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Home Cinema active Subwoofer malfunctioning

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

  1. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The Zenier goes in your position D5. It has a 10K to V+. There should be 5.6 V from ground to the cathode of D6.

    I hope you didn't replace the other diode on pin #1 of the OP amp?
     
  2. lemofus

    lemofus New Member

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    0.6V D6 to ground, regardless of ON/OFF mode.

    Nope, don't think so! I changed the diode on the section with the potentiometers, ON/OFF switch etc. Don't think there even is a D5 on the other board... D5 (currently with the zener) has roughly 0.5V, in both ON/OFF modes (and AUTO of course)
     
  3. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The diode is marked on the pic. Basically there is a + supply, a resistor and a diode and remotely a bad OP amp. The LED changes colors, so the OP amp has to be good.

    Trace that circuit. The band of the 5.6 V Zenier has to face up. Breadboard it with your 9V battery. Don't exceed 0.5W across the Zenier diode. This is what you have: Zener Diode Voltage Regulator - Electric Circuit

    My only guess at this point is that what you think is a Zener diode, isn't. What's the part number?
     

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  4. dave

    Dave New Member

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  5. lemofus

    lemofus New Member

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    Heh, I found the problem... The schematic doesn't correspond... D1 is actually D5 on my board, and D5 is D6... So yes, the inaccuracies on the schematic/board side didn't help! I put the old diode back in and the Zener in the correct place, in reversed position.

    With the Zener in there, it's really hard to gauge the latency; it still switches from red to green very swiftly, possibly with a slighly longer delay, although I may be imagining it. At least it's in the correct position this time...
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    With the 5.6 V Zener there, you should have a delay of about 1 second. Much better than you had and probably where the final value will end up.

    What do you say, we increase the delay to about 5 seconds and be prepared to bring it back to 1.

    1. Paralleling a capacitor makes capacitance add. Thus two 100 uf/16V would double the delay.
    2. Changing the 1K resistors to about 4.7K-10K (whatever you have)
    The 1K that you added with the cut should be for the OFF/ON delay.
    The 1K at the center of the switch will be for the AUTO/ON delay.
    Remember that OFF discharges the delay cap

    So, if it goes to 5 sec. or so you can see it and the delay gets tested with a delay you can see. Then return it to 1 sec when everything else works.
     
  7. lemofus

    lemofus New Member

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    Are you suggesting I should do both things, or start with one and see how it goes?

    Which 1K resistors? Only the one I added with the cut trace?

    So this is to add some delay, which could solve the "thump" problems, right? Do you still think Q2 is leaky? Still have Q8,Q9 and Q11 out out course, not sure what caused my BD139 and TIP36C to fry last time around....
     
    Last edited: Apr 25, 2011
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Replace the 1K across the cut trace with a 4.7K to 10K. This should only affect the OFF-ON delay and get something measureable. I'm on my phone so I can't provide details for the other OFF-AUTO delay.
     
  9. lemofus

    lemofus New Member

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    No problem. Thanks, that'll be a good start.
     
  10. lemofus

    lemofus New Member

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    Awesome. I found a 4.2K underhand, tried that, and the delay is now definitely noticeable. Still less than a second, but it's there.
     
  11. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I think a slight delay is needed there.

    Now, here is both of the delays defined.

    Not sure yet if we are dealing with a leaky component. Make the OFF/AUTO delay noticable.

    Well, increasing the 1K resistors increases the timing delay. Increasing the 10 uf cap increases the delay. That 2.7 M will change the discharge time.
     

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    Last edited: Apr 25, 2011
  12. lemofus

    lemofus New Member

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    Ah OK, I need to replace R41 with a greater resistance one, I assume ideally it should be the same as the one I added for the ON/OFF switch? So something like 4.3k? At the moment All I can find is 3.7k, need to stop and get a few between the ranges of 4 and 10k.

    Should I add a capacitor in parallele at C16 in addition? I can solder one on the other side of the board, no room on the normal component side...
     
  13. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    I initially mentioned paralleling, because paralleling is easier. 10 uf || 100 uf means 110 uf. Parallel caps add. You might consider replacing the 10 uf cap especailly if it's 5 years old. They leak too. I think you could test that one. We really don't want the cap to get bigger permanently.

    I want something larger for R41, JUST TO TEST, so 3.7 to 10K is good. Pick up a 47K too. They are cheap and small.

    I'm still confused why the AMP started thumping. What is the age of the amp? The calculated timing elements are very small compared to expected delays of seconds. It's possible that the designer wanted to minimize the amount of time that it takes to turn on.

    The amp itself has an internal built-in delay based on the filter supply caps and any associated circuit traces, fuses etc which add resistance. Those caps could have degraded too.

    As much as I'm trying to find a defective component, I'm now considering relative aging of components. Typically the power supply caps and the timing elements. Too bad I got side-tracked along the way. The blurry diagram didn't help.

    You should still go and TRY to measure the leakage current of the timing transistors which may or may not be possible. Remember to look at the voltage across either the Collector resistor or Emitter resistor when the transistor is supposed to be off. The ONLY reason why I seem to be chasing my tail is the plausability of "relative aging" which, I'll admit is a first. The only clue that this is happening is the calculated timing values.

    See RC circuit - Wikipedia, the free encyclopedia
     
  14. lemofus

    lemofus New Member

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    It started thumping because I shorted something on a circuit board, I suspect is was the Speaker + - Outputs...
     
  15. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Could it have been a co-incidence with DC appearing on the output of your audio system?
     
    Last edited: Apr 26, 2011
  16. lemofus

    lemofus New Member

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    Heh, good question. I suspected something was wrong with the amp output a few years back. That could possibly be my next project; I vaguely remember plugging the sub while the amp was on, or something like that, and something went very wrong. I played with connections (RCA I assume) and think I recall having a horrible sound and realising something was up with the amp. Now, I'm not sure what could have caused the sub issues. I hadn't used it in years because it had an awful mains hum. Having time on my hands, I decided to look into that hum. It transpired that the speaker magnet was too close to the transformer, and this is where the interference was coming from. There was an easy fix, simply to reverse the plate (aluminium bit with PCBs and transformer attached to to it); distancing the transformer from the speaker helped. But there was still a buzz. So, then I figured an earthed metal plate could help minimise interference between the transformer and the speaker. This is when I suspect I shorted the + - cables of the speaker out, which were not shielded at the end (I thought they were).

    All in all, can't say for sure what could have caused the problem, but I - at least initially - suspected the amp and sub were unrelated problem. Something I haven't seen here? Maybe they are related?

    One thing is for sure though, I won't be plugging the sub onto the amp once it fixed, until the amp is also fixed. In the meantime, testing is done with a phone playing music to be sure...
     
    Last edited: Apr 26, 2011
  17. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Give me some idea of the chicken and the egg.

    Hum, then thump?

    Thump, then hum?

    Did you use the speaker outputs from your audio system or the speaker outputs from your audio system?

    Did/does the mains hum happen with something connected or with nothing connected?

    Ground loops are a typical cause of mains hum.

    Hum with this thing alone could be pre or power amp related. I think we determined that this one is power amp related.
     
  18. lemofus

    lemofus New Member

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    Hum, then thump. Fixed most of the hum (by furthering transformer from 10" speaker magnet), fine tuning attempt resulted in thump. No idea where the remaining hum was coming from, but that quickly got overshadowed by the thumping!

    I was using the amp to test things initially, and noticed the horrendous thump would occur each time I switched channels on the home cinema amp (DVD to AUX to TV etc) - regardless of any sound playing or not. Then I noticed that horrendous hump would also occur with no source connected, when the switch was on the "ON" position and I wacked AC power on. I switch the sub on standby, and then immedialy back on (RED LED to GREEN), the thump would be hardly noticeable. The longer I left the sub OFF before reverting to ON position, the bigger the thump.

    The hum occured with nothing connected. No idea if the thing would still hum no though, since then I've complicated things quite a bit bit shorting a few too many things :D

    As you suggested earlier, maybe I can try changing the caps on the pre-amp, the 100pF ones, should be pretty cheap and simple to do... What worries me most at the moment (the thump will get fixed eventually) is how I burnt Q8 and Q11 (I think, it was possibly Q9, it was the BD139) last time I switched the power on with them in there. Stand-by mode was fine, RED was fine, and as soon as I switch to ON, light gizmo went crazy, LED died out and that was that. I'm sorry I'm really not helping with the sidetracking, having added on a few problems along the way...
     
    Last edited: Apr 26, 2011
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    DC on the input feeding the sub will cause this. No question.

    By design. the 2.7 M and 10 uf cap has to discharge. By design, it would take 135 seconds OFF for it not to matter at all.


    There is no delay in the ON mode without adding the 1K resistor across the cur trace. SO this too is expected behavior.

    Now back to hum.

    If you moved the transformer and didn't dress the leads (fancy name for route) you could have introduced hum by routing unshieleded signal leads near AC carrying leads and not necessarily mains.

    Hum in terms of a failure mode generally points to filter caps or regulator.

    In terms of an operational issue: ground loops

    Rules:
    Twisting (signal with return) cables removes EMF interference. Balanced transmission better.
    Shielding removed RF interference. Shields should be connected at the source end only.


    The longer I left the sub OFF before reverting to ON position, the bigger the thump.[/quote]
     
  20. lemofus

    lemofus New Member

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    OK, good to know. So as I suspected, once the sub is fixed, won't be plugging it back into that amp until that is fixed too!

    So, cap issue?

    Thanks, really good to know. I can give those a go once the thing is up and running again.

    Out of precaution, I got replacements for all of the caps on the pre-amp, only a few quids worth of caps; I also have the two necessary spâre transistors, Q1 and Q2, in case those are bad. So all that would be left is resistors on that board, and I can test those one by one. Unless I've missed something I guess we can then say the pre-amp bit will be fine, at least functional!

    How about the power amp section, think we-re ready to tackle this one head on again?
     
  21. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Recap it and get it over with especially C16?
    Replace R41 & the inserted 1K resistor with 10K resistors across the cut trace.

    Measure V across R38 in the OFF position.
    With no signal. Short C16 and measure V across R41.

    Remove short.

    Check delay to LED change from OFF to ON.
    Move to OFF. Wait 3 minutes.

    Move switch to AUTO.
    Apply music. Check delay to LED change.

    R43 and the internal leakage current of C16 allows the timing circuit to discharge when the mains are cut. At 5RC, the cap is about 99% discharged. I can fix that with a J174 FET and a resistor. The resistor across the cut must be used too.

    That ought to keep you busy for a while.
     

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