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Sunn Beta Bass steady-rate popping

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  • Sunn Beta Bass steady-rate popping

    Sunn Beta Bass amp head. Two channel inputs with a third combiner input to access both channels.

    Channel "A" works fine and output power seems normal. When plugging a 1/4" plug into either the "B" input jack or "both inputs" jack I get a steady-rate popping though the speaker. Sounds like a capacitor charging/discharging. Here are some notes on what I've done and some observations:
    • "Drive" control for Channel "B" AND "Master Volume" both will mute the popping noise
    • Increasing "Drive" control for Channel "B" to near max will eliminate popping noise, but introduces a fair amount of hiss
    • Patching a jumper cable into one or both FX loops makes no difference
    • Injecting a signal into the FX loop "Return" on channel "A" produces normal output
    • Injecting a signal into the FX loop "Return" on channel "B" produces minimal output, barely audible
    • Replaced out-of-spec C22 capacitor adjacent to channel "B" drive control
    • Tested both C107, C120 (680/25) filter caps on preamp board. Clipped in a cap in parallel across each to verify under voltage - no change
    • Tested both C16, C17 (3300/50) caps on power board. Clipped in a cap in parallel across each to verify under voltage - no change


    The attached schematic is a bit blurry, but it's the only one I could find. The troubleshooting guide suggests IC's 109 and 111 as possible culprits. I'll check some voltages at the test points and post them shortly.
    Attached Files

  • #2
    I would be looking at C112, Master Level IC.

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    • #3
      The weakest link in these amps is those 2.2uF 25V NP electrolytic caps. The most telling symptom to me is:
      Injecting a signal into the FX loop "Return" on channel "B" produces minimal output, barely audible
      That points to C135. It might not be the only bad component, but that should get the channel B "From Accessory" working. I highly suggest that you replace all those 2.2uF caps. Also, check all the solder connections to the pots.
      WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
      REMEMBER: Everybody knows that smokin' ain't allowed in school !

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      • #4
        The "B" channel will actually work if the "Drive" is increased to somewhere between 80-90%, but any other setting and it produces the steady popping that to me sounds like a cap charging and discharging. The popping occurs even with an unterminated dummy plug inserted into either the "B" input or "both inputs" jacks.

        Channel "A" works normally.

        I ordered 5 of those 2.2/25 NP caps and will swap them all and report back.

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