12v shorted, and P1100 pin 2 also shorted

Discussion in 'Hardware troubles and Repair support.' started by mig28, Feb 10, 2017.

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  1. mig28

    mig28 New Member

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    Hi Johnny and Guys,

    Nice to meet you again after the 8a8f8.com's cataclism!!!

    I've here my 500Hd with some shorts, i've created last sunday, which i don't understand how it has happened.
    Last Sunday, i tried to connect the box on another Television of my house. To avoid removing the original power supply, i used another power supply (with the same voltage and pinout on the jack but lower in current capability, 1A) and connected a WIFI DONGLE on the USB, backside (a Ridax mount installed years ago).
    What happened after powering the box, was a light "pff" sound and a very light smoke rising from the aeration aperture of the fan.
    After that it smelled as burnt but i can't see which component it is.

    At this point i have the mainboard alone without any connections on any devices (even fan), and i verified that the 12+ and - are shorted and also that pin2 on the P1100 (1.8v) is also shorted to ground!!!
    To start i'll focus on the 12v short
    I've already removed from the PCB, C1114 and D1102 but the short is still there.
    To avoid removing all the components that might have a + to - 12v connection as it can be seen in your 12v Input Circuit Drawing. Could you tell me, in a case like mine, what kind of component firstly could create this short?

    Thanks for your help
     
    mig28, Feb 10, 2017
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  2. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    I did a 12v/1amp test with two working boards and the buzzing sounds are from the U1108 (5v and 3.3v) and the U1101 (2.6v output.
    The only part that become hot is the U1102 which may burn eventually and could damage the U1100.
    In that case it creates a shortcut to the 12v input.
    So based on my test it could be the U1100, and probably the U1102.
    More about this, see this p1100-pin3-information drawing.

    Nevertheless since you say that you have a issue with the 12v and 1.8v, but others seems to work it is with the exception of the 12v shortcut a different situation as my test seems to show.
    But I think, remove first the U1100 or the inductors L1101-02-03 to isolate it from the 12v circuit but still there are some doubt because you didn't say that there is a issue with the 2.6v.
    In any case is this circuit output voltage related for the delivery to the lt1580 input which deliver the 1.8v output.
     
    Johnny B., Feb 10, 2017
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  3. mig28

    mig28 New Member

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    Well at this point i can't tell you if i have the 2.6v or anther voltage because it is shorted on the 12v and i don't want to worse the things.
    tomorrow i'll remove de U1100 and the L1101, 02 and 03 and tell you if the short has been isolated.

    Thank you. Give you more infos tomorrow
     
    mig28, Feb 12, 2017
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  4. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Ah okay, and you don't have to remove the U1100 and also the inductors L1101-02-03
    Only the U1100 or the inductors L1101-02-03 to disable it from the 12v circuit.
    However, suppose you have removed the U1100, and you still have a shortcut to the 12v, then it could also be one, or more of the 3 ceramic caps C1102,3,4 which is often also the case with a dm800hd.
    And if you have removed the 3 inductors, and you still have the shortcut, then it could not be due this circuit.
    So, I think that it's better to remove only the L1101,2,3 to see if it solves the 12v issue.
    For the location etc, see the dm500hd 12v circuit drawing.
     
    Johnny B., Feb 12, 2017
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  5. mig28

    mig28 New Member

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    Update:
    After the removal of the coils L1101, 02 and 03, i still have a short between +12v and ground
    so it's not or C1102, 03 or 04 or U1100
    I didn't remove yet C1115 and C1117, both are joining the 12v and the ground!!! but all the big electrolitic capa have been removed and i have the same result.
    Tomorrow i'll also see the U1102.

    Keep u in touch
     
    mig28, Feb 13, 2017
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  6. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Forgot to say,
    Sometimes it may help if you take a good picture of the motherboard, both sides.
    Because you then can see the parts bigger on the screen it could help to find the burned part.
     
    Johnny B., Feb 15, 2017
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  7. mig28

    mig28 New Member

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    Hi,

    Here are my pictures. As you can see, we can't see any damage component.
    PS : The 2nd problema i've mentionned " PIN 2 Short" is not a short because i've compared with another board and i've the same value 27Ohms
     

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    mig28, Feb 21, 2017
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  8. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Yeah indeed 27 Ohm is normal for this circuit, so it's only the 12v circuit that is shorted.
    In that case it could be every part which is directly connected between the +/- 12v circuit.
    Mostly the part closest to the input which than could be the U1108.
    Not a easy part to remove due the ground pad underneath, and a bit doubtful because mostly when it has become defective it doesn't give a full short between the 12v and the ground, and become fast pretty hot.
    Another possibility is the C1112 ceramic cap or smaller cap C1178 because these are also directly connected between the 12v and the ground.
     
    Johnny B., Feb 21, 2017
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  9. mig28

    mig28 New Member

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    Hi,
    I've removed U1108, C1112 and C1178 and the short was persisiting.
    Then i went to the C1215 and finally the short was interrupted!!!! so my C1215 was completly KO

    At the end of the day i placed again all the components i've removed before including a new C1215 BUT unfortunatly the box still does not show any LED lights on.
    Tomorrow i'll see the voltages and if any component was poorly solded principaly the U1108!!!
     
    mig28, Feb 22, 2017
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  10. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Nice job, luckily you have found the defective cap.
    And yes, the U1108 delivered the 3.3v and 5v, and if this does not work, the board will give no sign of life.

    I assume that you do have the 12v after the inductor L1104, and that the ground L1117 inductor is also good.
    Otherwise nothing will works.
     
    Johnny B., Feb 22, 2017
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  11. mig28

    mig28 New Member

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    Aparently it's working again (i have to make a try with antena i haven't here)!!! But it wasn't easy!!!!
    Soon, I assumed that U1108 would be poorly soldered because of the squared pad underneath (to the ground).
    And because the Hot Air Gun is not my funniest game toy, i warmed it a little too long that made jump off (correction, they flied literal over the board) the 5R3 Inuctor and D1105 and also C1114 seemed to be burnt (and with bad value).
    So i changed C1114 and slowly i put the inductor and the Diode (it gave me only 0.2v in the conductive direction and i thought first that it was KO but no).
    Then when i turned on, various lights appeared and saw my box starting normally.
    Now i have to test with antena if it's 100% OK.

    Thanks for your Help! Cheers
     
    mig28, Feb 24, 2017
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  12. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Great job, well done and congrats with the result.
    And yes working with hot air is risky, sometimes bad for the motherboard and surrounding parts.
    I use always kapton tape to protect the surrounding parts, and in some cases super flux to remove the defective part quickly with less heat.

    But still, there is always the risk with hot air to damage the motherboard.
    Thanks for telling your repair journey, enjoy your victory. :)
     
    Johnny B., Feb 24, 2017
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