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Circuit Protection
Forum HELP!!!.... Blown Electric Mistress pedal
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Related

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

Andy Betts
Andy Betts over 5 years ago

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)

Attachments:
imageMistress_v2_schematic.pdf
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  • jw0752
    jw0752 over 5 years ago +4
    Hi Andy, I wanted to understand the voltage regulator a little better so I built one on the bread board. My original intention was to also hook it up backwards to see how it failed. I have decided not…
  • Andy Betts
    Andy Betts over 5 years ago in reply to Andy Betts +4
    repaired... Transistor was around the wrong way
  • DAB
    DAB over 5 years ago +3
    Bad luck is a pain. I looked over your schematic and you will probably need to replace all electrolytic capacitors as they do not tolerate being hooked up wrong. If you smelled or saw smoke, you probably…
Parents
  • Andy Betts
    Andy Betts over 5 years ago

    Update...... I've found the culprit that fried...... The BC309 (in the pedal it's actually a BC212L, which is different to the schematic)

     

    It's in 2 halves, blown apart!! As a matter of course, and on John Clifts recommendation I'll buy in a new LM741, but i'll try just replacing the 309 first. second question, is the schematic calls for a BC309 but the board actual;ly has a 212L in it.... Any difference, or doesn't it matter?

     

    image

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  • Andy Betts
    Andy Betts over 5 years ago

    Update...... I've found the culprit that fried...... The BC309 (in the pedal it's actually a BC212L, which is different to the schematic)

     

    It's in 2 halves, blown apart!! As a matter of course, and on John Clifts recommendation I'll buy in a new LM741, but i'll try just replacing the 309 first. second question, is the schematic calls for a BC309 but the board actual;ly has a 212L in it.... Any difference, or doesn't it matter?

     

    image

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  • michaelkellett
    michaelkellett over 5 years ago in reply to Andy Betts

    BC309 isn't made any more.

    BC212L is an acceptable substitute here with slightly higher power rating an higher current rating.

    Don't apply power until you've replaced the 741 - if its a shorted lump it will turn the BC309 full on and there will be no voltage regulation.

    I would replace C11 and C12 as well.

     

    MK

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  • Andy Betts
    Andy Betts over 5 years ago in reply to michaelkellett

    There's loads of BC309's on EBay... fairchild ones too... I'll use a 212L, like for like

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  • Jan Cumps
    Jan Cumps over 5 years ago in reply to Andy Betts

    If you don't have desoldering tools, the 741 will be easier to remove if you cut all its pins first. Then remove each pin without force.

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  • Andy Betts
    Andy Betts over 5 years ago in reply to Jan Cumps

    Good idea..... I also plan to put a socket in there too

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  • jw0752
    jw0752 over 5 years ago in reply to Andy Betts

    Should you get the circuit up and running you should put a diode in the circuit as indicated to prevent reverse polarity problems in the future. If you use a Schottky diode it will only take 0.2 to 0.4 volts from the supply and will not affect operation.

     

    image

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  • Andy Betts
    Andy Betts over 5 years ago in reply to jw0752

    Will, that's a good idea.

     

    I've had a rifle through all the IC's i've got lying around and I have hit gold...  I've got a UA741CPUA741CP chip, I expect the CP doesn't matter?? The original is a UA741TC

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  • Jan Cumps
    Jan Cumps over 5 years ago in reply to Andy Betts

    CP and TC are package codes.CP and TP are both plastic DIP 8 through hole, should fit...

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