Crunchy C28 phono stages

[efhjr]

The two phono stages in my C28 are making a lot of noise, like a crunchy white noise. The noise is equally loud in both stages.

Considering how loud the noise is, I'd figure there's correct power going to the stages. Could there be a bad resistor in there?

Sound output through headphones does not exhibit the same crunchy noise; the sound is good, but there is some irregular low-level popping about every minute or so.

 

[jlovda]

Bad tantalum caps.

 

[AbSoundLabsMN]

From your description am I to understand that, outside of a popping, this issue is not present when you use the headphones. If that is the case, the issue is probably not the actual phono stage. What happens if you disconnect your turntable and then turn on your C-28? Is the noise still present?

Mark
McIntosh Factory Restoration Facility

 

[efhjr]

From your description am I to understand that, outside of a popping, this issue is not present when you use the headphones. If that is the case, the issue is probably not the actual phono stage. What happens if you disconnect your turntable and then turn on your C-28? Is the noise still present?

I've got two problems:

1. loud crunchy noise in phono stages, and
2. popping noise in other inputs (in this case, the Aux input when I listen to a CD player through headphones)

There could be popping in the phono stages, but the white noise is so loud I can't hear it. The phono noise happens whether a turntable is connected or not.

 

[dshoaf]

Yep, been there, done that. The power supply is the culprit. Logic is that both channels exhibit the same problem so the common point is the power supply. All electrolytics gotta go and swap out those diodes while you're at it. It isn't a conventional power supply design so pay attention to the regulator circuit.

Next, as some of the venerable Mac bench techs have noted, the ground drains are a mess at this age. That will contribute to a higher than normal noise floor once you get the PS fixed up. You can't hear that problem yet.

Let me tell you, if you're willing to put the work into it, the C28 is a great preamp. It was Mac's flagship product in its day.

Cheers,

David

 

[efhjr]

Let me tell you, if you're willing to put the work into it, the C28 is a great preamp. It was Mac's flagship product in its day.

Thanks for the great info, David and all you others.

I'm not technically qualified to do this kind of work, I'm afraid. I'm pleasantly surprised at the sound quality of this C28 and would like it to function optimally. But I don't know of any technicians who can do this work, and wonder what it'll cost.

McIntosh can be a cruel mistress, I tells ya!

 

[dshoaf]

Audio Classics or Terry DeWick are your guys. The good ones are busy so plan on standing in line!

Cheers,

David

 

[efhjr]

Audio Classics or Terry DeWick are your guys. The good ones are busy so plan on standing in line!

Calling around, I see I'm looking at between $500 and $600 for a full rebuild. I'll start saving my pennies and get this thing fixed sometime soon.

Thanks again for the help, gents.

 

[efhjr]

Now here's a funny thing: the noise is mostly gone.

I decided to let the thing just burn in, so I kept it on for about a week. And now I'm not hearing the popping and the phono stages aren't making that noise.

Granted, the noise could all come back, but hey, it sounds good and I'm enjoying listening to LPs with it.

Could just running some power through it have fixed it?

 

[slowpat]

Now here's a funny thing: the noise is mostly gone.

I decided to let the thing just burn in, so I kept it on for about a week. And now I'm not hearing the popping and the phono stages aren't making that noise.

Granted, the noise could all come back, but hey, it sounds good and I'm enjoying listening to LPs with it.

Could just running some power through it have fixed it?

That's just a temporary reprieve. Continue with saving your pennies, and take up a paper route on the side.

 

[twiiii]

I owned a C-28 and though I complained about its issues, when working was a rather sweet unit. If you want a warm smooth phono stage the C-28 is a winner. Where a C-29 is a touch quieter, more revealing, with tight bass, and works well with percussive instruments, a C-28 will make you think you are at an actual concert with a lot of recordings. Command recordings always sounded great if you can get your head to forgive some of the ping pong effects. C-28 also calmed some of Phase 4 London recordings that could be edgy. The 28 was an easy pre-amp to listen, too. Friends of mine used 28's with a Hafler D 500 in one case and another was using a 28 with his 7200 for years with Mac speakers . The first used a Grace F9E and was a fantastic analog performer. There isn't V-15 or Stanton that can match the combination. The other gentlemen was using Ortofons with a linear tracking arm and the sound was superb, but could not match the Grace. So a 28 is no slouch and if you have the divider panel on the phono circuit board you will have definite channel separation superiority. A C-28 is sure to please. But you need to have revealing amps and speakers to allow the pre-amp to show thru. Any clouding will really handicap a C-28. C-28's like cartridges with a lot out output. Certain Ortofons with step up transformers could make C-28's shine, a MC 30 for instance. With a Sl 15 Mk II the response was flat to beyond 30 kHz. The combination with a little 2 db dip from 1600 to 5 kHz as I remember could really help calm Altec, Klipsch, and Ev horn speakers. It even calmed my Concert Grands 209's a bit before I got room Equalizers.

Why did I get a C-29? The mis-tracking volume control, the intermittent low level input circuit connectors, and the alignment issues of the input select shaft that could cause intermittent failure of one channel on some inputs and the other channel on other inputs of the C-28. Getting the alignment right for consistent operation with the long control shaft could be an issue. The trim controls on top of the unit always needed cleaning . In that respect my C-34 is the best preamp I have ever had. Switching is quiet and the variable controls have never been an issue. But like the C-28 the stepped tone controls of the C-22, C-29, 26 and 27 were always noisy.

My former C-28 today is being used with a MQ 104 feeding a MC 2200 pushing a Pair of ML-2C.

 

[efhjr]

I owned a C-28 and though I complained about its issues, when working was a rather sweet unit.

Thanks for the great insight, twiiii.