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Old 30th January 2013, 04:52 PM   #1
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Default Sources of 120hz hum?

I have a Chinese amp (yaqin ms-20l) that has recently developed a slight 120 Hz hum in the right channel. What would be the likely source of the noise?
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Old 30th January 2013, 04:54 PM   #2
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Loose screw on a mounting lug, bad connector connection. Be sure to swap inputs.
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Old 30th January 2013, 05:03 PM   #3
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Hum remains if I remove the preamp tubes, so I guess that narrows it to the power amp side of the circuit? Bias is fine as well.
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Old 30th January 2013, 05:14 PM   #4
TheGimp is offline TheGimp  United States
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Possibly shift in bias of the output tubes. This causes the PS ripple rejection to go down and you get more hum.

How did you check the bias?

Otherwise, it could be a bad output tube. Swap them between the two channels.
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Old 30th January 2013, 05:56 PM   #5
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Looks like it's a noisy tube, thanks Gimp!
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