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Thread: vintage EHX Electric Mistress very strange issue

  1. #1
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    vintage EHX Electric Mistress very strange issue

    Hi everybody

    I am having a very weird problem with my Electro Harmonix Electric Mistress (vintage SAD1024 version)

    The pedal is not working properly, only the clean guitar signal can be heard, with a slight distortion and lfo ticking in the background. If i disconnect the input jack and reconnect it (this activates the power supply from the batteries) the effect sound briefly comes on, but fades away after a few seconds

    The strange thing is, if I connect the pedal to a signal generator, it works fine with absolutely no issues !

    I am really baffled with this issue. I have never experienced anything like this

    Obviously I have checked all the usual suspect. I have tried fresh batteries, and also an external power supply, double checked all connections. Not long ago I have replaced the electrolytic capacitors

    I am wondering this has something to do with the input load impedance. The signal generator has a 600ohm impedance so that is the only culprit I have

    Maybe the input opamp 4558 has some issues and it won't work properly with the guitar load impedance ?

    I am very keen to hear any possible explanation of this weird issue, and hopefully getting some suggestions on what to repair

    This is a schematic of the pedal

    http://www.metzgerralf.de/elekt/stom...-schematic.gif

  2. #2
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    When I see weird things like this I start looking at pots and jacks. Have you swapped the input jack out with a new one? I've seen the inner sleeve of an input jack have lots of dirt and debris coated on the on metal. As part of my repair process, I use a Qtip with isopropyl alcohol and wipe the inside sleeve of the input and output jacks. I'll do the same for the tip of the jack.

    Many guitar players grab the sleeve and tip of the guitar cord and transfer the dirt and oils from the fingers to the plug on guitar cord. Then this dirt and debris will stick to the inside of the guitar pedal jacks. I see this on old vintage units... not so much on newer pedals.

  3. #3
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    Quote Originally Posted by beatnik View Post
    Hi everybody

    I am having a very weird problem with my Electro Harmonix Electric Mistress (vintage SAD1024 version)

    The pedal is not working properly, only the clean guitar signal can be heard, with a slight distortion and lfo ticking in the background. If i disconnect the input jack and reconnect it (this activates the power supply from the batteries) the effect sound briefly comes on, but fades away after a few seconds

    The strange thing is, if I connect the pedal to a signal generator, it works fine with absolutely no issues !

    I am really baffled with this issue. I have never experienced anything like this

    Obviously I have checked all the usual suspect. I have tried fresh batteries, and also an external power supply, double checked all connections. Not long ago I have replaced the electrolytic capacitors

    I am wondering this has something to do with the input load impedance. The signal generator has a 600ohm impedance so that is the only culprit I have

    Maybe the input opamp 4558 has some issues and it won't work properly with the guitar load impedance ?

    I am very keen to hear any possible explanation of this weird issue, and hopefully getting some suggestions on what to repair

    This is a schematic of the pedal

    http://www.metzgerralf.de/elekt/stom...-schematic.gif
    Did it work after recapping the unit?

    nosaj

  4. #4
    Senior Member Enzo's Avatar
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    Unless this never ever worked, and all the rest of them with the same innards never worked, the chip they used in the design is not likely at fault.

    Every working guitar amp I ever plugged any working guitar into, it played. I never gave a moment of thought to what the guitar impedance might be or the input impedance of the amp. Such things might make subtle differences in bitsof tonal structure, but won;t be the difference between the thing functioning or not.

    SInce you get brief sound (does that brief sound have the echo effect on it?) that fades, my first thought is something charging up. And my first suspect is an op amp. Not due to impedance, but either the voltage reference is whacky or the thing wants to slam its output to one rail. Look at pins 1 and 7 of the 4558. Are they sitting at about half the supply. SUpply being 18v so we expect 9v? Looks to me like VR1 would center that. If one or the other drifts up to 18 or down to ground, that can cause this.

    Maybe the 4558 has some issues, forget the impedance part. Did you try a different 4558? I'd try most any op amp you got for this test.
    Education is what you're left with after you have forgotten what you have learned.

  5. #5
    Supporting Member Jazz P Bass's Avatar
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    I have seen a few repairs where the signal generator works but the guitar signal fails.
    It usually turns out to be a coupling capacitor.

    What signal 'amplitude' did you use coming out of the generator?
    T would not exceed 100mv's.
    g1 likes this.

  6. #6
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    actually it was a bad capacitor and / or wrong bias calibration setting. I have replaced the capacitors on the power rails and re calibrated, that fixed the issue
    thanks
    g1 likes this.

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