Powered PA speaker - fuses blown

Discussion in 'General Electronics Chat' started by eyesee, Aug 29, 2015.

  1. eyesee

    Thread Starter Member

    Oct 19, 2013
    33
    3
    Hi all,

    I'm trying to repair a power supply PCB from a 10" powered PA speaker. Both input fuses on the PCB have blown. The power supply is linear and the PCB is fed from a toroidal transformer. The power supply PCB components consist of two T3.15A fuses from both AC legs of the transformer fed to an RS 808 bridge rectifier and small bypass capacitor. The supply is smoothed with two large 3300µF capacitors on both DC rails.

    I have tested the forward voltages of all the diodes in the bridge rectifier and all seem to be OK. I have also tested the 3300µF capacitors out of circuit with an ESR meter - they check out OK. The power supply amplifier IC is a TDA 7293.

    On the rest of the board are filter components for the HF and LF speaker drivers and a couple of zener diodes from the main supply to feed the signal PCB on another board.

    I can't figure out what would have caused the fuses to blow? The only thing I have noted is that the resistance between the positive and negative TDA 7293 supply pins (+Vs & -Vs) is around 3 Ohms. I'm not sure if this is normal? Resistance from each of those pins to ground is high.

    Below is a photo of the power supply/amplifier board.

    J210A.JPG

    Any suggestions would be appreciated.

    Thanks.
     
    Last edited: Aug 29, 2015
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Is it possible to isolate the supply to the TDA 7293 ? This might mean cutting a couple of tracks.
     
  3. eyesee

    Thread Starter Member

    Oct 19, 2013
    33
    3
    I currently have the supply isolated to the TDA 7293. The resistance between the supply pins (13 & 15) on the IC still measures around 3 Ohms.
     
    Last edited: Aug 29, 2015
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    it is hard to tell from the picture, but I strongly suggest you closely examine the IC

    I seem to see a crack running horizontal across the face of it at about the half height mark.
     
  5. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Ok, do the fuses blow with just the TDA 7293 isolated?
     
  6. eyesee

    Thread Starter Member

    Oct 19, 2013
    33
    3
    That might just be smeared heatsink compound when I was trying to clean the face to read the part number?

    I haven't tried that yet, thought I'd eliminate all possibilities first.

    Is it normal to have such a low resistance between power supply pins but not to ground from each pin on these power amp. ICs?
     
  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    It may be a normal reading; when checking integrated circuits with a meter it often isn't possible to get useful readings, it all depends on the internal circuitry.

    Since there is very little else on the board and you have checked the power-supply components the power-amp IC seems to be the most likely suspect.
     
  8. JUNELER

    Member

    Jul 13, 2015
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    5
    HI, CAN I ASK A FEW QUESTION ABOUT THE POWER SUPPLY.FIRST IS THE POWER SUPPLY IS SPLIT TYPE SUPPLIYING A POSITIVE AND NEGATIVE VCC TO PIN 13 AND PIN15.SO IT ITS OK.YOU SAID A 3 OHM RESISTANCE READING ON PIN13 AND PIN15 ,THIS READING IS NOT NORMAL. IT SEEM BAD IC.TRY TO SWAP A NEW IC.PIN13 AND PIN 15 SHOULD HAD A HIGH READING IN MEG OHM.
     
  9. eyesee

    Thread Starter Member

    Oct 19, 2013
    33
    3
    The + and - outputs of the bridge rectifier feed +Vs (13) and -Vs (15) on the TDA 7293 respectively, with two large smoothing capacitors as mentioned above. The ground is taken from the central tap on the transformer.

    What would have caused such a fault in the first place?
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    A possible cause would be a short between output wires either when wiring up the amp or at the speaker.
    Speakers can develop bare places on the suspended winding and during a large excursion they can short out along the sides of the winding and momentarily short the + and - lines. This is not common but is possible. Check speakers, especially if old by using an ohm meter and moving the cone in and out slowly, any sudden change of reading lower than the normal resistance of the speaker indicates a bad speaker.
    The other cause would be an overheat condition, or just a plain old FAILURE of the IC for reasons unknown.
     
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