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Home theater subwoofer power supply fault

Discussion in 'Troubleshooting and Repair' started by Evaldas22, Oct 4, 2017.

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  1. Evaldas22

    Evaldas22

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    0
    Oct 3, 2017
    Hey guys, I'm trying to fix home theater subwoofer power supply. My friend said that all of sudden one day whole system (it is powered from power supply in subwoofer) didn't turn on (he noticed that standby led not glowing). Also, he mentioned that as he plugged it into mains while it was already dissembled some sparks came out of it - later as I saw he touched two of little transformer leads to metal housing and I think this caused short. But as I checked I think that transformer might be OK. As I came along to big transformer first I tried to find any data about it but with no luck. Then I tried to find primary winding of it. Found out that only 3 of 6 leads are connected together. The resistance between first and other two are 8.6 and 8.7ohms. The resistance between 2nd and 3rd around 0.3ohm. Then I tried to plug it into mains and see if AC even comes to it. I saw only 0VAC in multimeter. As I continue to search through circuit I notice there is a relay that connects 3rd transformer lead to black mains wire. My guess is that relay doesn't receive signal to turn on thus no AC is going to the transformer. I'm providing you with some photos and link to mains board schematics.
    Also, want to ask what that little transformer right after MAINS IN does??
    http://archive.espec.ws/files/PHILIPS LX8300SA(01,04,05).pdf
     

    Attached Files:

  2. kellys_eye

    kellys_eye

    4,279
    1,146
    Jun 25, 2010
    The little round 'brown' things are fuses (1902, 1904, 1907) - have you checked them?
     
  3. Minder

    Minder

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    Apr 24, 2015
    The 'little transformer' is a common mode choke, for suppression purposes.
    M.
     
  4. Evaldas22

    Evaldas22

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    Oct 3, 2017
    Yeah, checked them all.
    Can you explain more? What is it suppressing? Could it be faulty, since it was shorted? In it’s output I read 240V AC.
     
  5. duke37

    duke37

    5,277
    734
    Jan 9, 2011
    What do you mean by shorted? If it has 240V on input and output, it is not shorted and is working.

    Mains power is contaminated by high frequency signals. These are generated by switch mode power supplies, motors with brushes and power supplies with rectifiers. The common mode choke may be there to contain the filth within the device or to prevent it arriving from the mains. In the case of an audio amplifier, you do not want to hear the effect of any noise which is on the mains.
     
    bushtech likes this.
  6. Minder

    Minder

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    Apr 24, 2015
    Performs a similar function to the external ferrite chokes often seen on the power cord itself, Google should offer detailed description on common mode chokes and how they are wound, both windings are in series with the mains so it will show 240v in and out.
    M.
     
    Evaldas22 likes this.
  7. Evaldas22

    Evaldas22

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    Oct 3, 2017
    Can you please explain why those two leads of choke are black? Those sparks came after touching metal casing, which I think was 0 potential. And therefore a voltage drop of 240V occured, which caused sparks?
     
  8. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Simple carbonation from the shorting of the leads (copper under the solder). If you shorted 'properly' you'd have blown the mains lead input fuse (if fitted) or the external circuit breaker.
     
  9. Evaldas22

    Evaldas22

    12
    0
    Oct 3, 2017
    Well, the case is that after this, the braker in house main electric box was triggered and it popped, so had to go to that box and switch it back on
     
  10. kellys_eye

    kellys_eye

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    Jun 25, 2010
    The relay that activates the main transformer is powered by the standby supply which itself is powered through fuse 1906 and developed by the circuitry associated with the capacitive dropper (2908/2718), rectifiers and a circuit that drives a push-pull arrangement around transformer 5904.

    Measure the supply at LPS (RHS of schematic) to check for the correct standby voltage.

    Check inductors 5929 and 5931 too.
     
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