Marantz Model 27 troubleshooting

Discussion in 'Marantz Audio' started by mikepick, Apr 15, 2018.

  1. mikepick

    mikepick New Member

    Messages:
    3
    Hi folks! I just came into a Marantz Model 27 for a very reasonable price, but it did come with some problems.


    The reason it was being sold was that the left channel begins to distort and eventually drops out after a certain amount of time. I've verified this twice now by running it for a while. Eventually the left channel begins to softly distort, starting with a kind of burping and farting on the louder sounds and then the channel volume begins to fade. The rear top of the unit then becomes quite warm. The problem exists across all the inputs in my testing.

    The other problems are somewhat minor.

    The signal meter does not seem to work, it is lit from the left side but does not move at all when tuning stations.

    The tuner is a little misaligned as well, the local NPR station at 93.9 comes in at 92.75 or so on the dial. Looking through the service manual, it seems like adjusting the tuner is pretty tricky and so I'm willing to live with that particular issue.

    On an initial interior inspection, the unit appears quite clean. I did notice some suspect goop on a transistor heat sink on the power board, so I'm wondering whether a bad cap might be the issue.



    Any advice would be appreciated. I still need to do some tests to see if the signal is good from the preamp when the channel drops out.
     

     

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  2. Jailtime

    Jailtime Standin' on a corner Subscriber

    Messages:
    11,349
    Location:
    Velva ND
    I've never seen a Model 27, rare bird there. You might be able to slide the dial pointer on the string to get it to indicate the correct frequency. Most of them just have the pointer stuck to the string with a dab of glue.

    I'd suspect transistors in the driver stage of the amplifier are the cause of your problems. Would be nice to get it narrowed down to amp or preamp though.
     
  3. mikepick

    mikepick New Member

    Messages:
    3
    Yes, I've got the service manual from hifiengine and am going to test that as a next step.

    I've cleaned up that suspect goo, and on closer look it seemed more like hot glue. I have had a closer look and it looks like there is more goop in between the big heatsink and the power board, and it looks like wax, which makes me wonder whether someone set a candle on it at some point.

    According to the manual, there is a dynamic temperature sensing unit which regulates voltage to the big transistors. I'm wondering whether a failure in here might be a culprit.

    WRT the dial pointer, that's a good tip, thanks. I think I might check some other stations locations and see if the offset is even across the spectrum, and in that case maybe move the pointer, although I'm scared to touch the thing. to be honest. :)
     
  4. mikepick

    mikepick New Member

    Messages:
    3
    An update. The trouble with testing this unit is that I need to run it long enough to induce the distortion. Ran it for a while this afternoon with leads coming off J404 and J405 of the pre-amp board to another amplifier. The distortion appeared there as well, which seems to indicate the problem is further back than the power amp.
     
  5. wlhd1610

    wlhd1610 Penny and her new friend Subscriber

    Messages:
    2,131
    Location:
    upstate new york
    You could try a can of freeze spray and do one transistor at a time once it starts acting up.
    I have a couple of 27's here awaiting restoration.
    Bob
     

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