A Fool's Folly to repair a cellphone: Diode critical?

Discussion in 'General Electronics Chat' started by BobCov, Dec 6, 2014.

  1. BobCov

    Thread Starter New Member

    Dec 6, 2014
    6
    0
    Hi! First the meat of the matter, then the backstory. What is your best guess as to the criticality of a schotky (spellling?) diode in a cellphone with a dead screen? There's no way to replace it, so i can bridge the contacts where it was or just hope that cleaning all the corrosion in the area fixed something and put it all back together.

    So, a friend gives me a Galaxy S5 to have a look at. At a party, somebody spilled liquid into the headset jack and within 6 hours, the screen died. Everything else works.

    First problem, is the Galaxy s5 is waterproof to 1 meter for 60 minutes. So, I discovered this was an S5 clone.
    After finally getting it apart, I found evidence of corrosion in a small area of the board and a very tiny component marked "SL" just fell off from that area.

    While I've got this torn apart, I'm wondering should I bridge the contacts? Anybody have a guess as to what role this diode may have had in switching the screen on and off?

    Yes, I know this is really a stretch, but I've been given permission to trash it, so it's okay if I make the wrong guess. I just thought it would be fun to get some educated guesses first.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,773
    1,103
    Welcome to the forum!
    Just bridging the contacts is unlikely to help and may in fact cause more damage. The diode was there for a purpose. Manufacturers are not known for their generosity in supplying products with superfluous components.
     
    KJ6EAD likes this.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,991
    3,736
    If it is the first component leaving the battery, it is there to prevent any damage if a battery or charger is connected in reverse.
     
  4. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Everything works but the screen.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    My iPod suffered water contact (kayaking incident!) and now the screen shows nothing, although I can make the backlight come on. It is otherwise fully functional and I can even take screen shots of the screen I can't see.

    In my research, I learned that the problem could be just the screen, or it could be the board. Repair of either is more costly than the thing is worth, so I'm kind of stuck. No one mentions a potential easy fix by replacing a single component like a diode.

    How do you know the thing that corroded was a diode? Maybe take a good picture for more eyeballs here.
     
  6. Externet

    AAC Fanatic!

    Nov 29, 2005
    758
    57
    I suggest not to bridge nor remove parts.
    Instead, remove the battery and wash the circuits with abundant high grade alcohol and a tooth brush, attempt to clean any corrosion that way. Leave the phone open in front of a fan (or warm fan - no ovens!) for at least 24 hours and then place the battery and retry with fingers crossed.
     
    absf likes this.
  7. BobCov

    Thread Starter New Member

    Dec 6, 2014
    6
    0
    Hi,
    I'm not positive my identification procedure holds water (pun intended). I looked up surface mount device codes to try to find something like the bad chip, which was marked "SL." It said that SL is a diode. It's too small to take a picture of.
    As for it possibly being related to the battery, it's a good distance away from the battery and the USB port. But it is very close to the connector that runs the screen. It is also 1 mm away from a 6 lead chip that could have something to do with power, based on the thickness of the leads. I'll try to take a picture, but the parts are so small, I have to get the lens so close that there's little room for illumination.

    Looking for my card reader to post a picture.
     
  8. BobCov

    Thread Starter New Member

    Dec 6, 2014
    6
    0
    It's way too late for that. The user did NOT take the battery out right away and I didn't find out about this until recently. The incident was in August. And the part was not taken out, it FELL off when I removed the board. I'm guessing heavy electrolysis ate away at the part or high current fried it, but I don't see any burn marks. However, the part is very very small, and black, so I wouldn't be able to detect burn marks anyway.
     
    Last edited: Dec 6, 2014
  9. BobCov

    Thread Starter New Member

    Dec 6, 2014
    6
    0
    Hi,
    I appreciate everybody's help. I accept it's unlikely the screen will ever work again, but it's kinda fun to try. It boots, and the touch screen digitizer is active over the same connector. Sound works on boot. Here are some pictures. The damaged component was between the two red x's. The damaged component is identical to the one in the other picture in which, if you have good eyes, you can see "SL" written on it. Sorry for the quality.
    I removed as much debris as I could from the area in question and then dabbed it with alcohol to remove corrosion.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    Well at this point I guess you have little to lose. Got any scrap electronics laying around? You can probably find a schottky diode to scavenge and you could use this to attempt an experiment. The symbol is just like a diode, but with a tall "S" at the forward tip of the triangle, instead of the straight line for a regular diode. And actually, it's unlikely that the difference between a regular and a Schottky diode would make or break your experiment. I could be wrong.

    One more thing: diodes are polarity-sensitive and so the orientation matters. Is there any indication which way the diode was oriented? A band at one end of the device is typical, and a larger PCB will usually have a silkscreen to show you which way it goes.

    There is not much to lose trying it both ways, but it would be nice to see some verification that it might be a diode.
     
  11. BobCov

    Thread Starter New Member

    Dec 6, 2014
    6
    0
    Hi,
    I could probably salvage a diode out of something, but the board traces are so small that I would need a soldering tip as fine as a needle. Everything is so small I don't even know if I could bridge those contacts now if I wanted to. I put it all back together and the cleaning made no difference: Everything works, but the screen.
    The ultimate goal is to try to get into the phone and recover pictures for the owner. But since KitKat, Android requires an authorization from the phone interface to get into it over USB. So, since I can't see the screen, I can't authorize the USB access attempt.
    Maybe tomorrow, I'll try to rub a small bit of foil on the board with a toothpick to bridge the contacts and see if anything good comes of it.
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    Well that's different. I don't know Android but even for iOS there are utilities to get access to the file system. Forget the hardware and start searching for software.
     
    Last edited: Dec 6, 2014
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,991
    3,736
    There are a few companies offering cell phone repair. BatteriesPlus has multiple stores in most major cities and their tv commercials offer cell phone and tablet repair.
     
  14. BobCov

    Thread Starter New Member

    Dec 6, 2014
    6
    0
    I'm back for more punishment. I cut out a hole in the back of the case so that I can easily manipulate the contact points while the phone is assembled. I noticed that the voltage across the two points where the diode used to be goes from a few millivolts to 4 volts when I press the home button on the screen, and then, over the course of about a minute the voltage slowly goes down back to the millivolt range. I have some needle tip probes for my volt meter, so I'll go buy a diode tomorrow and see what happens if I put it into the circuit by attaching it to the ends of the lead set. Any clues what that 4 volts and slow drop to millivolts might indicate about function? Normally the home button wakes up the screen. If I keep tapping on the softkeys at the bottom of the screen, the voltage stays at 4.11. Maybe the screen has a 10 second timeout for inactivity and goes off and then the voltage starts to fall?
     
  15. flat5

    Active Member

    Nov 13, 2008
    403
    17
    I think you're on to something.
    Keep posting, please.
     
Loading...