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Thread: KRK Rokit 8 RPG2 head scratcher...

  1. #1
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    KRK Rokit 8 RPG2 head scratcher...

    Hello all, I wonder if anyone can help me here. I've got a Rokit 8 (special black goo edition) that has an intermittent fault in/around the TDA7294 driving the woofer. The low signal only farts through at low volume when its exhibiting the fault (the tweeter signal is unaffected) and this seems to happen more when the room's cold and it hasn't been on for a few hours. I thought there may be a fault in the mute circuit and I've changed out RL105 and RL106 as these were corroded from the black gunk, I've also cleaned the board and replaced some jumper wires that were rusted, but the fault persists and the mute/standby voltages seem ok.
    Tracing the signal it's fine until it reaches the 7294.
    Here are the voltages at the pins of the 7294 when its at fault:

    Pin V
    1 0
    2 0
    3 0
    4 0
    5 n/c
    6 0
    7 35.6 (Ripple seems fine on both rails)
    8 -35.6
    9 6.9
    10 6.6
    11 -35.6
    12 n/c
    13 35.6
    14 0
    16 -35.9

    When it's ok the voltages are all the same except pin 6, the bootstrap pin, which is 15v (I've replaced the bootstrap cap, CL106).
    I've tried replacing the 7294 but the fault persists and I can't see any broken traces or dry joints (but I've reflowed all the joints around there just to be safe).
    Any help would be much appreciated!

    RP8_AMPSCH-V6.pdf Mine is the version with the relay but the power amp section is the same as this schem.

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  2. #2
    Supporting Member Jazz P Bass's Avatar
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    Do you an actual signal at Pin #3 when the amp fails to operate?

    Pin 6 may be a red herring as it only charges when the amplifier is outputting a voltage.

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    Thanks Jazz, I was wondering how that could be. Yes, when its failing I get normal signal at pin 3 up until the amp starts outputing the strangulated waveform, whereby the signal at 3 jumps around slightly (will get photos when it does does it again). But I put this down to feedback via the limiter. I have tried injecting a signal via the ribbon cable from the preamp board, but the power amp still malfunctions, so I guess that rules out the limiter circuit.

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  4. #4
    Supporting Member Jazz P Bass's Avatar
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    Have you changed out CL103/ feedback dc decoupling cap?

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  5. #5
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    I have mate, yes. The original tested ok and there didn't look to be any corrosion on it. I've swapped out CL103,4,5,6 and it's misbehaved afterwards (although it won't now, of course).

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  6. #6
    Supporting Member Jazz P Bass's Avatar
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    Those TDA IC's are pretty much 'plug & play'.

    Hah.
    "special black goo edition"
    Is the black goo conductive?

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