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samson s6 distorted output

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  • #16
    Mark, I understand what you are saying and i really really appreciated you spending time on this.
    I checked my readings on the base of Q711 and Q710 many times over and with 2 very good meters and
    i get 12v and 14v on their respective bases.
    I connected to many different places where I measured at ground with an ohm meter
    I used P2 connector in the power supply and also tried D718 where it is connected to ground, among a few.
    Then I put the probe on either end of R721 which should give me the emitter voltages on these transistors
    and I get both positive .17v .

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    • #17
      Something on the negative rail is blown.
      Q707, 708, 711 must be verified good.
      Every resistor in the output section has a purpose.
      They all must be good.
      I know it's difficult without a 'layout' diagram, but I find it useful to go from the high side to the low side , checking readings against the schematic.(power off)
      Using the resistance scale & the diode check can help in finding the failed component (or open trace)
      It has been said before on the forum that you should know what the reading is likely to be before you measure it.
      That means a brief study of the circuit you are measuring.
      Hint: This is a Class AB amplifier.
      The outputs should be barely on at idle.
      Base= 0.6 volts. (+ & -)
      The drivers should be a diode drop above that.
      Base= 1.2 volts. (+ & -)

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      • #18
        I found Q711 had all of it's legs severed.
        Put a new one in now there's a different distortion,,
        Now voltages are as follows
        Q711
        B = - 1.3v
        E = .7v
        Q710
        B = .9v
        E = .3v

        Edit:correction Q710 had the legs severed by the transistor retainer

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        • #19
          I adjusted svr702 to balance the voltages on the driver base to 1.1v

          It still has a distortion
          Before Q710 was replaced the distortion was closest to an overdriven distortion.
          Now it has changed to a clipping when playing harder on an instrument or talking loudly on a mic.

          On the scope the wave looks really good!
          I don't get it.

          I know haven't mentioned this yet but this output looks like it had been rebuild.
          So there is probably another issue,

          Let alone the reverb oscillation, someone put in a belton tank.
          I've seen a number of these oscillate in some circuits.

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