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

Viewing 15 posts - 16 through 30 (of 37 total)
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  • October 16, 2010 at 4:32 pm #111998
    eggwater
    Member

    Well I’ve done some fiddling about today.

    I changed the zener and now get 6.65V at pin 2 on the 741. I wasn’t getting any readings on the 4558 so i’ve installed a new one of these. But still can’t get any voltage readings here.

    Any ideas?

    October 16, 2010 at 4:43 pm #111999
    eggwater
    Member

    holy freaking sheeet!! I just resoldered an iffy looking pin and it’s only now WORKING!!!

    October 16, 2010 at 5:14 pm #112000
    eggwater
    Member

    spoke too soon. It stopped again!

    October 16, 2010 at 5:38 pm #112001
    The EH Man
    Moderator

    Might recheck your solder joints.

    Voltage sounds good on pin 2. Make sure you’re checking the correct pin on the 4558. It’s pin 8. There’s a transistor by the 741 that helps regulate the output voltage. On 1 leg should be 18v and on another should be 12-13v.

    October 16, 2010 at 5:44 pm #112002
    eggwater
    Member

    it’s working again – it’s a bad joint i think!

    October 16, 2010 at 6:00 pm #112004
    eggwater
    Member

    I’ve pin pointed the issue to where I changed a component (he 2N4354) If I touch the joint with a multimeter probe where the two pins are then the pedal starts working. Any ideas?

    October 16, 2010 at 6:11 pm #112005
    The EH Man
    Moderator

    Just keep checking your solder joints.

    October 17, 2010 at 8:56 am #112008
    eggwater
    Member

    this is weird it’s like the transistor needs a little charge before it’ll work. no matter how many times i resolder it the same issue persists. I put the red multimeter probe on the joint and the flanger fires up!

    any ideas?

    December 16, 2010 at 12:04 pm #113122
    Scruffie
    Member
    Quote:
    this is weird it’s like the transistor needs a little charge before it’ll work. no matter how many times i resolder it the same issue persists. I put the red multimeter probe on the joint and the flanger fires up!

    any ideas?

    I’d still stick to what the EH Man said and keep at that joint, make sure it’s nice and shiney lookin’

    Other than that… ‘Charge’ to me says bad Polarized cap, i’d blame the 10uF Tantalum in parallel with the Zener first, could try replacing it with a standard electrolytic or a tantalum if you prefer, don’t forget it’s polarized though so it must be replaced the correct way.

    That or the 33uF from Pin 3 of the 741/ Collector of the Transistor.

    February 11, 2015 at 8:41 am #120526
    beatnik
    Participant

    hi everybody

    i have found this old thread because i have the exact same problem as the op

    my vintage electric mistress had a burnt LM741 in the power supply

    i have replaced it with a TL071

    the power supply circuit has a latch up problem and only starts working if i measure voltage from 2N4354 base to ground with dmm

    i couldn’t find a 2N4354 transistor. I’ve tried both 2N5087 and BC557 but the problem persists

    i am wondering what the problem can be. maybe these equivalents are not good ?

    February 11, 2015 at 2:43 pm #120533
    The EH Man
    Moderator

    I’d go back to the 741 and see if that helps.

    February 11, 2015 at 4:31 pm #120534
    beatnik
    Participant

    ordered one

    will keep you posted

    thanks

    February 12, 2015 at 7:10 am #120535
    eggwater
    Member

    I never did manage to fix mine. re-soldered it so much that the original track near the component I was trying to fit broke from the PCB and I wasn’t sure if i was even able to fix it. Still an unfinished project.

    :-(

    February 12, 2015 at 7:38 am #120536
    beatnik
    Participant

    cmon man try to fix it. it’s sad to have such a nice pedal collecting dust. to fix the broken trace just use a small piece of wire

    i will report if the lm741 fixes the latch up problem

    February 12, 2015 at 9:43 am #120537
    eggwater
    Member

    i know I really should. it was almost there. I was getting the effect to work but it just didn’t seem to trigger without touching the solder with the multimeter probe.

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