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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)

Attachments:
imageMistress_v2_schematic.pdf
  • in reply to jw0752

    its in a socket.... so not an issue to remove  it

  • UPDATE: still waiting for delivery of the replacement transistors... I've been waiting since Wednesday for the delivery, should due any day now. I've replaced 4 of the old transistors with new ones, and replaced the IC with a socket, and placed the replacement IC into the socket.. Just twiddling my thumbs now waiting for the transistors.

  • Hoorah, the transistors have arrived.... Next job is to solder it in, hold my breath and hope all is fine

  • in reply to Andy Betts

      wrote:

     

    Hoorah, the transistors have arrived.... Next job is to solder it in, hold my breath and hope all is fine

    Holding your breath won't help image :

  • UPDATE: I've replaced the blown transistor, and plugged it all in and tested. When I switch the effect on I get the usual volume drop as you normally get with these pedals but no effect at all..... I have signal through the circuit though obviously so I'm now concerned that the SAD1024 might be dead.

     

    So far I have replaced:

     

    IC LM741

    C9 - 100uf

    C10 - 10uf

    C11 - 10uf

    c12 - 33uf

    BC309 (this was replaced with a BC212L as per the original transistor)

     

    Any ideas?

  • in reply to Andy Betts

    As a first check: what is the voltage on CC212L' s collector (V+) ?

     

    2nd check: do you have small external PC desk speakers with built in amplifier?

    In that case you can use it as a signal tracer:

    connect ground of the speaker input to circuit ground (typilcal, on the cinch connection of these speakers, it's the biggest part of the tip close to the plastic housing.

    Then put the cinch tip to pin 1 of IC1 and check if you hear guitar signal. If yes, circuit before the delay is good.

    Then repeat at C5/R13 connection. You should hear the delayed signal...

  • in reply to Jan Cumps

    I'm actually in the middle of measuring the voltages on all the IC's and the transistors... I'll report back here when I have them

  • As a matter of interest, this web site has all the voltages etc for this pedal. For reference my pedal is a V2

     

    Electric Mistress Repair

  • Here are the voltages from the IC's

     

    SAD1024 socket (Chip removed)

    1 - 0

    2 - 3.38

    3 - 4.27

    4 - 0

    5 - 8.57

    6 - 0

    7 - 8.54

    8 - 4.26

    9 - 8.56

    10 - 4.27

    11 - 8.56

    12 - 0

    13 - 0

    14 - 4.23

    15 - 3.38

    16 - 0

     

     

    SAD1024 (Chip in socket)

    as above except

    6 - 2.85

    12 - 2.77

     

     

    IC1 4558

    1 -2.93

    2 - 3.01

    3 - 2.70

    4 - 0

    5 - 2.73

    6 - 3.08

    7 - 3.08

    8 - 7.86

     

     

    IC5 LM741

    1 - 0

    2 - 11.35

    3 - 3.85

    4 - 0

    5 - 0

    6 - 3.29

    7 - 2.40

    8 - 0

     

     

    IC4 LM339

    1 - 5.82

    2 - cycles from ~0.60 - 7.51

    3 - 8.21

    4 - 4.22

    5 - 4.19

    6 - 1.26

    7 - 0

    8 - 0.05

    9 - 4.95

    10 - cycles from ~0.55 - 7.55

    11 - 0.27

    12 - 0

    13 - 0

    14 - cycles from ~0.20 - 1.30

     

    The voltage on the 212L collector is 23.9v

  • in reply to Andy Betts

    The V+ should be 12V - 13.5V maximum. You measure 23.9V.