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Thread: MXR-Distortion Plus

  1. #1
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    MXR-Distortion Plus

    My MXR Distortion +, burnt by over voltage and R9 on the board burnt and don't know the value.Close to R9 is R8 and it's 3.6K.
    All resistors are 5bands resistors and I searched on Google and only found diy schematics not original sch.Do you guys know it and Pls help me.

  2. #2
    Senior Member Enzo's Avatar
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    I was looking for my MXR files for someone else here, and finally found where I had put them a couple days ago, I think I have that one too, I'll check when I get home.
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
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    THKS Enzo

  4. #4
    Supporting Member Jazz P Bass's Avatar
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    Here is the schematic that I have.

    Unfortunately, there are not any component designators.

    It should not be all that difficult to figure out which resistor is R9.

    MXR Dist +.zip

  5. #5
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    Quote Originally Posted by Jazz P Bass View Post
    Here is the schematic that I have.

    Unfortunately, there are not any component designators.

    It should not be all that difficult to figure out which resistor is R9.

    MXR Dist +.zip

    thks,I finally found out that It was 100 Ohms resistor,from v+..........thks anyway.

  6. #6
    Supporting Member Jazz P Bass's Avatar
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    Hmmm.
    From V+ to where?

  7. #7
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    Hmmm.
    From V+ to where?
    To everything else, it's in series with the +9V supply jack, to act as a fuse ... which it did.

    Cheaper/smaller/lighter than a fuse and a fuse holder.
    Juan Manuel Fahey

  8. #8
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    The over-voltage may have damaged other components, as things must have drawn a lot of current to blow the resistor.
    They may be dead, dying, or just compromised.

  9. #9
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    yes, 100ohms res,100uf filter cap and 741 IC,all 3 components replaced and It came back to life,thks for the supports......
    J M Fahey and The Dude like this.

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    Junior Member Chrisfromiowa's Avatar
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    I know that this thread is old, but not that old, so here goes. How did you determine that was the problem? I love the D+ and have several, one stopped working completely, and one started sounding "farty" like a very low battery or the synth mod on a Boss DS-1. I've checked all the usual suspects, bad joints, loose wires, etc. On the completely non working unit I get 9v using the multimeter on pin 7 of the ic chip, but nothing from the other points. Any ideas? Thanks

  11. #11
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Chrisfromiowa View Post
    I know that this thread is old, but not that old, so here goes. How did you determine that was the problem? I love the D+ and have several, one stopped working completely, and one started sounding "farty" like a very low battery or the synth mod on a Boss DS-1. I've checked all the usual suspects, bad joints, loose wires, etc. On the completely non working unit I get 9v using the multimeter on pin 7 of the ic chip, but nothing from the other points. Any ideas? Thanks
    1) post here the schematic, even if not the original one.
    Some are close, some are modded or show veroboard layouts or whatever.
    The "reissue" one (made by Dunlop?) "should" be close, besides added 9V DC jack for pedalboard supply (which the original never had)
    2) that said, it's a 741 Op Amp, so it "should" have +9V on pin 7, 0V on pin 4 and about 4.5V on pins 2 - 3 - 6 , please recheck that.
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    Juan Manuel Fahey

  12. #12
    Junior Member Chrisfromiowa's Avatar
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    Thank you Mr. Fahey,
    I'll try that when I get home tonight. I realized that I'm talking about two different D+'s a 1979 Block logo without a power adapter although I used to use a reversed battery clip to hook it to a dc adapter, and the second one is I think a 1981 with an LED and a 2pdt footswitch instead of the spdt switch. Here is a schematic of the non LED one, the two have the same PCB. Thank you for your help. Regards Chris
    1980mxrd-.jpgClick image for larger version. 

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    Ok, well first, it is a 741 in both of them, the one with no sound only gets voltage on the 7 pin, the rest are dead.
    Last edited by Chrisfromiowa; 10-11-2017 at 12:12 AM. Reason: Home from work

  13. #13
    Junior Member Chrisfromiowa's Avatar
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    The 9v going in to pin 7 is straight from the power supply, right? So if nothing is coming out of any other pin, does that mean the chip itself is bad? I'm going to check the "farty" D+, but its chip seems ok. Thanks

  14. #14
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Chrisfromiowa View Post
    The 9v going in to pin 7 is straight from the power supply, right? So if nothing is coming out of any other pin, does that mean the chip itself is bad? I'm going to check the "farty" D+, but its chip seems ok. Thanks
    Not necessarily: see that there is a voltage divider made out of 2 x 1M resistors going from +9V to ground; to make 1/2 of 9V at their junction, to bias the 741, check 9V actually reaches the top one and you have 4.5V in the junction.
    Then those 4.5V travel to 741 pin 3 , you should also find them there.
    Actually a little less, around 4V, but thatīs fine.
    Pin 3 will straight have that, and pins 6 and 2 "follow" that, you will also have around 4.5V on them.
    2 possible defects:
    * if +9V does not reach the divider, of course you will not get 4.5V ... you might have a broken/burnt track or cracked connection, follow the path to see where you lose voltage.
    As you see, it would not be a "bad parts" problem ... unless you consider the PCB "a part".
    Thatīs why Pros are against "shotgunning", you may replace ***ALL*** parts in a dead amp or pedal and still not repair it .
    Troubleshooting, which basically means checking different circuit blocks are working as intended , is the best way to go.
    And it includes voltage measurements at least.

    * If you have +4.5V (or 3V) at the voltage divider but not at 741 pin 3 , then that IC melted and is now a blob of shorted metal, replace it.
    I am against shotgunning but since that IC costs some 40 cents and is a big suspect anyway , just replace it.
    And you have *already* troubleshooted and found wrong voltages on its pins, so itīs not actually shotgunning, at all.
    Different case is when people starts replacing ICs and transistors without reason, just to expect they get lucky
    FWIW, if luck ever worked for common people, instead of against, Las Vegas would still be a cowboy ghost town, population 85
    Chrisfromiowa likes this.
    Juan Manuel Fahey

  15. #15
    Junior Member Chrisfromiowa's Avatar
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    Ok, waiting for some fresh chips. Testing D+ #2 and I get 9 volts from #7, and around 1 from the other live pins. So that means that I'm glad I ordered a few chips, right. I'm guessing that an old power supply went bad as the D+ was always first in the chain and almost always on. Well, once I get them changed I'll let you know. Thanks again Mr. Fahey, regards Chris

  16. #16
    Junior Member Chrisfromiowa's Avatar
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    Hey Mr Fahey,
    Got the 741's in, replaced them on both D+'s and both are working just like new. My wife suggested that since I have 4 working pedals that are exactly the same that I could sell a couple of them now, you'd think after 17 years she'd know that wasn't going to happen... thanks for the information, I am indebted to you, but really, thanks to everyone. Much appreciated. Regards Chris
    J M Fahey and g1 like this.

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