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MX110 channel imbalance from Main Out, not Tape Out

Discussion in 'McIntosh Audio' started by monkboughtlunch, Jul 9, 2018.

  1. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    1. Output from "Tape Out" is perfectly balanced from input sources.
    2. Output from "Main Out" is slightly but noticeably louder in the right channel. Can hear this in headphones and set it on scope. When selecting inputs, both the "Aux" and "FM" are slightly too loud in right channel from "Main Out". Haven't yet tested Phono inputs.

    Thoughts on where the imbalance could lie?
     

     

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  2. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    When using "Main Out", if I mono or reverse the stereo signal using the "Mode Selector" control, the channel imbalance still exists, suggesting the problem is occurring after the "Mode Selector" control.
     
  3. c_dk

    c_dk Addicted Member

    Messages:
    6,240
    Location:
    West Michigan
    Basically anywhere in the preamp.......the tape out has no gain if not a phono or tape hd input...... main has all the tone circuits, balance and volume control......
     
  4. Steve O

    Steve O Super Member

    Messages:
    1,167
    Location:
    SE MI
    As already noted, the imbalance could be coming from just about anywhere in the preamp/tone section. Typical are mistracking betw sections of the volume control and balance control where physical center is different from electrical center. I also found imbalance in my early version 110 was originating from the high and low filter PECs where internal resistor values had drifted way off value. I replaced them with little pcbs I made using discrete components.
     
  5. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    Are there any pots on the MX110 that adjust the the L / R output from the "Main Out?"
    Thanks - what are "betw sections" of the control? The imbalance stays they same through the full range of the volume control. So it's not worn carbon.

    If the physical center is different from the electrical center, how did you track this down and how can it be corrected?

    This below photo shows the amount the balance control needs to be rotated left to offset the imbalance and even out the sound.

    IMG_8597.jpg
     
  6. meggy

    meggy AK Subscriber Subscriber

    Messages:
    36,169
    Location:
    Phoenix
    In testing have you swapped the L & R inputs from whatever source you're using?
     

     

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  7. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    Yes - the same imbalance occurs if FM (originating within the MX110) or an external source (such as Aux for example) is selected. Thus the issue appears to be source independent.
     
  8. twiiii

    twiiii Addicted Member

    Messages:
    6,222
    Location:
    west Texas
    Thats not very much and I would be suspicious of the volume controls. All Mac pre-amps before the C-29 had tracking issues. Some models that used two stage volume controls could be horrendous and that especially applies to MX-110's. My MA 230 and C-22 were the worst offenders followed closely by my C-28. Davie Obrien was able to fix the C-22 but after the third control in the C-28 I gave up. He called me when Mac was going to release the C-29 So I got one of the first 50. I was so happy with it I kept it for over 30 years, untill I got my Pre-owned C-34. I wanted a 40 but it wouldn't fit and a C-46 didn't have a 10k control. It might be fun to have a C-22 III for all the tube lovers out there.
     
    monkboughtlunch likes this.
  9. gkargreen

    gkargreen Active Member

    Messages:
    480
    Location:
    Fairfax, VA
    Going thru the same problem, a huge difference (50%) between the rt and lf channel out, shows up around the vol pot but that pot is tracking much better than the difference in volume, still puzzling it out, thinking now is the loudness tap circuit...
     
  10. gkargreen

    gkargreen Active Member

    Messages:
    480
    Location:
    Fairfax, VA
    OK, found my issue, its with the loudness taps as when the wiring to the loudness taps are disconnected, the volume tracking is greatly improved. That would seem to suggest that the caps and/or resistors that connect from ground to the loudness taps are wonky. You will have a great deal of difficulty if you do not have a scope and a sig gen to trace out the problem.
     
  11. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    Good troubleshooting. I'm not seeing any difference in stereo balance when any of the slider/rocker buttons are "in" or "out." Was the loudness issue noticeable when you rocked the switch back and forth -- or only when you desoldered the taps?
     

     

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  12. kaplang

    kaplang Works for me ! Subscriber

    Just rotate the knob on the shaft so it’s centered when the volume sounds the same from both channels. That should do it!
     
  13. 62caddy

    62caddy Trust but verify Subscriber

    Messages:
    9,463
    Location:
    NEPA
    The knob position cannot be changed relative to the shaft on this model. I think most McIntosh balance controls are designed this way.
     
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  14. c_dk

    c_dk Addicted Member

    Messages:
    6,240
    Location:
    West Michigan
    Without a scope and a analyser you could be guessing for weeks......

    Have you moved the tubes from channel to channel.....cleaned the tube sockets?

    Is it confined to one frequency band ie, bass treble ok but out of balance in the midrange?

    Most likely tech time......
     
    monkboughtlunch likes this.
  15. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    I swapped V16 and V17 -- no difference. I listened on headphones -- doesn't appear to be confined to a frequency. No change when bass, treble, loudness, LF, HF controls are adjusted.
     
  16. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    Interesting. On headphones I do hear some slight stereo image shift. Seems like the image shifts to the right somewhat above the 3 o'clock position. Not sure if this is the root issue or not.

    Does McIntosh make replacement volume pots designed for the MX110 today? Or is the best approach to go aftermarket -- like an Alps pot?
     

     

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  17. Steve O

    Steve O Super Member

    Messages:
    1,167
    Location:
    SE MI
    A few years ago I had a problem with the volume pot in the MX110 I was restoring: bad loudness taps causing imbalance and other strange behaviors. At the time Mac had nothing. I found a NOS repl part @ Audio Classics...last one they had. It didn’t track perfectly but better than what was there.

    The problem with finding a replacement now is that the physical configuration, resistance value and loudness taps combined with the tight fit on the front panel eliminate just about every modern possibility unless you’re willing to compromise on a couple of characteristics. There may be a resource out there that can rebuild pots like this but hopefully your issue is not with the volume or balance pots.
     
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  18. kaplang

    kaplang Works for me ! Subscriber

    Audioclassic rebuilt the balance pots of my MX110 couple years ago at a cost of $125.00. The audio taper is not the same as the originals but it was the best option I had at the time and it’s close enough.
     
  19. monkboughtlunch

    monkboughtlunch Super Member

    Messages:
    1,812
    What were the symptoms of the defective pot?
     
  20. kaplang

    kaplang Works for me ! Subscriber

    Channel cutout. Deoxit didn’t help. Tried several times. I believe the wiper wore away some of the carbon from constant use.
     
    rollcentre and monkboughtlunch like this.

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