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1978 Electro-Harmonix Electric Mistress High Supply Voltage

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  • 1978 Electro-Harmonix Electric Mistress High Supply Voltage

    Hi everyone,

    I'm currently working on a 1978 V5 Electro-Harmonix Electric Mistress. Thanks to the help of this forum, I have already made a lot of process, sorting out a short at the power supply. I biased the trim pots using online instructions (http://www.metzgerralf.de/elekt/stom.../align.shtml#2) and an oscilloscope, and the pedal does work now.

    Working on the circuit, I have found that the supply voltage is significantly higher than it should be, and I just can't figure out why. It should be regulated to 12-13.5V, but reads at nearly 16V.
    I have attached the schematic, with my voltage measurements. The effect is powered using an 18V power supply (9V Truetone with voltage doubler).

    So far I have tried:
    • Replacing Zener with a new 1N5235B
    • Checked all electrolytic capacitors, replaced the high ESR ones
    • Replaced the 22k resistor in the voltage regulating circuit with 15k and 10k, but voltage didn't lower
    • Checked the 10uf Tantalum cap parallel to the zener, it seems to measure fine
    None of the steps led to any change of the supply voltage.

    Any help would be greatly appreciated!


    Click image for larger version

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  • #2
    Originally posted by clarisso11 View Post
    [*]Replaced the 22k resistor in the voltage regulating circuit with 15k and 10k, but voltage didn't lower

    None of the steps led to any change of the supply voltage.
    Makes me suspect IC5.

    Generally with opamps the voltage at their output pins is most relevant.

    - Own Opinions Only -

    Comment


    • #3
      Originally posted by Helmholtz View Post

      Makes me suspect IC5.

      Generally with opamps the voltage at their output pins is most relevant.
      Thank you! Just ordered a new LM741, will report back with the results.

      Comment


      • #4
        Success everyone! The new (NOS) LM741 chip arrived, I swapped it in, following Helmholtz advice above. I now get a solid 13.15V of supply voltage!

        I re-biased the chip, and it sounds great now! Interestingly, the low frequency 'thump' noise I had reported on another thread has also completely disappeared, maybe it was a sign that the LM741 was failing.

        Thanks once again for the great help on this forum!

        Comment

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