Mcintosh C28 Hum

Rudes · 3rd May 2013, 03:07 PM

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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Rudes · 5th May 2013, 11:54 AM

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!

Vintage Amp Builder · 5th May 2013, 12:15 PM

Hey I hav the same problem with my Marantz PM-26 amp. Im not bothered about it since i never use the phono inputs.

dirkwright · 5th May 2013, 12:24 PM

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.

Rudes · 5th May 2013, 05:15 PM

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.

dirkwright · 5th May 2013, 05:19 PM

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.

Rudes · 5th May 2013, 06:12 PM

Yes, I shorted the left and right phono input to ground and the hum stops.

djoffe · 5th May 2013, 06:14 PM

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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dirkwright · 5th May 2013, 06:15 PM

So, that's good news then. Your preamp is probably OK.

Rudes · 5th May 2013, 11:49 PM

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?

3rd May 2013, 03:07 PM	#1
Rudes diyAudio Member Join Date: Sep 2009	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! Mcintosh Diagrams, Schematics and Service Manuals for FREE

5th May 2013, 11:54 AM	#2
Rudes diyAudio Member Join Date: Sep 2009	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!

5th May 2013, 12:24 PM	#4
dirkwright diyAudio Member Join Date: May 2011 Location: Virginia	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. __________________ http://www.jeffersonsudburyschool.org

5th May 2013, 05:19 PM	#6
dirkwright diyAudio Member Join Date: May 2011 Location: Virginia	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. __________________ http://www.jeffersonsudburyschool.org

5th May 2013, 06:15 PM	#9
dirkwright diyAudio Member Join Date: May 2011 Location: Virginia	So, that's good news then. Your preamp is probably OK. __________________ http://www.jeffersonsudburyschool.org


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5th May 2013, 12:15 PM	#3
Vintage Amp Builder diyAudio Member Join Date: Feb 2013	Hey I hav the same problem with my Marantz PM-26 amp. Im not bothered about it since i never use the phono inputs.

5th May 2013, 05:15 PM	#5
Rudes diyAudio Member Join Date: Sep 2009	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.

5th May 2013, 06:12 PM	#7
Rudes diyAudio Member Join Date: Sep 2009	Yes, I shorted the left and right phono input to ground and the hum stops.

5th May 2013, 06:14 PM	#8
djoffe diyAudio Member Join Date: Oct 2008	Just to eliminate assumptions. May I assume that you have the ground wire from the turntable connected to the ground screw on the preamp? Akitika GT-101 Audio Power Amplifier Kit Update My Dynaco

5th May 2013, 11:49 PM	#10
Rudes diyAudio Member Join Date: Sep 2009	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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