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MC-2105 right channel barely audible - meter and headphones fine

J

Jason Berk

New Member
I am hoping for some advice on my MC 2105 that's not functioning correctly.

The right channel suddenly became extremely quiet. Abnormally quiet. When the volume is turned way up on that channel you can hear the music but it's extremely distorted and very, very quiet. The left channel is working correctly. This behavior remains the same even when swapping left and right inputs on the preamp. In addition, everything functions correctly through the headphone jack and the meters appear to be functioning as expected as well. The problem is just on the output to the right speaker (regardless of which ohm terminal is being used).

I tried swapping the driver boards from left to right and see if one of the boards was bad. When I performed the swap, both channels started working again. I thought I lucked out and put the amp back together and started using it. After about 1 hours right channel failed again and now I'm stuck.

Any thoughts on where I should start diagnosing?
 
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Hmmm ... have you worked the speaker ON/OFF switch as well as the DIRECT/NORMAL switch on the rear panel? Both are known culprits as the amps age.
 
I have, but wouldn’t those two affect both channels simultaneously, not just a single channel? The left channel works perfect and the right isn’t totally off, just extremely quiet.
 
Thanks for the recommendation, but I swapped driver boards from the left channel to the right channel and the problem persists. Perhaps an issue on the power output section?
 
Jason Berk said:
I have, but wouldn’t those two affect both channels simultaneously, not just a single channel? The left channel works perfect and the right isn’t totally off, just extremely quiet.
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If there's some oxidation on those switches one channel could be affected and not the other - they have separate contacts. Maybe try cleaning the switch contacts with DeOxit if you can get at them.

Also, did you verify that you don't have an intermittent short in your right speaker or right speaker cable?
 
I though about that as well, but McIntosh, when I spoke to them on the phone didn't think there were relay's in these late model 2105s. I can't find one, unless it's hidden somewhere and I'm just an idiot (very possible). If it was the relay though, wouldn't all sound be cut off to the speaker. I'm still hearing sound, just VERY low and distorted.
 

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OK. I cleaned the speaker switch and it worked great for a few minutes and now crapped out again. I cleaned again but no resolution. Same problem as before. Could this be an issue with the driver boards?
 
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It's such a great machine. I'd advise to take it to a reputable tech if you're not equipped and experienced, as I am, not. If deoxing the pot helped, then didn't, I think the problem is there. But I don't know, really. So I'll shut up and watch the thread. All best.
 
There are several ways that an MC2105 can die a smoky death, have it checked out including the bias resistors and driver board update as well as a proper re-cap.
 
I don’t understanding what the OP is suggesting. Tinker Man already showed in the schematic that if both channels work fine all the time, then the problem is after that, and can not be the driver boards, meaning either the speaker switch or the autoformer is suspect. The speaker switch is a known problem, and people have jumpered it out intentionally for this very reason, as I believe there are no replacements available.
 
perryinva said:
I don’t understanding what the OP is suggesting. Tinker Man already showed in the schematic that if both channels work fine all the time, then the problem is after that, and can not be the driver boards, meaning either the speaker switch or the autoformer is suspect. The speaker switch is a known problem, and people have jumpered it out intentionally for this very reason, as I believe there are no replacements available.
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I agree with this. Try a jumper on the switch if you are comfortable doing so.
 
Just an update. Took it George Myer AV, our McIntosh authorized service center here in LA. They diagnosed it as needing a driver board update and new capacitors. Well guess what. All things replaced per their instructions, full driver board update and still DOESN'T work. I've checked voltages all the way down the line and everything is seemingly fine. Obviously not the caps because they've all been replaced. Jumpered speaker switch and no dice same problem. How do you test the audioformer? Is that worth looking into?
 
Jason Berk said:
Just an update. Took it George Myer AV, our McIntosh authorized service center here in LA. They diagnosed it as needing a driver board update and new capacitors. Well guess what. All things replaced per their instructions, full driver board update and still DOESN'T work. I've checked voltages all the way down the line and everything is seemingly fine. Obviously not the caps because they've all been replaced. Jumpered speaker switch and no dice same problem. How do you test the audioformer? Is that worth looking into?
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Did you ever rule out the possibility of an issue with the right speaker by hooking it to the left channel and hooking the left speaker to the right channel? Did you verify that you have no shorts in the right speaker wire?
 
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