did it provide? Full battery for a reasonable lifespan? Sometimes AC pulses will give you what looks like a revived charge circuit. You can sometimes bring a dead battery back to life by pulsing the charge voltage to it. If the battery only took a little more charge, it is possible that this is what you saw. If the battery now boasts a full charge and the charging system is no longer failing, then I would agree with the comments above about a cold joint or cracked solder pad. In this case, you should get out the magnifier and see what needs reflow.

MP

 

[peteec]

hi guys,thanks for your useful pointers,
i also had the battery pack apart to start with thinking this was the problem,but i then found would accept charge from a different power supply (got 12V supply on my bench trickled charged it a bit,windows then reported 4% charged instead of nothing)which then lead me on to a charging fault,i then had a look round board for bad/dry joints but couldn't see anything.... anyways.... i desolidered the actual battery pack away from the pcb (the one for charging curcuit inside battery) and solidered in a led so i could see when the laptop actually started providing power to the battery to charge it,in the process of checking different solider points on the board (hoping to find the, what i thought bad component on the board) i touched a resistor and my led illuminated,i then resolidered the joint (thinking it was a dry joint),but its still the same,i guess i've gotta have another look round the board at what is supplying the resistor??
i also have a hot air blower (its a heat shrink gun)that (not ideal but )will melt solider enough to get a new decent joint,however i'm a little worried about the air blower melting the solider then blowing components about!!!!! if i decided to go sh*t or bust with this route would i need to flux the whole board too?

thanks again for your help

pete

 

[MP1]

Not really a good idea, but if you do this, you will need to clean the board with alcohol, flux all parts that you will heat, then clean all the flux off the board when finished and apply a conformal coating to the board to re-seal all of the solder connections so they do not corrode.

MP