Biasing Question on Mx110 Preamp Section

Discussion in 'McIntosh Audio' started by Elemental, Jan 8, 2018.

  1. Elemental

    Elemental New Member

    Messages:
    10
    Location:
    Fort Worth, Texas
    Technical question: I'm troubleshooting distortion on a McIntosh Mx110Z and wanted to rule out incorrect biasing of the 12AX7 tube (V15) which is the first stage of the preamplifier section.

    Looking over the schematic, something doesn't make sense. The schematic says that on, say, the right channel the grid should be at 10 volts (pin 7) and the cathode at 13 volts (pin 8). Similar for the left channel. Fine, but where does the grid voltage come from? Per the schematic, the grid ONLY connects, through the high and low frequency filters, to a 0.1 uF capacitor coming from the input selector switch (the filters are bypassed when turned off). There is no DC path to either ground or B+ on either channel, regardless of the filter switch positions. It seems that this would leave the grid floating relative to cathode and anode.

    The distortion is in both channels in the form of "crackling" on bass notes and overall mushy sound, and it only began recently. I'm guessing it originates somewhere in the preamp section (tubes 15, 16, & 17) since it is present with both FM tuner and phono inputs. I tried swapping the two 6U8 tubes (V16 & V17) with no change, and I've ruled out the power amp as the problem.

    Any thoughts from the experts out there?
    Thanks
     
    Last edited: Jan 8, 2018
  2. Elemental

    Elemental New Member

    Messages:
    10
    Location:
    Fort Worth, Texas
    Update: I'm no longer convinced that the 12AX7 is the culprit, and in any case it is in the second preamp stage, not the first. The first stage is the 6D10 tube (V10) which performs triple duty amplifying FM audio (prior to demux) as well as the right and left external inputs. The 6D10 can be bypassed using the tape monitor input. I did this using a separate preamplifier going into the tape monitor input. The distortion remained, which appears to rule out the 6D10. However, I now find distortion present when using this second preamp as the sole input to the power amplifier, thus removing the Mx110Z completely from the system. This points to the power amp (an Mc275) as the possible culprit. Very frustrating and elusive, since the distortion is intermittent, mainly happening on low bass notes at higher volume, and I don't have spare tubes to swap out to help isolate.
     
    Last edited: Jan 10, 2018

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