Subwoofer amp blows fuses

Discussion in 'The Projects Forum' started by Jack_K, Jun 22, 2014.

  1. Jack_K

    Thread Starter Active Member

    May 13, 2009
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    I've had a Polk speaker system with subwoofer in my home for over ten years. Recently the subwoofer blew an internal fuse. A check with an old analog ohmmeter seemed to indicate the TIP31 (Q1) was shorted. I happily replaced it AND the TIP32 (Q5). It still blew fuses.

    I then checked the bridge rectifier and it seems OK. I removed the two filter caps, C2 and C6, and checked them. They appeared OK, too.

    Not knowing anything about FET transistors, I ordered a new pair for Q2 and Q7. With those two transistors out it didn't blow fuses. Then I checked the other transistors and the diodes with the ohmmeter. All appeared OK.

    When the new transistors, Q2 and Q7 arrived, I replaced them and tried the amp again. It still blows fuses.

    Then I pulled the bias jumper, 100con2, and tried it. It didn't blow the fuse and drew very little AC current.

    Now I'm stumped and need help.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could try this:
    Temporarily substitute bias jumper 100con2 with a 10k resistor.
    With no audio input, measure V+, V-, +LV, -LV, and voltages at the bases of Q3 and Q6 and both ends of R14. Post the results.
     
  3. Jack_K

    Thread Starter Active Member

    May 13, 2009
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    I'll do that tomorrow.
     
  4. Jack_K

    Thread Starter Active Member

    May 13, 2009
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    After inserting a 10K resistor at the bias jumper, I made several measurements as requested.

    V+ is 51.6, V- is -51.6, VL+ is -0.55, VL- is 0.

    The base of Q3 is -0.54, the base of Q6 is -10.

    The base of Q1 (cathode of zener) is 0. I guess the zener is shorted. It's a 1N5245B, 15 volts.

    The base of Q5 is -0.08.
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Yup. Replace Z1 and repeat the measurements (still with the 10k jumper).

    Edit: It's odd the zener failed, as it would normally carry only ~3.5mA. Check that R4 (10k) hasn't gone low resistance.
     
    Last edited: Jun 24, 2014
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Without the system powered up, check the following, which all have a major effect on the bias current of the output FETs:
    1) Diodes D2,4,5,6 are good,
    2) There is no open circuit or dry joint in the current paths between the R1/R3 junction and the R19/R20 junction,
    3) R1,20 haven't gone low resistance,
    4) R3,19 haven't gone high resistance,
    5) Pot R7 looks undamaged (don't alter its setting).
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Alec is steering you on a good course and I do not want to interfere but I would like to suggest the following:

    If Q1 shorting is the root cause of the failure then a lot of parts got subjected to higher voltage than what they can survive. Anything that was connected to +LV could be damaged. If you begin seeing several bad parts that are connected to +LV then you might consider a "shotgun" approach.

    Mark
     
    Last edited: Jun 24, 2014
  8. Alec_t

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    Sep 17, 2013
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    Mark's right. Q5 shorting would have a similar effect re -LV.
    Since the circuit is essentially symmetrical, comparing the values of parts which should be the same may help to identify culprits.
     
  9. Jack_K

    Thread Starter Active Member

    May 13, 2009
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    The diodes are all good.
    No opens in those circuits.
    I lifted one leg of R1, R3, R19, and R20 and verified all to be good.
    R7 looks good and reads 98K.

    How can I check the MOSFETs?
     
  10. bwilliams60

    Active Member

    Nov 18, 2012
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    Hi, just following along so hope you don't mind me asking a couple of questions. Q1 and Q5 are NPN and PNP bipolar transistors. Can they be tested as diodes and compared to each other, only opposite? Why is there no voltage on LV-?
    Thank you, trying to learn more about stereo repairs.
     
  11. Alec_t

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    Sep 17, 2013
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    The results in post #9 point to those tested components being ok.
    Have you replaced the zener and re-measured as per post #5?

    Yes. A useful test, albeit not conclusive of full function of the transistors.

    Because Z1 is dead.
     
  12. bwilliams60

    Active Member

    Nov 18, 2012
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    Jack,
    Were you able to fix this unit? Just curious.
     
  13. Jack_K

    Thread Starter Active Member

    May 13, 2009
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    I'm waiting for the zener and some other parts to arrive. Then we'll see.
     
  14. Jack_K

    Thread Starter Active Member

    May 13, 2009
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    I finally got my parts in.
    I replaced Z1 with a 1N4744 15 volt zener. I checked voltages and it appeared that Q7 the IRF640 was shorted, so I replaced both Q2 and Q7.

    Now I have decent voltages:
    1. V+ is 51 and V- is -51 (+/- a few tenths)
    2. +LV is 14 and -LV is -14 (+/- a few tenths)
    3. Q3 base is 1.63, emitter is 1.03, collector is 37.3 (40.3 volts at 100CN, pin 2) 10K inserted between pins 1 and 2
    4. Q6 base is -1.51, emitter is -.9, collector is -48
    5. Both sides of R14 are 0 volts.

    This looks promising, but I only have one fuse left so I'm hesitant to insert the Bias jumper.

    Will it hurt to power it up without the speaker connected?
     
  15. Lestraveled

    Well-Known Member

    May 19, 2014
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    Not connecting the speaker should be no problem, in fact I recommend it.

    What you can do on the bias jumper use a 10 to 50 ohm 1/4 watt resistor to jumper it. It will act as a fuse.

    Mark
     
  16. Lestraveled

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    OK, now I get it, you have 10K across the bias jumper. Yes, it is looking good. A 10 ohm resistor is still a good precaution.

    Rock on.

    Mark
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Agreed, it's looking good so far.
    Yes, recommended as a replacement for the 10k. Put ammeter in series with the 10 Ohm, cross fingers for luck, say prayers and switch on :).
    Do you know what the output stage bias current should be? That may need setting using pot R7 (at minimum resistance to start with). Simulation suggests that the pot adjustment may be quite critical ....a small shift making a big change in bias current.
     
  18. Jack_K

    Thread Starter Active Member

    May 13, 2009
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    Made some progress. No more blowing fuses.

    When I power it on, while connected to the receiver, I get a WOOMP from the speaker, but no other sounds. I guess something in the preamp portion is dead.

    I sure wish I had a signal generator or something to use as a signal source so I could scope out the circuitry.
     
  19. Lestraveled

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    May 19, 2014
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    Jack
    I suspect that your output amp is OK. However, I agree, you have other parts that are bad. They were probably damaged when the power supply went. I suggest that you replace every op-amp on the board. If it is not bad now, it will be bad later (as in soon).

    Measure the voltage across the speaker terminals.

    Mark
     
  20. Lestraveled

    Well-Known Member

    May 19, 2014
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    The LM324s are available at Radio Shack.
     
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