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DC detection circuit failure on Subwoofer

Discussion in 'Troubleshooting and Repair' started by Durenth, Nov 27, 2020.

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  1. Durenth

    Durenth

    2
    0
    Nov 27, 2020
    Hello,

    First of all sorry for my English and for my poor knowledge in transistor behavior, I’ll do my best to be clear…

    I have a problem with the DC detection circuit of a Panasonic 40W Subwoofer, model SW-P270.
    The relay in the DC detection circuit never turns on thought there is no DC in the audio output signal.
    Output signal is ok, controlled with an oscilloscope.

    I found the service manual (main diagram attached to this post) and measured the voltage as shown below, pink is the expected value indicated in the manual, red is my reading:

    upload_2020-11-27_15-34-16.png

    First of all, I suspected a failure on Q13 (NPN Transistor), so I removed it and got those new readings:
    upload_2020-11-27_15-48-2.png

    Getting closer but still doesn’t trigger the 24V relay and created a problem with the R19 resistor which began to overheat.

    I have multiple questions to ask you:

    1. About the overheating resistor:

    Voltage at B+ being +35V, if Q10 is on then, if I’m not mistaken, there should be a +35V voltage drop at R19.
    R19 being a 330 Ohm / 2W resistor it cannot dissipate U²/R = 35²/330 =3.7W so I must be missing something…

    Could you tell me what I got wrong? There is no voltage drop in the relay right?​


    2. A curiosity question:

    In the schematics I see that Q11 is supposed to be on (Vce = 0V and Ve=0.8V) with only Vbe = 0.5V when I understand it should be at least 0.6V to turn on a transistor… Maybe there’s a tolerance but then why creating a circuit so close to the limits.

    Again, I must get something wrong, do you have any explanation for me?​

    3. About my main problem:

    The schematics says I should get Vbe = 0.7V on Q10, in my circuit I have reading of 0.6-0.1= 0.5V that, I believe is not enough to fully trigger the transitor and explains the 10.6 V voltage drop on Vce.
    I also get lower voltage on many other rails as shown on the second schematics.
    I thought of replacing Q11 and Q10 but I fear there are other problems in my circuit…
    I double checked my measurements with to different multimeter one Fulke and one ChauvinArnoux.

    Do you think changing Q11 and Q10 will do? Then I fear for the over heating R19 resistor…​

    Thanks for your time and for your help, it was a long post!

    Bye Bye
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,999
    2,502
    Nov 17, 2011
    That seems to bee the problem. There should be a voltage drop across the relay. With 0 V across the relay it cannot be activated.
    There are 2 possibilities:
    1. The relay coil has a short circuit - unlikely but not impossible.
    2. The diode D4 across the relay coil has a short circuit. Imho the likely issue.
    With either of the two having a short circuit, the full voltage will drop across R19 and it will get hot.

    The datasheet states Vbe = 0.65 V typical at Ic = 2 mA. But Ic is only on the order of 1 mA and Vbe will be lower. Unfortunately the datasheet doesn't seem to show a graph where one could read a good value. In addition, the values from the schematic should be taken with a grain of salt, i.e. +- 10 % tolerance.

    My guess, apart from a possibly defect relay coil or D4 is that the voltage on the trace on the upper right is incorrect. Lift one leg of R36 and check the voltages on Q10 and Q11:
    upload_2020-11-27_17-58-51.png
    R16 and R34 form a 1:1 voltage divider between the -32 V (top left) and the +12 V rail leading to -10 V at the base of Q13 as indicated on the original schematic. But even wit only -5.5 V Q13 should be definitely off.
     
  3. Durenth

    Durenth

    2
    0
    Nov 27, 2020
    Hi Harald and thanks for your reply!

    So you were right, D4 is short dead...
    I'm currently looking for a diode to replace it as I don't want to power up the board without any flyback diode on the relay..
    I'll probably put a regular 1N4001 or 1N4148 diode, I unfortunately don't have a 24V zener at hand...

    The trace on the upper right is the audio input for the speaker, I controlled it and it seems ok (I looked for unexpected DC and checked the output signal when an audio input is connected to the board via the jack).

    I put Q13 off the board because I thought it was leaking and consequently dropped the expected 1.3V at the base of Q11 to -0.5V.
    Once I'll have my diode replace you recommend me to putting Q13 back in place if I got you well?

    Again thanks for your help Harald !

    Bye Bye
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,999
    2,502
    Nov 17, 2011
    Any of these should work.
     
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