Home › Forums › Vintage EHX › Electro Harmonix Electric Mistress repair (polarity fry?!) – hints or tips required!
- This topic has 36 replies, 5 voices, and was last updated 9 years, 2 months ago by Scruffie.
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October 16, 2010 at 4:32 pm #111998eggwaterMember
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 #111999eggwaterMemberholy freaking sheeet!! I just resoldered an iffy looking pin and it’s only now WORKING!!!
October 16, 2010 at 5:14 pm #112000eggwaterMemberspoke too soon. It stopped again!
October 16, 2010 at 5:38 pm #112001The EH ManModeratorMight 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 #112002eggwaterMemberit’s working again – it’s a bad joint i think!
October 16, 2010 at 6:00 pm #112004eggwaterMemberI’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 #112005The EH ManModeratorJust keep checking your solder joints.
October 17, 2010 at 8:56 am #112008eggwaterMemberthis 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 #113122ScruffieMemberQuote: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 #120526beatnikParticipanthi 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 #120533The EH ManModeratorI’d go back to the 741 and see if that helps.
February 11, 2015 at 4:31 pm #120534beatnikParticipantordered one
will keep you posted
thanks
February 12, 2015 at 7:10 am #120535eggwaterMemberI 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 #120536beatnikParticipantcmon 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 #120537eggwaterMemberi 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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