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Yamaha G100 - crossover distortion

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  • #16
    Update: Replaced TR17-23 with NOS components, checked surrounding components off TR24 and TR16. Tested good. I replaced TR16 too and verified continuity as I went to check for any broken pads for good measure.

    Condition is the same, distorted grainy output. Couple differences I did find -- DC circuit board test point now maxes out at 00.2mv with VR1 turned completely clockwise and across D1 is -1.6v

    I am fairly confident it was not *negative* 1.6v when I initially checked. It would seem no progress was made.

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    • #17
      Another change: The emitters of TR17 read 50v and TR18 emitter is 49v. Both will drift 1v or so and swap voltages between each other e.g. TR17 is 49v and 18 is 50v and vice versa. Previously these emitters read 119v

      I also noticed that the collector of TR23 is 50v while TR22 is 128v.

      The amp also now makes a farting noise when it has powered down.

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      • #18
        TR17 and TR18 cannot have different readings, they come from the same point.
        TR22s collector should be at rail = 128v ish.
        TR23s collector should be about 50volts only if the collector of TR17 is about the same.
        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
        If you can't fix it, I probably can.

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        • #19
          TR17 Base - 121v
          TR17 Collector - 121v
          TR17 Emitter - 51v

          TR19 Base - 121v
          TR19 Collector - 121v
          TR19 Emitter - 48v

          I observed the Emitters of both drifting up and down from a cold start. The point you speak of is TR16, correct?

          The Base, Collector and Emitter of TR16 122v, give or take a volt. I don’t think that’s right, is it?

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          • #20
            The BCE of TR16 *is* 122v, sorry. Surely that can’t be correct?

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            • #21
              TR16 is a constant current source for the differential input pair and will have a few volts between the collector and emitter. The voltage across the emitter resistor determines the current available to let the differential pair work correctly, TR17 & TR18.
              No transistor will survive more then 1volt between the base and emitter, NPN positive with respect to the emitter and PNP negative with respect to the emitter.
              Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
              If you can't fix it, I probably can.

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