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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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    Hello all !

    Before I start, I am pretty much a newbie in electronics, I know a few basics, the simple things (Power/Voltage/Intensity/Resistance), and can just about identify resistors, transistors, capacitors and LED bridges (and that probably covers it !). Here goes the problem:

    Symptoms
    - When I switch the subwoofer on, from "cold" (transformer and capacitors unloaded), a big power jolt is sent through the 10" speaker. (see Video 1)
    - Now, the interesting thing is, once the source is constant (film playing in DTS for example), the sub behaves normally. If, however, I pause (amp returns to Stereo/PCM default mode) and resume the film, another huge jolt is noticed.
    - Similarly, when I switch from one channel to another, on the amp, an identical jolt is noticed each time. (see Video 2)

    Video 1 : YouTube - Subwoofer amp problem 1/2
    Video 2 : YouTube - Subwoofer amp problem 2/2

    A few notes
    - As far as I can tell, the audio source is not the issue : I've tried using a walkman as sound output, into the sub, and a few other things. The fact the jolt happens when the sub is not connected to any audio source (from a cold switch on) also tends to confirm that.
    - The electricity supply in the house is fully earthed and matches expectations
    - Importantly : this never used to happen, I know this comes from a mistake I made. I shorted something while the sub was on, having dropped a bit of foil inside the sub (silly, I know). I suspect what I shorted was the output wires (black and red) to the speaker, afterhand having noticed the wire connections were bare (from the connectors to the speaker core)

    I have been doing a fair bit of reading, and have started by testing the two biggest transitors, the NPN and PNP transistors most obvious on the pics/videos. Both were fine (I followed a tutorial) apparently. Could it simply be a capacitor discharging each time, due to something else malfunctioning? Or, could it be a capacitor that is simply not doing its job anymore, and the jolt coming directly from the transformer?

    I'd love to here your thoughts on this. And beyond repairing this subwoofer, i would love to use this as an opportunity to acquire some basic electronics skills and learn from this, the amp and board being pretty simple overall (it would seem!). I have a multimeter and soldering iron ready for action!

    Any advice would be much appreciated :)

    Tony
     

    Attached Files:

  2. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    It's hard to guess what's happening. For one, take a look at the DC voltage at the speaker terminals with no input.

    A bad coupling cap may cause this especially if the DC voltage is high. > 0.75V or so.

    Suppression is sometimes actively done on power on or when inputs are switched.

    With a service manual in hand it would be easier to determine where to look.
     
  3. lemofus

    lemofus New Member

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    Hi,

    With no inputs, the speaker output is at 0.08V. When I switch the sub off, the voltage remains unchanged for 1-2 seconds, and then switches to -0.032V, and then gradually goes to 0.

    It's hard to gauge the output when the speaker goes "crazy", since I have no hold function on the multimeter, and it's a very short burst, but the voltage reaches at least 0.4V (before finding its constant 0.08V).

    I've asked the vendor to provide me with a service manual, I assume this would have the detailed electronics plans for the board etc? Let's hope they send that across. In the meantime, thank you for the suggestions!

     
    Last edited: Mar 21, 2011
  4. dave

    Dave New Member

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  5. lemofus

    lemofus New Member

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    This afternoon I also seriously considered getting a new multimeter, with capacitance testing, but it seems it would be a useless bit of kit to get in this particular case, since the coupling caps are 6800µF at 50V, and most affordable multimeters don't get close to that mark! Any (cheap) idea of how I can test those big capacitors?
     
  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Multimeter capacitance ranges are useless for testing electroytics anyway, you need an ESR meter - but in any case, those 6800uF's are VERY unlikely to be your problem.
     
  7. lemofus

    lemofus New Member

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    Ah... OK, thanks... So if it's not the coupling caps, and not the main power transistors (I tested those), what could it be, with those symptoms?
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The voltages on the speaker < 0.75 would be considered OK. 5 mV would be excellent.

    6800 uf capacitors are generally not coupling caps. They are probably on the order of 1-10 uF.

    Capacitance only tells part of the story. ESR (Effective Series Resistance) is generally a better predictor of capacitor failure.
     
  9. lemofus

    lemofus New Member

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    Ah ! So more likely I damaged a capacitor in that capacitance range, and probably close to the speaker outputs? I'll check to see what capacitors match that range.

    Sorry - I wrongly assumed coupling caps would be the big ones !
     
  10. carmusic

    carmusic New Member

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    what parts are on the other board?? i had a similar problem with old sub and it was a faulty relay, but im not sure you have any relay on this sub pcb
     
  11. lemofus

    lemofus New Member

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    The orange capacitors are 47µF, the isolated small black one is 47µF, and so is the one of the same size and colour right against the red cable. The other smaller black capacitor next to the red cable is 100µF apparently.

    I can't see any sign of visible damage anywhere on the board though. How should I about testing ESR, is there a cheapish way of testing them? I have no oscilloscope underhand...

    Could it be one of the resistors in the circuit that failed? If so, is it possible to test those while they are still on the PCB?

    Thanks !
     
  12. lemofus

    lemofus New Member

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    Let me take it apart. I (possibly wrongly) assumed the problem would be close to the speaker output cables, but let's check. The other board has the various connections, everyelse but power, so the speaker inputs, outputs, volume, crossover, phase, power button... I'll take some pics !
     
  13. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Capcitively coupled output stages have gone out of favor. Here LM1875 - 20-W Audio Power Amplifier is an IC based amp. With a capacitively coupled speaker, a large capacitor would be used between the amp and the speaker. This is typical for a Class A amplifier.

    Class AB is also called push-pull and in this system, the pair of transistors are partially conducting so there is a smooth transition through zero volts.

    Class D is a digital amplification technique, where the outputs are fully on for varying lengths of time. The output is then filtered to remove the high frequency component.

    C1 is a coupling cap. C6 and C7 is a power supply filter cap. C4 and C3 are bypass caps. They provide high frequency stability.

    The R2/C3 combination taylors the low frequency response. In this case the -3db point for the low freq response is about 7 Hz. =1/(2*3.14*0.000022*1000)

    There are different Classes for amplifiers. Class A, AB and D are common for audio amps. In many audio amps there is also a network that sets the high frequency response limit as well.

    C1 should be of a metalized polyester type (best cost). Usually it's an electrolytic which starts to leak and generates a DC offset.

    What's not shown here in this circuit would be a system that would effectively mute the input briefly when a signal source is powered up or switched. Many times that is a FET shorting the audio input to ground.
    In the old days, input selecting switches were called "shorting rotary switches". The proper name is really "Make before break". Thus when you switched from Tuner to Aux, for instance, both would be connected for a short period of time. This prevented the "thump". This is the area to explore first.

    "Thump" supression is sometimes employed as part of the protection circuitry. There are turn-on, turn off and switching thumps that can be surpressed. Generally a relay is placed in series with the speaker. If DC is detected on the speaker, the relay opens.

    In your case, the thumping when switching is likely a pre-amp problem.
     
  14. lemofus

    lemofus New Member

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    Last edited: Mar 21, 2011
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    You have no relays. For fun, check the voltage between the bases of what looks like Q10 and Q11? The transistors at the edge of the board. Zero isn't a good number.
     
  16. lemofus

    lemofus New Member

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    Interesting stuff, thanks. I've much to learn! I don't think there is a relay anywhere in this system; the two PCBs within the pics show all of the components. Could it be one of the FETs (smaller ones) that has given up on me then, simply?

    Great news though, a friend of mine has found the service manual online:

    http://dl.dropbox.com/u/8396442/Eltax-Atomic-A10.pdf
     
  17. lemofus

    lemofus New Member

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    I had unsoldered those both a couple of days ago and tested them, they were apparently OK, but I went ahead and looked at Q11 while it was plugged. It showed 37.6V roughly, and then... I slipped and shorted the two base nodes of Q11 (2 out of the 3). That showed a beautiful spark and now the power LED doesn't even switch on, I fear I might have bust the transformer...

    Wow I'm really not cut out for electronics it would seem....
     
  18. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Oops! There is a fuse. You might get lucky. Placing a 60 W incadesent lamp in series with the mains can lessen damage when powering back up.

    Check Q8, Q9, Q10, and Q11 for shorts with an ohmmeter. Does your meter have a diode test mode? Do you know how to use it and what it does?

    I suspect your thumping issue is between Q3 and the CONT terminal. Right now, that's a secondary problem.
     
  19. lemofus

    lemofus New Member

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    OK, so I checked, and found a hidden fuse in the back socket I hadn't seen last night - great news. The transformer is delivering 27.5V, so that's fine. As soon as I switch it on, with the PCB connected, it blows the fuse. So I must have shorted a component on that PCB...

    Testing time ! My meter doesn't have a diode test mode; the way I had originally checked the two big transistors is by unsoldering them from the board, and following some instructions from the web, looking at voltages between each of the 3 terminals. Would a diode test mode allow me to test those components without unsoldering them from the PCB? I'm not familiar with this mode...

     
  20. lemofus

    lemofus New Member

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    Actually, I think my meter has a diode testing mode, but I just didn't realise I was using it... It's the mode that feeds a small current and beeps with the resistance is is 0?

    I used this site for transistor testing: Basic Amplifier Repair

    I unsoldered Q11, and whatever combination I try on the 3 nodes, including reversing polarity, I get 0 resistance. Am I mistaken in assuming that this is a bust transistor? (Possibly one of a few?) Incidentally, this is the one that I had shorted and that had gotten really warm from it...
     
  21. lemofus

    lemofus New Member

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    Last edited: Mar 22, 2011

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