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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:
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.
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
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