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Help with repairing a Samsung HT-BD8200 Soundbar SMPS

Discussion in 'Power Electronics' started by eigiman, Oct 10, 2019.

  1. eigiman

    eigiman

    3
    0
    Oct 10, 2019
    Dear EEV Friends,

    I have many years' experience in SMPS and general electronics design and repair, and generally know my limits, but am a little stumped by this little bugger (see attachment).

    Basically, there is a 5.6V standby circuit, which is used to produce a P_ON signal which switches everything else on.
    Everything else does come on and is well within tolerance, except the +/-28V and -5V supplies generated by the MR4040. This will come on intermittently, most often not though.

    All the standard culprits have been checked - electrolytic capacitance and ESR checked, diodes, optocouplers, resistances, dry joints. MR4040 has been replaced. Nothing. I've partially localised the issue (I think) - the measured Vcc on pin 4 of the MR4040 is at around 7V when it doesn't switch on, but is at +11V when it is functioning. I can't find any info on the Vcc requirements for the MR4040, though.

    You can see that during the transition from standby to on, initially the Vcc is supplied from a rectified output on the primary of the ICE3B1565J via RB16, DB6 and RB10 (all checked), but I don't understand how this voltage can be too low if all the secondary voltages are correct.

    I'm beginning to suspect the ICE3B1565J, a little out of desperation rather than logic...
    Any suggestions or ideas?

    This appears to be a common failure mode for this particular board based on internet searches (power LED flashes white/red, sometimes starts if allowed to "rest" for a few days), however I could not find a solution...

    Cheers
    Ilya
     

    Attached Files:

  2. thedoc298

    thedoc298

    8
    0
    Oct 4, 2019
    Run down the pin 4 some more with the 50v cap and 24v zenner I would be expecting more than 11 volts. See what is happaning on ub2
     
  3. eigiman

    eigiman

    3
    0
    Oct 10, 2019
    Thank you, I've fixed it.

    First, I tried the feedback components around the MR4040 - no change.

    Then I replaced the ICE3B1565J - no change.

    Then I reduced the value of RB16 until the MR4040 was switching on reliably - 47R. Reconnected the power supply. The flashing red/white was much slower and eventually the machine went into standby. I could then switch it on, but it wouldn't read discs nor open/close the tray.

    I checked the supply voltage to the tray motor control IC - nothing. Traced it back to a very hot 7808 V regulator. Then checked all the wiring connected to the +8V line on the main board, disconnecting the flex cables one by one. The short was coming from the 40-way cable to the optical sled in the BD/CD drive...

    Dismantled the drive and ... just where the first 180 degree bend is in the flex coming from the sled, on the outside the +8V and AGND lines had touched and come loose/broken from overheating.

    I stripped back just enough of the flex to get decent copper, bridged the tears and strenthened everything with PVC tape perpendicular to the flex. It's not perfect, but the connections are there, without a short.

    All working now, let's see how long it lasts. In any case, I can easily get a replacement flex.

    So now for the fault analysis... The extra load on the +12V line from the SMPS (which feeds the 7808) was causing a change in the voltages on the primary side, so that the MR4040 wouldn't start up.

    Thanks for all your help! We may have been barking up the wrong tree, but your encouragement was very important for me.

    Another device which won't end up on the dump for another few years!
    Ilya
     
  4. thedoc298

    thedoc298

    8
    0
    Oct 4, 2019
    Did you change the rb16 back to original. Glad its working
     
  5. eigiman

    eigiman

    3
    0
    Oct 10, 2019
    No, I've left it at 47R for now,Thanks!
     
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