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C37 PreAmp Issue Wits End

Discussion in 'McIntosh Audio' started by nowinowski, Apr 11, 2018.

  1. nowinowski

    nowinowski New Member

    Messages:
    36
    My C37 is having an intermittent issue where its like it turns on and off really quickly while music is playing. It happens very quickly - sometimes the music level drops, sometimes it is most noticeable as the faceplate dimming and other times there is a click like on the traditional turn on with the switch relay sequence. I brought it back to the shop where i bought it and another tech and though they got observed it they both claimed they couldn't fix it as it was intermittent and couldn't be diagnosed.

    Any thoughts or help would be greatly appreciated.
     

     

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  2. c_dk

    c_dk Addicted Member

    Messages:
    6,311
    Location:
    West Michigan
    Well, why are your techs bailing on you......intermittents are all a part of any real world electronic repair.

    If you bought it recently you are due a refund.....

    For your at home diagnosis I would have you start with blocking off the IR remote sensor. A cd case stood up in front of the preamp has worked in the past to keep out stray IR.

    Your tech could have tested for low voltage with his variac, looked and measured for power supply issues etc. but for what ever reason they have seemed to hung you out........
     
  3. nowinowski

    nowinowski New Member

    Messages:
    36
    yeah - i'm not too happy about it. Stray IR is an interesting thought. Oh, the one thing i could think of that might trigger it is the cold - that gear is in the basement and it gets 60ish down there.
     
  4. c_dk

    c_dk Addicted Member

    Messages:
    6,311
    Location:
    West Michigan
    Sunlight has always been a remote sensor pollutant....high intensity halogen lighting has been known to false trigger sensors. So has a obscure code on a TV or VCR remote.

    Conductive build up on circuit boards due to proximity to the ocean has also caused lots of false triggers.

    Had a C38 that had a cat puke into it.......that was interesting......
     
  5. louisjames

    louisjames The "real" Louis James Subscriber

    Messages:
    2,005
    Not sure if you have already but if not I would give Chuck at McIntosh a call. He's the tech guy and usually very helpful. Ask for support when the receptionist answers. Barring that send it off to AC and have the repair done right. Not the cheapest route to go but the best for piece of mind. Just my 2c's, YMMV. Best of luck!
     
  6. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    2,212
    Location:
    France
    Try to check the soldering of the power transformer (T1) , tags and other connections inside the "black box" PCB 047233

    PCB-047233.JPG
     

     

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  7. c_dk

    c_dk Addicted Member

    Messages:
    6,311
    Location:
    West Michigan
    I would think his local tech(s) would have at least taken a look at such things as possible cold solder joints or detritus on the logic boards but from the OPs first post who really knows.

    Does sound like there is some hubris on the techs side though.
     
    Last edited: Apr 11, 2018
  8. twiiii

    twiiii Addicted Member

    Messages:
    6,333
    Location:
    west Texas
    Are you sure the wall connector the unit is plugged into is OK? Make sure the spades on the ac plug are making good contact in the receptacle. Do you have any aluminum electrical wire in the house?
     
  9. nowinowski

    nowinowski New Member

    Messages:
    36
    its plugged into a power strip but has had the same issue in multiple outlets - the wiring in the house is a bit of a mish-mash so definitely a possibility that there is aluminum wiring.
     
  10. 62caddy

    62caddy Trust but verify Subscriber

    Messages:
    9,553
    Location:
    NEPA
    Symptoms observed by the tech at his place so probably safe to say extraneous IR, house wiring and/or sockets are not part of the puzzle. Sound like something along the lines of the power relay circuit or possibly worn/corroded power button contacts. The front panel buttons on this unit are momentary electronic type.
     
  11. SomeGeoffGuy

    SomeGeoffGuy New Member

    Messages:
    30
    Location:
    Metro Detroit
    Try wiggling your ribbon cables while it is playing and see if anything happens. Those things are notorious for failing on the older Macs. And find a new tech...

    Good luck!

    -Geoff
     

     

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  12. clinic-audio

    clinic-audio all on YAMAHA untill 1990

    Messages:
    2,212
    Location:
    France
    Do not use long top cover screws because you will damage the flat ribbon on the side !
    I saw it once
    This will affect commutations only
     
  13. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

    Messages:
    3,477
    Location:
    NE Ohio
    My MX130 has a backup power supply very similar in design as this one. Although it did not fix my problem (it was an IC), when I took it apart, the circuit board area under the small transformer was darker in color from heat. Might be worth reflowing the transformer pins.
     

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