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Old 3rd May 2013, 03:07 PM   #1
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Default Mcintosh C28 Hum

I just picked up a Mcintosh C28 preamp. The line section works fine. In the Aux and Tuner mode there is no hum. When I turn the selector knob to phono, mic or tape head, there is a hum present. So there is a problem in the low level section. I checked the electrolytics on board 044-309 and they are good. Would appreciate any advice!

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Old 5th May 2013, 11:54 AM   #2
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Default ANYONE?

Since the hum is in both channels I replaced the power supply caps. The hum in just the phono, mic and tape head line is still there. Low level section only!
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Old 5th May 2013, 12:15 PM   #3
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Hey I hav the same problem with my Marantz PM-26 amp. Im not bothered about it since i never use the phono inputs.
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Old 5th May 2013, 12:24 PM   #4
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Did you short the inputs to ground when you tested it? Those are very high gain channels, so they may be sensitive to hum pickup.
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Old 5th May 2013, 05:15 PM   #5
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OK...so you're saying to short the phono inputs to see if the hum goes away? If the hum stops, that means what? There was a turntable connected and the hum was there.
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Old 5th May 2013, 05:19 PM   #6
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Just to be clear, I mean to short each one to ground, not to each other.

If there is no hum, then everything is fine.
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Old 5th May 2013, 06:12 PM   #7
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Yes, I shorted the left and right phono input to ground and the hum stops.
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Old 5th May 2013, 06:14 PM   #8
djoffe is offline djoffe  United States
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Just to eliminate assumptions.

May I assume that you have the ground wire from the turntable connected to the ground screw on the preamp?

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Old 5th May 2013, 06:15 PM   #9
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So, that's good news then. Your preamp is probably OK.
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Old 5th May 2013, 11:49 PM   #10
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I have not hooked a turntable up to this but the owner said that he did and with the ground wire connected. He said there was a hum.

Maybe he has a bad ground on the table?
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