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Home Cinema active Subwoofer malfunctioning

Discussion in 'Repairing Electronics' started by lemofus, Mar 19, 2011.

  1. lemofus

    lemofus New Member

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    Done, fully recapped. R41 and Rbonus were replaced with 8k resistors, all I had underhand was 8 or 20k...

    V = 0V

    V = 0V, in both ON and OFF positions. interestingly, the LED did not turn green when switched to ON, with C16 shorted...

    Hard to estimate, I'd say 0.5-1 sec.

    I'd say a good couple of seconds here, longer than OFF-ON change.

    I'm just confused as to why ON wouldn't show a green LED with C16 shorted?! Expected?

    I'm off to Paris on sat, south of france on wednesday, spain on Friday.... Then I have a few days of work, and I'm off to Italy for a few days... Next few weeks will be a bit more on and off so I'm trying to speed things up, or at least stop somewhere reasonable! Can't complain about holidays though...
     
  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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  3. lemofus

    lemofus New Member

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    Yep - but an amp specifically, and not simple an mp3 player or something else like that? Should be the same for testing?

    0.003V AC

    DMM has a "Frequency measurements" mode, I assume that's what I'm after? If so, nothing is showing.

    R13 = 3.3k (unchanged)
    R14 = 3.3k (1/8W, a bit larger, I reverted back to this one after frying things by letting the extra pcb touch the transformer...

    Yep, would be a good idea, back with a 3.3k resistor (so 3.3 + .5, potentially). This time I'll glue gun the ends so that I don't short anything with the extra PCB.

    Let me roam through my growing draw of electronic components, see if I have a 1k resistor underhand. R being Resistance? If so, current will be around 0.036A, so that means with a 1/4 W resistor I shouldn't exceed 7V. Should be ample, I recall Q7 being around 2.5V roughly.

    EDIT: Just realised we are talking about Q8 and Q9 voltages, no idea what to expect there. Good chance they will be higher, in which case I'd have to go and get some resistors in town tomorrow, current ones would be too flimsy...
     
    Last edited: Apr 27, 2011
  4. dave

    Dave New Member

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  5. lemofus

    lemofus New Member

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    I have two 1K resistors measuring slightly short of 1cm long, and 3mm in diameter. No idea what wattage that would be, probably 1/4W, or at best, 1/2W...
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    (0.036^2) * 1K, so a 2W resistor will be fine. I know the leads won't fit, but it's not permanent. 3mm sounds like 1/4W.

    Based on the frequency meter and some of the tests we did WAY back, I don't expect hum the pre-amp section. It's just an additional check. You can also extract a signal right before it's applied to the sub's amp too on the connector. Use the MP3 player as the signal source; the output being the LINE out and the SG points separately.

    Warning, the glue gun may not be a good fastening method. Been there. Done that.
     
  7. lemofus

    lemofus New Member

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    I'll go and pick up a couple of 2W 1K resistors this afternoon, 1/4W would have been a little flimsy if the voltages are anywhere close to those of Q10-Q11...

    Thanks. Will take your word for that. I'll isolate using electrical tape, and once everything is setup permanently, I can glue the PCB add-on properly with an epoxy; will be cleaner.
     
  8. lemofus

    lemofus New Member

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    Done. Q10-11 still out of course.

    LED on RED (standby):
    Q7 EC = 0V
    R15/R11 and base of Q7 = 0.05V
    R16/R12 and base of Q7 = 0.15 to 0.30V

    LED on GREEN (ON):
    Q7 EC = 0V
    R15/R11 and base of Q7 = 0.05V
    R16/R12 and base of Q7 = 0.15V

    A few observations:
    - 2W resistors in EC of Q8 and Q9 got very warm in a short period of time; made sure I didn't leave the thing on too long whilst doing measurements!
    - Obvious discrepancy between R11/R15 and R16/12... Why?
    - R15/R11 and R16/R12 measurements were done using a fixed range, the DVM went crazy in autorange... Not reassuring.
    Both R13 and R14 are 3.3k resistors at the moment, although R14 should apparently be 3.6k.
     
    Last edited: Apr 29, 2011
  9. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Stupid me. That's what a migraine does. the Q8/Q9 E-C resistor does absolutely nothing except get hot. +V to 1K to 220 to 1K to -V. Oops!.

    Go for broke. Leave out Q10 and Q11 and use the 25 W lamp.

    Get the R14 combination going and set to 3.3K or less. Monitor voltage across Q7 E-C. Don't let it get above 2.4V

    If successful. Reduce Q7 E-C to the minimum value you can get. Install Q10, Q11 and the heatsinks. Again, monitor Q7 E-C.

    No light bulb. Do a brief sound test. There could be cross-over distortion.

    FINAL BIAS ADJUSTMENT.

    No signal. Monitor voltage across R18 or R19. Increase your bias POT until 10 mV or so appears. Continue monitoring for at least 1/2 hour. If it starts to creap up rapidly, it too high.

    MUSIC test. No light bulb.
     
  10. lemofus

    lemofus New Member

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    I wonder why we bother with E-C then :D

    More seriously, not sure I follow here...

    OK, so something like 3k + 0.5k pot, and then we can fine tune.

    You bet; will be keeping an eye on that one for sure!

    Sounds good to me; this could be mean either increasing, or decreasing R14 resistance? Or will it necessarily go one way or the other?

    I hope nothing else is burned somewhere... Got through a few too many Q10 and Q11 so far!

    Will give those things a go tonight or tomorrow if I can, before going to Paris... Hope the migraine is fixed. I bet a leaky transistor is responsible for that too.
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    It turned out to be nothing but a voltage divider with 80 V across it. Basically a heater. It wasn't useful at all.

    3.2K or 3.3K might be the ideal value, but go for 3K. You can always change it.

    That's the initial key voltage. At 0 to maybe 1.5 V+ volts there is no way to overheat the outputs. It's always essential to monitor this voltage first. You should be able to use the light bulb to at least determine if you can vary it. But don't have a high voltage here without heatsinks on the outputs for very long.

    Ideally, what you want is when the potentiometer is turned clockwise, the voltage goes up which means the bias goes up, which, I think, means R4 goes up.

    Tuition is expensive!

    I've had about a week of daily migraines. In the southern US there was 300+ killed because of tornadoes. Even that's way to far from me, my area was still bothered by the weather system. The day before it hit in the south, I had my worst one.

    Have a fun trip.
     
  12. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    If you think this repair is fun, what about this one:

    Dog Story

    A Kansas farm wife called the local phone company to report her telephone failed to ring when her friends called - and that on the few occasions, when it did ring, her dog always moaned right before the phone rang.

    The telephone repairman proceeded to the scene, curious to see this psychic dog or senile lady. He climbed a telephone pole, hooked in his test set, and dialed the subscriber's house.

    The phone didn't ring right away, but then the dog moaned and the telephone began to ring.

    Climbing down from the pole, the telephone repairman found:

    1 . The dog was tied to the telephone system's ground wire with a steel chain and collar.

    2. The wire connection to the ground rod was loose.

    3. The dog was receiving 90 volts of signaling current when the number was called.

    4.. After a couple of jolts, the dog would start moaning and then urinate.

    5. The wet ground would complete the circuit, thus causing the phone to ring.

    Which demonstrates that some problems CAN be fixed by
    pissing and moaning.


    Thought you'd like to know.
     
  13. lemofus

    lemofus New Member

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    Ah aha ! Love it !
     
  14. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Next time you go shopping.

    Shopper: I need a 3 Volt 2 Watt light bulb?
    Salesman: For what?
    Shopper: No, two.
     
  15. lemofus

    lemofus New Member

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    Hello there! Back for a few days... Before heading to Milan this time. Good traveling !

    So, where were we?

    That means sticking Q8 and Q9 back in? I hope I won't burn them....
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Welcome back. Yep. go for broke. The moment you see decent adjustability, back it off. Put the heatsinks back. Do a brief sound test and final adjustments.

    You could. but be VERY careful make some small heatsinks out of a cooking aluminum throughoway container or multiple folds of aluminum foil. Clip it into place with a clothespin. Watch for shorts.
     
  17. lemofus

    lemofus New Member

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    With Q10 and Q11 still out, Q7 EC voltage is 0, on or off.... expected? No apparent heat... I need Q10 and Q11 back in? R14 is set to 3.28k at the moment.
     
  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I don't think Q10 and Q11 have to be back in. Remember that the "proper value" is 3.6K and a higher value i=of R should translate into a greater voltage across Q7.

    You need to adjust it while monitoring E-C. My guess is it will "suddenly" start to work while turning it.

    If you can get to 2V and still have some travel left your probably OK. I'd turn off the amp and measure the total R. Set ot back to 3.3K and re-assemble with a heat sink.

    For the final adjustment you need to be concerned about Q7 (E-C) and the voltage across the low value emitter resistor which will probably be near 10 mV.

    If that low voltage starts to creep up rapidly, then the bias is too high. With no signal, monitor it continuously for the first 5 minutes and then check every 5 minutes for an hour.
     
  19. lemofus

    lemofus New Member

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    Unfortunately, nothing yet. This is the situation now : R14 replaced with 3.3k + 0.5k pot; Q10 and Q11 are out, everything else is back in (including additional resistor to add delay).

    Power on is just fine, bulb acts normal, LED gows both RED and GREEN depending on switch position. But regardless of R14 value, Q7 E-C remains at 0V (I varied between 3.3 and 3.8k). At least nothing is burning, I was worried about killing another Q8 or Q9, but no cause for worry if voltage remains null !

    I'd love to think I'm close to fixing this :)
     
  20. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Voltages across R4 and R9?
    and
    Voltages across R11 and R12?
     
  21. lemofus

    lemofus New Member

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    R4 = 2.2V
    R9 = 1mV...

    R11 = 0
    R12 = 0
     

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