MX120 seems confused

Discussion in 'McIntosh Audio' started by smartin53, Apr 15, 2018.

  1. smartin53

    smartin53 Active Member

    Messages:
    487
    Location:
    Denver
    So when it rains it pours. While working on other gear my MX120 seems to have become confused.

    It started playing audio randomly from one channel or another, volume fluctuates, and at one point the front panel buttons stopped working. Different inputs behave differently but odd. Right now I’m getting digital audio from the CD player...ok just cut out again...and now just quietly from one channel. Front panel buttons seem OK still.

    I’ve tried doing an all settings reset from the menu but no joy. Also completely disconnected the unit from all power and connections but that only gave me the front panel buttons back. Is there a more thorough method? There doesn’t seem to be a service manual out there for the MX120 that I can find. Thanks
     

     

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  2. smartin53

    smartin53 Active Member

    Messages:
    487
    Location:
    Denver
    Ok found the service manual at least. Not sure why it didn’t come up on my first search. We’ll see if it helps any.

    Update: NOPE! Didn't really assist in this particular instance. I also tried the 2 minute "microprocessor reset" mentioned in the manual to no avail. The way it glitches it almost seems like there is a suspect charge somewhere in the unit that's playing havoc with the thing. I'll let it sit unplugged overnight and see what happens.
     
    Last edited: Apr 15, 2018
  3. Yamaki

    Yamaki Not For Hire Subscriber

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    3,693
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    Any luck with the long power down?
     
  4. smartin53

    smartin53 Active Member

    Messages:
    487
    Location:
    Denver
    Unfortunately not. Maybe 8 hrs just isn't enough time to dissipate charges. I have a bad feeling when I was testing the MR55 that something wrong with the tuner might have zapped something on the input PCB (and god knows where else from there). The reason I feel there is an abnormal charge somewhere is that is wreaking havoc is that I lost button control again, but disconnecting and reconnecting the front panel reset it to working order.

    Signals seem to be getting in, especially clean digital ones, but getting them back out is the issue. Audio directly into the input board is distorted and quiet when output on both the unbalanced and balanced outputs. Audio coming from digital signals is either just dead, comes in fine, or flips between channels before settling on quiet output from the right channel. I'm gonna run a few more functional tests.

    I'm going through the schematic now trying to find the best way to test this guy. IC based stuff is no fun. Also, I wanna shoot whomever decided that an adhesive strip under the top cover and under the lip of the face panel was a good idea. That was not fun to get loose.
     
  5. smartin53

    smartin53 Active Member

    Messages:
    487
    Location:
    Denver
    In addition to the above, one behavior I noted now that I spent more time with the unit was regarding inputs/outputs and how they behaved. Output from the Balanced and Zone A Front outputs would work assuming a digital input was being fed, but the behavior was about 5 minutes of silence, followed by each channel kicking in at normal volume, and then each stepping down to a lower volume and staying there. There is no input or output from the main input PCB which is all analog.

    As such I decided to run through a process of elimination panned out quickly. To a certain extent the unit acts like a PC where you can disconnect components and others keep working. The first thing I did was disconnect the main I/O cable from the J9 connector on the input PCB which powered it down and took it out of the loop. Immediately the unit returned to normal behavior audio wise. Guess now I need to figure out where the failure was on the input PCB. Wouldn't be much of an issue except I can't use my record player right now.

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  6. ron-c

    ron-c AK Subscriber Subscriber

    Messages:
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    Check power supply voltages.

    Ron-C
     
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  7. feinstei

    feinstei AK Member Subscriber

    Messages:
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    I would call McIntosh and ask to speak to a service technician (not customer service). One of them may have run into a similar problem and could give you a lead as to which IC circuits might be destroyed. Look at my thread from about two years ago regarding similar issues with my MX-119. Under the advice from one of the McIntosh service technicians, I replaced 3 IC's on the main board and everything was beautiful....
     
  8. smartin53

    smartin53 Active Member

    Messages:
    487
    Location:
    Denver
    So I got around to looking at the board again. I discovered that resistors R111 on the -15V line and R113 on the 15V line were burnt. I checked the incoming voltages and all was fine on those (hence why nothing else seems to be affected down the line) so I swapped out the 2 resistors and the new ones burnt up immediately. Definitely something in the line causing it though I need to do more review of the circuit diagram to see how those link to the various parts. Haven't called McIntosh directly yet but I might try that if I can't sort this out soon.
     
  9. twiiii

    twiiii Addicted Member

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    west Texas
    Pack it up and send to Mac, yesterday.
     
  10. smartin53

    smartin53 Active Member

    Messages:
    487
    Location:
    Denver
    Welp all fixed finally. Found the culprit was one of the .1uf ceramic caps in the input buffering circuit.
     

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