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C-28 MC2105 Help

L

Lamont

New Member
Hello gang,

I've done some searching on the forum, but there is so much information to sort through I thought I'd start a new thread - apologies if this is an easy deduction or has been asked before.

I inherited these Macs and had them refreshed about 4 years ago, but I don't think they did a good job...

Here's my big issue: one of the channels sounds great and the other sounds drastically different. Super muddy like I'm listening to the music through a wall. Any ideas what I can do to address this? I'm pretty novice to this stuff so thanks for any help you might have. c_dk, are you still around? You were tons of help last time I had issues...

Thanks again!
 
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Have you tried switching speakers around to eliminate them as the problem? At the back of the amp swap you L&R speaker wires to see if the issue follows the speakers.

If it's not them try cleaning the amp speakers binding post and strip clean wire.
 
That's the same gear I had with my first Mc system. You might be having problems with the C28.
I never had that symptom with the 2105.

As 4-2-7 suggested, make sure it isn't a speaker problem first.
 
Yep. Do the easy stuff first like 427 said and clean the back panel switch with DeOxit as well as the volume pots. You'll have to flip it over and take the bottom off.

While you have the bottom off, remove the cage screws, flip it back over, pull the driver boards and clean the contacts with same stuff.


Murray
 
Let's assume they did a good job, took care of the basics, and something else has happened.

By swapping the speaker outputs as well as the preamp outputs, determine which piece is causing the sound differences. Could easily be a dirty card contact, or some other minor problem, but first figure out which piece is causing the problem.
 
Flush the loudness compensation switch on the C28, a good place to start with DeOxit D5. Four years is long enough for unused switches to oxidize and cause problems, especially if not used.
 
I will suggest something so obvious, sometimes it is missed with the C28. On the top there are a row of small controls. Turn them to max so all are fully on. On the amp, turn the volume controls fully up so the volume is controlled wholly through the volume control of the C28. Does it make any difference. Then turn the amp's volume controls down about 3/4rds. This will require turning up the preamps volume control to a higher level. Did this change anything? Because of these topside level controls that are not usual, many new owners do not do anything with them without realizing they are not adjusted equally or properly in some manner. A great idea but, a pain when we sold them and new owners did not spend anytime reading the manual.

The 2105 is easy to test by itself as it has volume controls. Connect your tuner or digital player to the inputs of the amp and use its volume controls to control the volume. Did this clear up the problem. If yes, then you need to dig into the preamp. If not then likely the 2105 is the issue.
 
Thanks of much for the help everyone.
I have ruled out the speakers as being the problem.

1. I found a vendor for DetoxIT and picked some up today. Will follow 4-2-7 and skippy_ds instructions tomorrow.
2. dewickt, how do I "flush the loudness compensation switch"?
3. Brian, I read over your suggestions and I'm stumped. I admit I'm a total newcomer and novice. I tried your first suggestion of adjusting those top dials, but no change. I can't see how the 2105 has any volume controls, though. Sorry and thanks if you can clear it up for me!
 
Is the problem in the preamp or power amp? Swap the pre amp outputs to see if the problem switches sides.
 
I swapped the L/R inputs on the back of the 2105 power amp and the problem moved from one speaker to the other.
The problem moved from one speaker to the other.
Does this tell us the same thing?

PS - these things are HEAVY!
 
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Really until you do some cleaning of contacts we will all be guessing.

I don't think anyone asked did this problem just all of a sudden happen? Has it been slowly happening?
To me it sounds like a dirty contact.
 
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Lamont - since you swapped the interconnects on the rear of the MC2105 and the problem changed sides, this is an indication that the problem lies upstream of the amplifier. Think about that and it will make sense.

I'll also recommend that you have both units professionally serviced in an effort to get the problem fully resolved. The C28 happens to have more switches, dials, knobs, and levers than any other McIntosh preamp and some of them are somewhat difficult to access let alone clean properly. The cluster of selector switches is particularly difficult to service. In addition, C28s are also noted to have grounding problems that creep up with age.
 
So the issue is being caused by the preamp. Time to manipulate all the controls, including those on the top, to see if any are causing, or change the issue. Of coarse if a particular switch affects the issue, Deoxit is called for. Use fader lube, green can, for potentiometers.
 
I manipulated ALL the controls of the C-28.
Nothing seemed to make any change in the issue whatsoever. I discovered that the left channel "BASS" adjustment switch caused some popping and static, but no change in my main issue.

I've got the bottom off now...

Thanks again for walking me through this, gentlemen...


PS - I added some extra pics of my setup for all to see if I'm doing something else dumb. Never hurts to ask!
 
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Wanted to pop back in and let you guys know that I appreciate all your help.
After following all of the suggestions (including Brians to simply connect a digital player to the 2105 itself), it's clear that the C-28 is the culprit and needs professional help.
Local company quoted me $300 - $400 for servicing - this is in Canada. Does that sound about right? Because it sure sounds expensive to me...

Thanks again.
 
Being around here (specifically in the Mac forum) the relatively short while I have, I fully concur with damacman's comment. That thing is complex.

You can always send it to Terry at Macintoshaudio.com. You will pay a fair price (not the cheapest price) for the best McIntosh service there is.
 
I should have remembered. The volume controls on the MC2105 are labeled Gain. From the photos, I problem could be yours out world is upside down :) .

It looks like someone has already been into the C28. I would send it to Terry. These are a bit of a bear to work on as they have long signal paths due to the topside controls and from experience it does not take much harness movement to introduce more noise. These units benefit from competent repairs and will give decades of great listening joy.
 
I assume you have corrected the output level difference as shown in the top view.

The bass and treble controls are always in the circuit of the C28. A claim of a "muddy" sound causes me to suspect a problem there.

To do a C28 correctly can easily take 6 hours.....local labor rates apply.
 
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