Marantz 1150 Left Channel Distortion.

Discussion in 'Marantz Audio' started by RuairiMutt, Mar 27, 2018.

  1. RuairiMutt

    RuairiMutt Member

    Messages:
    57
    Location:
    Lake Geneva, Wisconsin
    Presently, I'm attempting to repair a sick left channel on this extremely nice looking stereo.

    When it came to me for repair, the left channel plays, but with a lot of distortion. It had been previously worked on. Used components were used, which I went ahead and replaced with brand new parts instead.

    Presently, I have replaced the following with new components.
    R742, 2K Trimmer, was fully burned out.
    R741, 2K Trimmer, this was not burned, and still tested as working but replaced anyway with new.
    R747, Was replaced, but board shows this burned out previously.
    H713, replaced with new MJE15032G
    H714, replaced with new MJE15033G

    Both output transistors are brand new as well, I used MJ21194G & MJ21193G.

    It's still functional with the same symptom of very poor, distorted output. Adjustment of the trimmers in either direction renders no difference in quality of output, it remains completely unchanged.
     

     

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  2. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    27,348
    Location:
    uk.. the middle bit
    is it possible to use those trimmers to set the voltages correctly as per manual ?
     
  3. raidermc

    raidermc It Is What It Is <> Subscriber

    Messages:
    239
    Location:
    Western New York
    Might be an issue in the preamp or bad internal contact switch in the (pre out amp in jacks).
    Grab a set of RCA cables and plug them into those jacks to connect the pre to the amp externally to see what happens.
    You could also swap the left with the right and if the problem doesn't change its an amp issue.
    Also make sure the relay contacts are clean.
     
  4. RuairiMutt

    RuairiMutt Member

    Messages:
    57
    Location:
    Lake Geneva, Wisconsin
    Minor Update.

    It took awhile to arrive but I replaced the 2N4923 transistor hidden below the driver heat sync, H709, and while the one there was bad, it did not correct the issue.

    I connected up my DVM and checked the bias, Idle Current between J701 & J702 calls for 10mV at idle. I can't get it to adjust anywhere below 155mv.

    I probed around, and all the resistors are well within their tolerances. I replaced out the three electrolytic caps, as well. Also nothing is getting excessively hot either.

    I'm stumped.
     
  5. raidermc

    raidermc It Is What It Is <> Subscriber

    Messages:
    239
    Location:
    Western New York
    I would replace the differential input transistors they are known to go bad and might just be the cause of you not being able to obtain the 10mv adjustment.

    I gotten into the habit of replacing them as they are cheap.

    Good luck
     
  6. RuairiMutt

    RuairiMutt Member

    Messages:
    57
    Location:
    Lake Geneva, Wisconsin
    Sounds like a good place to start. Still learning the ins and outs, which transistors would these be? I know this is dumb to ask, but still learning the finer details.
     

     

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  7. raidermc

    raidermc It Is What It Is <> Subscriber

    Messages:
    239
    Location:
    Western New York
    On the power amp board, they are the two that have their flat sides with heat sink compound held together with shrink tubing.
    H701 and H702
     
    Last edited: Apr 14, 2018

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