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C28 Humming

Robel

Robel

Active Member
My C28 preamp has developed a hum. Took it to the AnalogStore in the Raleigh area, but Alex could not find the source of the humming. My question is, "does anyone out there know where I can get this serviced?" I'm in North Carolina and have run out of options here. I've tried the DeOxy route before taking it to the AnalogStore, but of course that didn't work. Anybody know of a qualified tech who is good at servicing old McIntosh equipment? Thanks!
 
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I don't know what's available in our area, but if you can't find a local tech and have to ship it, make sure you pack it very well as shippers will do their best to break it - especially the glass.

These folks in upstate NY are very highly recommended. They can give advice on packing and shipping as well.

Audio Classics
 
So, how do I overcome that issue? Run a ground wire to the amp? I'm not a technical guy. But the tech who worked on it, couldn't find the source of the problem. I understand the volume pot goes bad on these units.
 
Volume pots wear out, different symptom.

Grounding can be at the inputs on the back panel, also there are problems with the grounds/shields on the shielded internal cable, documented here, some C28 searches will bring up results.

Getting a C28 right can be tricky, don’t leave it with someone who is not very experienced in McIntosh C28 service or you will regret it.
 
SaturationPt said:
Volume pots wear out, different symptom.

Grounding can be at the inputs on the back panel, also there are problems with the grounds/shields on the shielded internal cable, documented here, some C28 searches will bring up results.

Getting a C28 right can be tricky, don’t leave it with someone who is not very experienced in McIntosh C28 service or you will regret it.
Click to expand...
Thanks for the info, Jeff. The trick will be finding that experienced tech.
 
I know, it’s getting harder unfortunately.

Many people here have given up and just ship to Audio Classics, but there are success stories and there might be a good McIntosh Tech in N.C., hopefully it will work out.
 
I have always heard about grounding issues causing this in this unit. This would be for a low background hum that follows the volume knob. You could try some simple fixes if you are not comfortable going too deep into the wiring. Sometimes you can reverse the plug at the wall outlet and find a position that is quieter. Shorting plugs on the phono section inputs may help.
If the hum is a loud 120 Hz hum you may need to replace the can capacitors.
 
SaturationPt said:
I know, it’s getting harder unfortunately.

Many people here have given up and just ship to Audio Classics, but there are success stories and there might be a good McIntosh Tech in N.C., hopefully it will work out.
Click to expand...
I'll keep looking. Will keep in mind the Audio Classics route. Thanks!
 
Refugee said:
I have always heard about grounding issues causing this in this unit. This would be for a low background hum that follows the volume knob. You could try some simple fixes if you are not comfortable going too deep into the wiring. Sometimes you can reverse the plug at the wall outlet and find a position that is quieter. Shorting plugs on the phono section inputs may help.
If the hum is a loud 120 Hz hum you may need to replace the can capacitors.
Click to expand...
The fact that the tech couldn't find the source of the issue has me thinking this is probably a pretty tough nut to crack. He was really good at fixing my other Mcintosh gear.
 
FWIW, maybe try completely unhooking the amp, and any other connected devices from the preamp and plugging in head phones in its HP jack and seeing if the hum is still present which in doing so would eliminate any externally connected ground loop causing devices, if they are in fact the cause of the hum.

Be aware that if the preamp is selected to the phono inputs while the preamp`s volume is turned up during this checkout without shorting RCA plugs inserted will usually result in some hum, as would be expected due to the high gain nature of that circuit.

If you still have your hum, then there is internal grounding issues, and or power supply capacitor, component failings, especially if the caps are original factory.

Good luck on getting this fine Mc. preamp`s issues resolved, Robel, Sir.
 
Bill Ferris said:
FWIW, maybe try completely unhooking the amp, and any other connected devices from the preamp and plugging in head phones in its HP jack and seeing if the hum is still present which in doing so would eliminate any externally connected ground loop causing devices, if they are in fact the cause of the hum.

Be aware that if the preamp is selected to the phono inputs while the preamp`s volume is turned up during this checkout without shorting RCA plugs inserted will usually result in some hum, as would be expected due to the high gain nature of that circuit.

If you still have your hum, then there is internal grounding issues, and or power supply capacitor, component failings, especially if the caps are original factory.

Good luck on getting this fine Mc. preamp`s issues resolved, Robel, Sir.
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Bill, thanks for your reply. I actually did try that. The hum is still there listening through phone with just a CD player plugged into it.
 
Well I don't know where you are in NC, but we have a very good tech here in Williamsburg VA. he has worked on my C28s. Replaced volume controls and fixed a hum problem. People bring gear to him from further away than you are. Store is Action Electronics. Guy is Bob Phone# 757 229 1880. Call him.

On an older electric plug that has non-polarized spades with no ground prong, its always a good idea to try reversing the plug. You can try that on any piece of gear equipped that way. I had a hum on a Thorens TD125 and reversing the plug solved the problem believe it or not.
 
The neutral (larger slot/blade on a 120V outlet) is bonded to ground, somewhere (depending on your home’s age) whether it is in your electrical panel, at the meter, and certainly at the transformer outside. This means that in equipment one side will be closer to ground potential than the other, reversing the plug can make this better/worse in older equipment.

Also, having the chassis ground or signal ground on one conductor in one piece of equipment (ex: RCA cable shield) and the other conductor in a connected piece of equipment (ex: the RCA cable center conductor) can cause hum so reversing the plug on either one can remove this mismatch.

It isn’t anything I see in modern home-audio equipment but prior to the polarized plugs (1970s? 1980s?) and some code/UL changes it was fairly common. In pro equipment that might be plugged into multiple power circuits it is common to see a “ground lift” switch to eliminate hum at times.
 
jcidave said:
Well I don't know where you are in NC, but we have a very good tech here in Williamsburg VA. he has worked on my C28s. Replaced volume controls and fixed a hum problem. People bring gear to him from further away than you are. Store is Action Electronics. Guy is Bob Phone# 757 229 1880. Call him.

On an older electric plug that has non-polarized spades with no ground prong, its always a good idea to try reversing the plug. You can try that on any piece of gear equipped that way. I had a hum on a Thorens TD125 and reversing the plug solved the problem believe it or not.
Click to expand...
Thank you for the contact. Good to know that he was able to fix the volume control. I've heard that is a tricky fix. I'll give him a call.
 
SaturationPt said:
The neutral (larger slot/blade on a 120V outlet) is bonded to ground, somewhere (depending on your home’s age) whether it is in your electrical panel, at the meter, and certainly at the transformer outside. This means that in equipment one side will be closer to ground potential than the other, reversing the plug can make this better/worse in older equipment.

Also, having the chassis ground or signal ground on one conductor in one piece of equipment (ex: RCA cable shield) and the other conductor in a connected piece of equipment (ex: the RCA cable center conductor) can cause hum so reversing the plug on either one can remove this mismatch.

It isn’t anything I see in modern home-audio equipment but prior to the polarized plugs (1970s? 1980s?) and some code/UL changes it was fairly common. In pro equipment that might be plugged into multiple power circuits it is common to see a “ground lift” switch to eliminate hum at times.
Click to expand...
Seems the problem has grown. Since I brought it home from being serviced, it no longer plays loud enough to hear a hum. Volume is turned all the way up and is barely audible through the speakers and headphones. Checked all the levels on the top and they are where they are supposed to be. Just out of curiosity. It seems you know quite a bit about electricity. If the problem were related to grounding and reversing the plug could fix it, is the workaround still valid if using a line conditioner such as the Furman, which I am. Seems the Furman should handle the grounding. All the other components connected to it seem to work well without humming.

Thanks for your feedback. Much appreciated.
 
It no longer sounds like a simple power plug problem but:

Have you tried a different input / different cables? How about cycling all of the switches on the front panel as well as those on the top? If a switch or pot is making poor contact you will hear it when you start to move the (culprit) switch. Both channels it sounds like so probably not a cable or RCA jack, but try everything.

As far as a line conditioner, typically a good idea, especially if you have unusually noisy power. However, most line conditioners do not have an isolated ground, I don’t know if the Furman you’re referencing does.

Look elsewhere to find the diminishing output, might be the same source / might not but it certainly does need to be addressed. If cycling switches and pots, changing sources and cables, do not affect the symptom it is time for a qualified service Tech.
 
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