Home › Forums › Vintage EHX › Electro Harmonix Electric Mistress repair (polarity fry?!) – hints or tips required!

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  • October 11, 2010 at 5:39 pm #80844
    eggwater
    Member

    Hi

    A good few years ago I got sick of powering my old 18V Electric Mistress (not deluxe) with batteries and decided to stick a power supply in. Being young and foolish I didn’t check the voltage or polarity. It was so long ago I forgot what actually happened (possibly a brief whining noise), it just stopped working with batteries after this and I consigned it to a drawer.

    Less than a few years ago someone took the filter matrix switch out to fix an old hot tubes pedal and it was placed back in the drawer.

    Enough back story… I’ve decided the fix it using my very ropey electronics skills and guess what I’m clueless.

    I fitted a new switch to get the ‘easy’ part out of the way.

    Anyone have any ideas what I may have fried?

    I read around and people mention the 741 opAmp so I bought an equivalent (not fitted yet)

    To be honest I really don’t even know how to test components or circuits with a multimeter so all help/pointers would be appreciated.

    Cheers

    Adam

    October 11, 2010 at 6:05 pm #111905
    The EH Man
    Moderator

    Most likely the 741 but possibly the zener as well.

    Check for 6.8v on pin 2 of the 741 and approx. 12-13v on pin 8 of the 4558.

    October 11, 2010 at 6:10 pm #111906
    eggwater
    Member

    Thanks Ron

    I’ll check this out tonight.

    I’d love to get it working again, Too many non-experts have told me it can’t be fixed. I used to believe them, but now it’s time to prove them wrong.

    Adam

    October 11, 2010 at 6:11 pm #111907
    The EH Man
    Moderator
    Quote:
    Thanks Ron

    I’ll check this out tonight.

    I’d love to get it working again, Too many non-experts have told me it can’t be fixed. I used to believe them, but now it’s time to prove them wrong.

    Adam

    Yeah, don’t listen to them. I’ve fixed plenty of these.

    October 11, 2010 at 6:51 pm #111908
    eggwater
    Member

    here’s some images:

    ehem1.jpg

    ehem2.jpg

    ehem3.jpg

    I had to wire a DPDT switch instead of a SPDT ‘cos it was the only one in the store that fitted the pedal. Have i wired it correctly?

    October 11, 2010 at 7:01 pm #111909
    The EH Man
    Moderator

    Looks right to me.

    October 11, 2010 at 8:02 pm #111910
    eggwater
    Member
    Quote:
    Check for 6.8v on pin 2 of the 741 and approx. 12-13v on pin 8 of the 4558.

    Can’t get a reading on pin 2 of the 741. Getting 12.48 on pin 8 on the 4558 (though I’m not 100% sure i’m testing these correctly – sorry I haven’t done this since high school – 18 years ago)

    When i’m testing these with a multimeter I’m connecting black to the negative on one of the batteries (is this correct?)

    hopefully if I just swap out the 741 it will be back in business.

    *fingers crossed*

    October 12, 2010 at 11:40 am #111917
    eggwater
    Member

    if the reading at pin 2 on the 741 was 8.6v would this mean anything?

    October 12, 2010 at 12:15 pm #111918
    The EH Man
    Moderator

    It should 6.8v on pin 2 because of the zener diode.

    You can also use the chassis as a ground. I just put the negative probe in one of the screw holes.

    Have you replaced the 741?

    October 12, 2010 at 12:39 pm #111919
    eggwater
    Member

    Not swapped it out yet. I found out I needed some more equipment. De-solder pump (just bought one at lunch).

    Sorry I realise I’m a complete amateur :-/

    Adam

    October 12, 2010 at 7:22 pm #111924
    eggwater
    Member

    OK I switched the 741 for a TL071

    still not working.

    I can’t get a voltage reading on that pin 2 at all.

    :-(

    October 12, 2010 at 7:25 pm #111925
    The EH Man
    Moderator

    Your 6.8v Zener may be shorted. With the pedal’s power off, check it w/ a multimeter for a short.

    October 12, 2010 at 7:39 pm #111926
    eggwater
    Member

    the meter reads 830. Is this normal?

    October 12, 2010 at 8:12 pm #111927
    The EH Man
    Moderator

    Did you check it both ways?

    October 12, 2010 at 8:53 pm #111928
    eggwater
    Member

    It was 830 one way and 1 the other.

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