Powered Subwoofer (JBL Venue Sub12) Makes Pop Static Noises When Loud Troubleshoot

Discussion in 'Technical Repair' started by troymario, Nov 7, 2017.

  1. troymario

    Thread Starter New Member

    Nov 7, 2017
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    I have a JBL Venue Sub 12 powered subwoofer that I bought (knowing the amp had some issues). I just want to learn a little about troubleshooting a broken circuit and was wondering if anyone had any tips about what could be wrong with the amp or how I can troubleshoot it. I've tried checking it out with a multimeter and it seems fine, but I don't know all the techniques for checking the circuit. The caps all look like they aren't busted as well. There is a video of the problem posted below. I have hooked the amp up to a different speaker than the sub12 JBL speaker for convenience sake. The same noises are made regardless of which speaker it is attached too.

    Basically when the volume is turned up slightly loud it starts making these popping static noises. There is also a small crack in the circuit board (off R9), not sure if this may be causing the problem. It is shown in the picture below.



    Anyway, I would appreciate any insights. I am happy to attach any other pics.

    The schematic diagram is attached.

    CircuitCrack.jpg
     
  2. JoeJester

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    Apr 26, 2005
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    There is a common connection between Q3 collector, R8, and R9. Use an ohmmeter to ensure there is zero ohms. Make sure you de-energize the equipment and wait sufficient time for the power supply capacitors to bleed off.

    R8-R9-Q3.jpg
     
  3. troymario

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    Nov 7, 2017
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    I took a look at this. There is no resistance in the circuit between those points.
     
  4. JoeJester

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    Does no resistance mean ZERO resistance aka a short?
     
  5. troymario

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    Nov 7, 2017
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    This could be accurate. It read 0 when I checked the resistance.

    I attempted to replace diode D3 as recommended by Harman customer support (after I started this thread) only to realize that the diode I had for replacement was not the right type. I melted the old diode, so the system currently does not make any noise because it is missing a diode. I ordered a replacement diode, so it is on the way. I'm thinking that when I put it back in, the system will start working as normal again (meaning that it may still make weird noises).

    Could this be the reason for the short? Or is that unrelated? I used the test probes between Q3, R8, and R9 (in all pair combinations between the three). To get the reading of 0 each time. I assume that the reading would only be effected by the circuit between the probes, but I am not an electrical expert so I am not sure.
     
  6. JoeJester

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    The short I had you measure was expected. You asked about that burn hole in the board between the collector and R9.

    Once you get a proper diode installed, when you power it up with the volume all the way down, measure the +15V with the negative lead on the ground side of the nearby capacitor and also measure the -15V as well. The meter may read 14.3 and that too will be acceptable.

    Make sure you install the diode in the proper manner.

    Since there was excessive heat in the area of Q3, you might as well check it using the diode function on your meter. If you don't know how that works, the ebooks here can show you how.
     
  7. troymario

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    Nov 7, 2017
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    I installed the diode, and it fixed one of the problems, where the standby would flicker on and off at very high volumes, however the other problem still persists.

    I measured the voltage across C4, C5, C9, and C10 they measured 11.8,11.8,14.8,and 14.9 Volts respectively. Is this what you wanted me to measure? I wasn't sure where to measure the +15V and -15V on the circuit, so I just measured across the caps.

    I also used the diode function to check Q3. Current was able to flow through Q3 and did not produce a short in any combination of tests.

    I am interested to know, what is the general method for finding the problem? Are we just checking for irregularities in voltage / current? And was the 14.3 calculation made from doing circuit analysis or is it on the datasheet somewhere?
     
  8. JoeJester

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    Generally, I would ensure the power supplies we're proper or reasonable (within a couple of percent maximum). If I hear hum, I would check the ripple voltage and test the capacitors.

    Initially, I would make a complete description of the problem. This is important on multiple input and multiple output systems because you can eliminate a lot of the circuits quickly.
     
  9. troymario

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    Nov 7, 2017
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    Ok, as far as the "hum" goes, does that mean a hum coming from the board or power supply? Mine has a hum coming from the supply area.

    Other than that, is there anything else you recommend I check specifically?

    As for as the problem description, i didn't know there were multiple problems until after I started the thread. As far as I can tell, there is something wrong with the part of the circuit that boosts the signal, that being because the system doesn't make any output noise at all when it is on and no signal is being inputted. I could be wrong though.
     
  10. JoeJester

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    Hum, as I remember is a synchronous to the line frequency. This is caused by excessive ripple on the DC supply.

    Do you have a spectrum analyzer or an android app that can display the sound from the speaker?

    Multiple problems can occur, especially if your not the original troubleshooter opening the box. That's why I start with power first. You can check the power output transistors or mosfets to ensure they are not shorted between elements. What was the part number of the diode you replaced and what was the part number of the replacement you blew up? We can check the datasheets.

    Since the diode blew up. I would check the load side of the diode to ensure there were no shorts (zero or very low resistance). This, coupled with looking at the datasheets, can confirm you inserted a much lower rated part into the circuit.

    Once you have the power problem sorted out, you can do the procedures located at page of the service manual.

    While you are waiting for that diode, it's a good time to review the service manual to familiarize yourself with the unit. I would grab the user manual as well for review.
     
  11. troymario

    Thread Starter New Member

    Nov 7, 2017
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    I was able to get it working! Thank you for your help. Turned out it was a dried up capacitor. Cap C12.
     
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