Dm800se not starting

Discussion in 'Hardware troubles and Repair support.' started by iridium, Jul 8, 2021.

  1. iridium

    iridium New Member

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    Hi:
    I am trying to repair a DM800se that doesn't power up (no power - blank display).

    1. I've started following the 12V diagram and I see that both Q1100 and Q1101 receives 12V but no 5V in any of the pins
    2. I've switched off and disassembled PCB and started to check resistance

    Pin5 (5V) has 1.2 ohms
    Pin4 (3.3V) has 91 ohms
    Pin3 (2.5V) has 3.35k ohms
    Pin2 (1.8V) has 29.8 ohms
    Pin1 (1.2V) has 409 ohms

    3. As far as I've read, it "seems" that there could be a problem with the 5V supply so I started removing "jumpers".
    3.1 I've removed R1131.........same resistance values as mentioned before. So I've soldered "jumper" again.
    3.2 I've removed R1112........here pin5 gives me 1.18k ohms (pin4 still 91.8 ohms).

    What should I do next? What this could mean?

    Thank you so much for your guidance in advance.

    Br,
    Francesc.
     
    Last edited: Jul 9, 2021
  2. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    By removing the R1112 zero ohms resistor you separate the 5v source from the other circuits.
    More about this see this U1101-1102 schematic.
    Suppose that you ohm value rises to 1.18k on one side of the R1112 when removed, but the other side has 91.8 ohms, than is the issue most likely on the 5v Dc to Dc (U1100) side.
    More about the 5v source, see this U1100 schematic.

    In order to exclude more sectors which are needed this 5v to work, you can consider to remove the R1125 inductor and to check which side the ohm value says low.
    When no better results, do the same with the removal of the zero ohm resistor R1150 to see if it's still is a issue from the 5v side, or other R1150 side.

    When you have the issue on the 5v delivery, you could consider to try to repair it, or to use a dc to dc converter as you can see on this topic, when scrolling down.
     
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  3. iridium

    iridium New Member

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    Thank you so much for your quick response. Highly appreciated.

    All ohms values I was talking about are measures from P1100. Never took values on the removed R1131 & R1112. I was stating that resistance in P5 from P1100 increased to 1.18K by removing R1112 :)

    So......now:
    I removed R1125: Left side from inductor(the one closer to the edge): 1906 ohms.........right side (closer to U1100): 1.1 ohms

    What would be your recommendation here?

    BR,
    Francesc.
     
  4. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Than it's most likely a issue from the U1100 LTC3850, to be sure it is, you could consider to remove the R1150, just to exclude that it's not be caused due the U1103/Q1111.

    When not, you probably need to replace the U1100 for a new one, but since there are other surrounding parts connected to it you need to check them when the U1100 has been removed.

    Other option is to replace this 5v delivery circuit with a Dc to Dc converter as shown here.
    Or add a own 5vDc power source to this circuit to see if the box will work.
    But again, you still need to remove the parts before you want to do it.
    And, be sure that the R1125 has been place back before feeding the motherboard , otherwise you may damage the other Dc step down circuits.
     
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  5. iridium

    iridium New Member

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    Okey. So let's go for a Bingo, then.

    Keeping R1125 removed, I also removed R1150.
    Resistance is the same in R1125 points (1906 on the left, 1.2 on the right).

    So........let's remove U1100 and take some measures? Or do you recommend some other checks before it?

    Best regards,
    Francesc.
     
  6. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    Yes, based on the info, the next step is the removal of the U1100 and check the surrounding parts before replacing a new one, or to do the Dc step down mod.

    These U1100 voltage chip is used also for a internal hdd, and when using a older type hdd, one which uses more power consumption could damage chis chip.
    But also when the usb consumption was to high it may damage it.
     
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  7. iridium

    iridium New Member

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    Checking the surrounding parts next to U1100 (resistors, capacitors and inductors seem ok)
    U1100 removed.

    However I still have 1.2 ohms on the right part from R1125. Is it normal once U1100 has been removed?
     
  8. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    No, sorry but in that case could be the Q1103 or D1103 or Q1102 or D1101 be faulty.
    Because these are still on that circuit, and has a ground pin.
    But if one of these are faulty, then the U1100 will most likely also be damaged.
     
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  9. iridium

    iridium New Member

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    Good morning Johnny:

    I've removed the mentioned 4 (Q1103,D1103,Q1102,D1101) but still 1.2 ohms on the right side R1125).
    So getting crazy I've also removed Q1100 &Q1101.........same shitty result.

    Not sure if I would need to stop removing :)
     
  10. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    When all other sections are separated from the 5v source, as the removal of the R1125, R150, R1131, R1112.
    As also removed the U1100 etc, and there are no other things connected to the motherboard as the front panel, tuner, and you still have the wrong ohm value on the 5v source side, than the only thing I could think of is a defect with the C1105 or R1112/C1106 ceramic cap, or else it could be a (internal) print track issue.

    -
     
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  11. iridium

    iridium New Member

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    Thanks........after some days trying to figure it out, I think I will go for plan b........external 5V supply.

    Thanks Johnny for your time and patience.
    BR,
    Francesc.
     
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  12. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

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    No problem, always willing to help.
    And keep in mind that this 5v circuit is close to 1 amp consumption, so when using an external 5v supply it is advisable to use at least a 2Amp Dc/Dc converter.