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HELP!!!.... Blown Electric Mistress pedal

Andy Betts

I have made a huge school boy error... i was looking at running my original Electric Mistress pedal for a recording I'm doing and needed to run it off an external power supply. I had a 18v supply to hand which I just needed to connect up to a DC plug so that I could power the pedal. In my haste, I accidentally connected the negative and voltage wires the wrong way round...... connected the pedal, switched on the power and I immediately heard a small pop and the smell of burning. I have blown an original 1977 Electro Harmonix Electric Mistress effects pedal worth around $500!!! however, i'm sure it's not dead yet, and it can be brought back to life... Hence this post.

 

For your reference I have attached the schematic.

 

can anyone advise the best place to start troubleshooting, I'm assuming anything on the ground side of things could have been the first thing to blow, and I'm thinking a transistor.... bearing in mind the 18v supply was connected to the ground, and the ground was connected to the voltage in. Anything can be replaced, the only issue is if the culprit ends up being the SAD1024 chip that's it... the pedal is a gonner. However I very much doubt the voltage got that far in the circuit to be honest (hopefully)

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imageMistress_v2_schematic.pdf
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  • I think that Jon is right about the 741 and I would also test the BC309 before I panicked. This a voltage regulator section that usually takes the 18 volts and reduces it to the 12 Volt. Depending on how it failed it may not have directed the reversed power to the rest of the circuit. Unless an electrolytic cap has exploded or is bulging they are probably OK. If this was on my bench I would put my analog multimeter on RX1 and put the positive probe on the 12 volt rail and the negative probe on the ground to see if the 12 volt rail is showing any short to ground. Keep in mind the red multimeter lead may or may not be the one with the positive voltage. You will have to use another meter to verify. If the 12 volt rail is not shorted replace the BC309 and the 741 and give it a try. If there is any chance you would make this mistake again you can add a diode that would protect the circuit from reverse polarity. If there are any resistors that have been over heated they would be a clue to where you might find other problems like DAB indicated.

     

    John

  • in reply to jw0752

    I've been looking at the circuit and also posted on another forum and as luck would have it an old Electro Harmonix employee has chimed in with pretty much what you guys have said. he's said "I doubt there's much wrong beyond the voltage regulation circuit.". i thought the same and have already thought of replacing the LM741 and the BC309... After that I'll look at the zener diode as a possible issue.

     

    beyond that I'm in the dark really. he has also suggested making a small secondary regulator circuit on some perf board and wiring that into the pedal, effectively bypassing the one in the circuit

  • in reply to Andy Betts

    The best way to check the rest of the circuit (apart from the voltage regulator bit)  would be to remove the BC309 and apply power very carefully from a current limited bench supply.

    If you don't have such a thing it's harder to be sure of not frying things.

    Certainly look very carefully for any cooked parts - if you find some then report back and people may have ideas about the significance.

     

    MK

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  • in reply to Andy Betts

    The best way to check the rest of the circuit (apart from the voltage regulator bit)  would be to remove the BC309 and apply power very carefully from a current limited bench supply.

    If you don't have such a thing it's harder to be sure of not frying things.

    Certainly look very carefully for any cooked parts - if you find some then report back and people may have ideas about the significance.

     

    MK

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