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1979 Electro-Harmonix Electric Mistress - No supply voltage

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  • 1979 Electro-Harmonix Electric Mistress - No supply voltage

    Hi everyone,

    I am currently working on repairing yet another Electro-Harmonix Electric Mistress, after having repaired one in the past with the help of this amazing forum. This one was in a real state, wires disconnected and jumbled up. Using my working Electric Mistress, I was able to restore the wiring, and confirm that the SAD1024 chip still works.

    Where I was having issues with high supply voltage on the last one, this one has no supply voltage at all (or very close to 0 V)

    Here are the symptoms:
    • No sound at all when engaged
    • Supply voltage 0.3V instead of 13V
    • Bad voltage at LM741 pin 3 (non-inverting input): 0.1V instead of 6.6V
    • Bad voltage at LM741 pin 6 (output): 1.36V instead of somewhere around 13V
    • Audio signal stops completely at RC4558 pin 5
    • RC4558 voltages are mostly 0

    Here is what I have tried so far:
    • Checked all electrolytic capacitors for ESR and capacitance
    • Replaced LM741 chip
    As far as I can tell, the issue is with the voltage regulator circuit, specifically with the low voltage at the LM741 output pin, which in turn reduces supply voltage. With no power supply to the RC4558 chip, the audio signal stops completely (I think?).
    But what is causing LM741 to misbehave? Unless the replacement I tried is also defect, could this have something to do with the bad voltage at the non-inverting input, and what could be causing this?

    I have attached the schematic with my voltages in orange, and the correct voltages in red.



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  • #2
    The 741 is just an opamp. It does not produce and voltages, it runs off the rail voltages like anything else. With the 741 removed, what voltages do you get a the supply pins? Work backwards from there. (note: I am at work and don't have time to dig into your schematic)
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

    Comment


    • #3
      Originally posted by eschertron View Post
      The 741 is just an opamp. It does not produce and voltages, it runs off the rail voltages like anything else. With the 741 removed, what voltages do you get a the supply pins? Work backwards from there. (note: I am at work and don't have time to dig into your schematic)
      Thank you for your reply! I have just unsoldered LM741, here are the pin voltages: 6.6V at pin 2, 19V at pin 7, 0V everywhere else.
      Not sure where to go from here to be honest, but I'm happy to post more voltages if that helps.

      Comment


      • #4
        I'm NOT familiar with the 741 as the heart of a voltage regulator. But there's few enough components to check out. The next suspect would be the transistor. Please note that when the 741 is in the circuit and operational, the voltages at pin 2 and pin 3 will be equal. Also without the 741, no voltage gets to pin 6 (control the for regulated output) so there's no voltage to anywhere else in the device. That said, there could be something else that's pulling the voltage down when the 741 is installed. Did you notice anything getting warm when the replacement 741 was installed?
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

        Comment


        • #5
          What might be a useful test is (while the 741 is out) apply a regulated 12vdc supply to V+ to see if the unit springs to life. Take care to avoid immolating the entire board with unlimited current, place a resistor (start with maybe 1k) between the power supply and the V+ rail. Absent a 12v supply, a 9v battery would prove out whether something else in the circuit is drawing down the supply. Measure the voltage drop across the 1k resistor so see what the device load is. I have no idea what the typical load is, find a dropping resistor that gives good data. If you reduce the dropping resistor to 220R and are still dragging the rail down to nothing (0.3v) then the current is 9v/220R = 41ma and that's gotta be way too high for a circuit like this, IHMO. If that's the case, you've got a different device (not the 741) shorting out.
          If it still won't get loud enough, it's probably broken. - Steve Conner
          If the thing works, stop fixing it. - Enzo
          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

          Comment


          • #6
            Also if you have a diode check function on your meter, you aught to be able to make checks on the two transistors in the circuit. At least the BC309 while the 741 is out of circuit. Note that the BC309 does all the heavy lifting for the supply section.
            If it still won't get loud enough, it's probably broken. - Steve Conner
            If the thing works, stop fixing it. - Enzo
            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

            Comment


            • #7
              Originally posted by eschertron View Post
              Also if you have a diode check function on your meter, you aught to be able to make checks on the two transistors in the circuit. At least the BC309 while the 741 is out of circuit. Note that the BC309 does all the heavy lifting for the supply section.
              Hi eschertron, thank you for all your help! Just saw your messages and figured I’d start with the BC309- I think we’ve found the culprit! Will check how the circuit performs once I have found a replacement, and report back!

              Click image for larger version

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              • #8
                Everything working now! In the end there were two faulty components in the voltage regulator circuit: The LM741 chip, and the BC309. I have re-calibrated the unit, and it works perfectly.
                I had skipped checking the BC309, since I had never encountered a shorted transistor before - always something new with these repairs!

                Many thanks once again to eschertron for all the help and explanations.

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