Where to start repairing DM800HD

Discussion in 'DM800HD Hardware troubles and Repair support.' started by Arthur, Dec 29, 2019.

  1. Arthur

    Arthur New Member

    Messages:
    6
    Likes Received:
    0
    Trophy Points:
    1
    Gender:
    Male
    Love this little box but it suddenly stopped working. Seems completely dead. Read through this forum for help, started measuring voltages at P5 as per some advice read in the threads. All voltages are out of where they should be:
    p1 should be 1.3V I measure 0.02
    p2 should be 2.6 v I measure 1.06
    p3 should be 3.3 V I read 0.6v
    p4 should be 3.3v I read 0v
    p5 should be 5V I also get 0v
    p6 should be 12 I get 3.3V

    Read great threads on how to investigate when certain voltage is shorted but here them all are out of where they should be.
    Any hint how to start please?
    Thanks in advance.
     
  2. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

    Messages:
    2,523
    Likes Received:
    1,301
    Trophy Points:
    113
    Gender:
    Male
    Location:
    Netherlands
    Home Page:
    When all voltages drops then the first thing is to check the 12v input diode D14.
    And check also if there are no burned spots on the print side, the 12v input section.

    Further, it may also be caused due a defective ceramic smd cap as the C283 near the u21 transistor or C313/C312 or C314 or more of them.
    When you use a good ohm measurement device and check these caps by placing the minus probe on the gnd side and the plus probe on the plus side, you can sometimes see which cap is shorted due the lowest ohm value.

    In any case see this 12v (input) issues. see the 12v shorting on the pcb, most defective spots and the more advanced drawing, in this you can see all the connections to the 12v circuit.

    On note, just to be sure you already uses it, use a 12vDc power supply which must be at least a 2 Amps.
     
    raza05 likes this.
  3. Arthur

    Arthur New Member

    Messages:
    6
    Likes Received:
    0
    Trophy Points:
    1
    Gender:
    Male
    Thank you Johnny, that gave me a start, very appreciated. Measured continuity on the ceramic mentioned with components still in the pcb but found none shorted. Removed D14 and measured it, seems to be ok, at least neither open nor shorted and allowing current in one direction only. Removed also C285 which seems to be on the other side of L36 and the same. Only (maybe) relevant thing is 1) following the mentioned 12V thread it looks like measuring ohms at pin 6 of P5 it should give ~9.8 Ohms and I get ~980 ohms. Also 2) in the same diagram from that thread I see a reference to measure voltage at the LM, which seems to have 5 pin only. Mine has 7 pins (LM2599). My PCB is Rev J maybe different layout/components (?). Re the PSU I'm using the original one that came with the unit. Measured it and still gives 12vdc. Just in case I used a second 12vdc supply (2250 mA) and it made no difference. Thanks in advance.
     
  4. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

    Messages:
    2,523
    Likes Received:
    1,301
    Trophy Points:
    113
    Gender:
    Male
    Location:
    Netherlands
    Home Page:
    I think, the best next step is to measure the pins by ohms, and to give me the results.
    Just measure it between a ground spot, and to the pins.

    [​IMG]

    As for the LM, yes there are two versions, one 5 pins, the other 7 pins as you can see here.
    It depends a bit on how it's connected, but at least the 7 pins is only active when the SD /SS pin 7 voltage is between approximately 1.3V and 1.8V, when lower, then it's inactive.
    But I have not the idea that this could be the issue for dropping all the voltages.
     
    raza05 likes this.
  5. Arthur

    Arthur New Member

    Messages:
    6
    Likes Received:
    0
    Trophy Points:
    1
    Gender:
    Male
    Thank you for the additional detail, mentioned the LM not as a possible cause but to confirm measures where applicable also to the 7-pin version I have. Measures of the ohms at P5:
    p1 210 ohm
    p2 570 ohm
    p3 597 ohm
    p4 765 ohm
    p5 1095 ohm
    p6 980 ohm

    I realize now my ohms measure at pin 6 seems to be correct according to the table. Understood from the tester photo from the 12v-input-issues.22 thread it should be 9.8 ohm, my mistake.
    Big thanks!
     
  6. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

    Messages:
    2,523
    Likes Received:
    1,301
    Trophy Points:
    113
    Gender:
    Male
    Location:
    Netherlands
    Home Page:
    Hmm, strange ohm readings, 1.2v is higher than normal, thus looks as if some part has been interrupted and the 2.6v is scary, because 1.2v and 2.6v are mainly from the cpu.
    In any case, this could be a cpu issue because it uses the 3.3v 2.6v and 1.3v to operate.
    Normally, I would check the cpu (L parts) inductors to see if one of them are interrupted, or remove them to isolate the cpu from the voltage circuit.
    But that is not so easy to do, and when it has been removed, and the voltages come back, then it's the end of the repair job.


    But check at least the 1.2v circuit, see if it has some caps issue, check at least if these caps are flat on the top, and if so, check the solder status of these caps.
    Further, to known if some circuit (dc to dc step down) ic's are working, it's better to disable them from the circuit, the output sides.

    For example, to see if the 3.3v still working, you can remove the R118 zero ohm resistor, and to see if the 5v delivery still works, the R119.
    When removed, and it gets the 12v input, then it should deliver the output voltages.
    Of course, when the 12v still has been collapse, it will not deliver the outputs.
    More about this, see here the u6 info.

    However, since the 12v collapse, but the ohm value seems to be fine, it's a bit of strange issue.
    Normally you should expect that this 12v ohm value is wrong.

    But still, your main goal is to find the reason why the 12v collapse to 3v, which normally could happen due one of the caps connected to this 12v circuit, or the U6 12v input has some issue, or u7, or it's unfortunately caused due a defective cpu.
     
    raza05 likes this.
  7. Arthur

    Arthur New Member

    Messages:
    6
    Likes Received:
    0
    Trophy Points:
    1
    Gender:
    Male
    wait, likely my fault. I still have the two fuses removed, which likely have some influence (?) in the readings. Let me undo the changes and I'll measure ohms again.
     
  8. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

    Messages:
    2,523
    Likes Received:
    1,301
    Trophy Points:
    113
    Gender:
    Male
    Location:
    Netherlands
    Home Page:
    The F1 fuse only disable the dvi 5v sector, and the F2 fuse disabled only the 5v sim card sector.
    If one of these makes a difference, then it's again a strange issue because first of all, these fuses are self-healing and only cuts the 5v when there is a heavy load from the output side which are connected to the L19/C220 and C221, then the dvi pin 14.

    As for the F1, this is very odd when there is a issue from this circuit, and the F2 may only cuts off when there is a defective sim card which could happen, and even if, then the other voltages should still be working because the fuse should be cut off the defective circuit.

    Nevertheless, since you say that the ohms measurements between the ground, and the 12v circuit is 980 ohm, then you may assume that it's not a part which are connected to the 12v directly, thus I can not think of any reason why the 12v collapse, while the ohm readings of this 12v circuit reads fine, or somehow, on a strange behavior a dc to dc converter ask more power consumption than the 12v can deliver, and thus collapse to you say 3v.
    Then I'm thinking of a issue with the U6, the 5v side output, which you can disable from the 5v circuit by removing the R109 zero ohm resistor.
    When removed, and it is a issue with one of the 5v parts on the U6 5v output circuit, then you should have back the 12v, and the u6 outputs 3.3v and 5v should also be back if the U6 is not defective.
     
    raza05 likes this.
  9. Arthur

    Arthur New Member

    Messages:
    6
    Likes Received:
    0
    Trophy Points:
    1
    Gender:
    Male
    It doesnt look good. I put all back in but as you expected no difference. Removed R109 but still measure 3.3V in p6 instead of 12V. I still need to do the test you proposed above, remove R118 and see if the 3.3v still working, and R119 for the 5v delivery. If still relevant. Thanks for the assistance anyway.
     
  10. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

    Messages:
    2,523
    Likes Received:
    1,301
    Trophy Points:
    113
    Gender:
    Male
    Location:
    Netherlands
    Home Page:
    Yeah it's still a strange thing that the 12v collapse while the ohms value of this 12v circuit side seems to be normal.
    I would rather thinking that is collapse due a defective power supply, but since you also measure some wrong ohm readings on the other circuits, it could be not the case.
    But in any case, by disabling the dc to dc output converter from the output circuit, it's often that at least the dc to dc output side come back to life.

    But as I told before, dc to dc converters which uses the 12v input to work, could also not working after the output has been disabled due the to low 12v circuit.

    To find it out, I would normally remove the parts which are directly connected to the 12v circuit.
    This is often due a smd cap, but may also be a ic which use the 12v to operate.
     
  11. Arthur

    Arthur New Member

    Messages:
    6
    Likes Received:
    0
    Trophy Points:
    1
    Gender:
    Male
    Would it be safe to power up the circuit with D14 removed? Looks like some sort of reverse power protection (?)
    I removed it and tested it for shortcut out of the board but was not brave enough to power it up and measure if the 12V where there.
     
  12. Johnny B.

    Johnny B. Technical Support Staff Member Moderator

    Messages:
    2,523
    Likes Received:
    1,301
    Trophy Points:
    113
    Gender:
    Male
    Location:
    Netherlands
    Home Page:
    Yes, no problem.
    This diode is placed to give some wrong polarization protection.