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Thread: Mxr distortion+ help!

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    Mxr distortion+ help!

    I recently bought a older mxr distortion+ pedal at a pawnshop for $30. Dont know how old it is but its not one of those -plastic cased ones. It has a red LED light on it. I cannot get it to play. If I run a guitar through it without pushing the foot pedal i can play the guitar with no distortion but when I stomp the button it goes dead silent and the LED doesnt light up at all, not when the pedels in use or not. I tried the obvious things like changing the battery and examining the wires to try make sure they are all connected and as far as I can tell they are. I dont know what else I can do. am I doomed? I've long since forgotten ho to read schematics and am such a terrible solder-er anyway that I often fry the wires trying to solder them. Is there anyone that can help me make this work? I have a copy of the schematic saved on my computer. It just seems unlikely that I will be able to solder all those wires in such a tiny place. Thank you for takin he time to read my post.

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    Lifetime Member Enzo's Avatar
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    A good battery won't do anything if the circuit it is in is not complete. First thing to do is turn it on and start looking to see if the operating voltages are present. If not, we then try to find out why.

    It sounds like you need to find a ffriend who knows some electronics, if you can;t solder wires even very well.

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    Education is what you're left with after you have forgotten what you have learned.

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    Senior Member guitician's Avatar
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    That pedal is nothing but a ua741. Buy that chip at an electronics store and replace it, and you may be in business. Other then that a shorted 1uf cap in the bias circuit may be the culprit. good luck

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    how would I go about checking the voltages when I turn it on? one of those voltage meter things? I can solder decent but not am not very confident soldering in such a small area that requires a lot of percision. Ill look for an electronics store in my area.

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    Senior Member guitician's Avatar
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    They call it a multimeter because they can check ohms and AC/DC volts. De-soldering bulbs, solder wick, or a spring loaded solder sucker are the ways to remove the solder from the component before removal. If you press too hard with the hot iron you can damage the foil circuit trace, so be careful. Pins 3 & 6 should have about half of the battery voltage and pin 7 should have all the voltage. A substitute part number is LM741CN.

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    I bought a multimeter but am unsure as to which setting I shoud put it on to check the curcuit. They are: ACV which has either 750 or 200. 1.5V(4.0mA)9V(25mA). DCA which has either 200u, 2000u,
    20u, 200m. 10A.hFE. Ohm which has an option of 2000k, 200k, 20k, 2000, 200.and DCV which has options 200m, 2000m,200, 1000.
    just messing around with it some settings give readings and some dont. What do you mean by "Pins 3, 6, 7? which components or whatnot are they? Sorry I'm quite dense when it comes to things like this, i can read schematics, can solder ok, but other than that Im lost when it comes to electronics. I have a friend that may help me if he can figure it out.

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    Senior Member guitician's Avatar
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    This is the IC viewed from the component side of the board.
    Click image for larger version. 

Name:	741Pinout.gif 
Views:	431 
Size:	4.3 KB 
ID:	15085
    And the schematic for the MXR distortion + circuit is in this tread about making a replacement PCB for one. It shows the traces which is good for knowing what's what.

    MXR Distortion+ PCB Trace

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    thanks. that diagram helps me a lot. still not sure which setting to put my multimeter on when testing this circuit

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    You want to test for DC voltage, so set your meter to read this at whatever scale is convenient to read 9 volts DC.

    Place the black lead into one of the screw holes in the case. This will be your ground connection. Insert a guitar cord into the input jack to turn the battery on. It doesn't matter if there is a cord in the output jack.

    Now take the red lead from the meter and touch it to pin 7 of the chip. You should get a reading of about 9 volts DC. Next touch the red lead to pin 6 of the chip. You should get a reading of about 4.5 volts DC here. Also check pins 2 and 3.

    If you get no voltages at all, check to see that the black lead is making contact with the metal case. You can also touch the black lead to the metal ground of the in or out jack to make the above tests.

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    Thanks a lot for really spelling it out for me. thats what I needed. I'll test it, see what I get...

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    Old Timer J M Fahey's Avatar
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    You want to test for DC voltage, so set your meter to read this at whatever scale is convenient to read 9 volts DC.

    Place the black lead into one of the screw holes in the case. This will be your ground connection. Insert a guitar cord into the input jack to turn the battery on. It doesn't matter if there is a cord in the output jack.

    Now take the red lead from the meter and touch it to pin 7 of the chip. You should get a reading of about 9 volts DC. Next touch the red lead to pin 6 of the chip. You should get a reading of about 4.5 volts DC here. Also check pins 2 and 3.

    If you get no voltages at all, check to see that the black lead is making contact with the metal case. You can also touch the black lead to the metal ground of the in or out jack to make the above tests.
    *This* is the kind of answer that was needed from the beginning, approppriate for the original poster's reality.
    PS: I would only have added: "set the multimeter to the 20V D C scale"

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    damn straight its appropriate for my reality.
    So, I did the above tests and the pins that should be getting half the voltage and the pin that gets all of it are reading between 0.3 - 0.6 volts. I tried checking the wires starting at the battery to see what the problem is and i barely get any readings anywhere. the first wire from the battery gets a little voltage however not much.
    Thats why I wanted explicit directions on doing the voltage test. I figured I was doing something wrong to get the readings I"ve been getting but it appears it aint my doing...
    So, what? rewire the whole thing? Dont need to replace the chip? What is my next step?

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  13. #13
    Senior Member guitician's Avatar
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    That's why I said in my 1st post that all that needs to be done is... replace the LM714. Not much else can go wrong with such a simple circuit. A thorough inspection of the wires and solder joints should cover most of the other problems you may be having with it.

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    Old Timer J M Fahey's Avatar
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    Before chip or any other part change, I'd make certain voltage reaches the board, which apparently is not happening.
    Forgive me for suggesting some apparently dumb checks, but no stone must be left unturned.
    1) Measure the battery voltage *outside* of the pedal. Multimeter on 20V DC scale.
    You should read around 9V, no less than 8.2 or 8.4 V
    2) Plug the battery into its clip which has 2 wires, red and black.
    Follow them to the points where they are soldered at the pedal, red should go straight to the board, black probably to one leg of the input jack, measure right at the solder blobs. Voltage?
    3) Plug a guitar cable into the input, even a spare unmounted plug will do.
    Red still to the positive solder blob at the board, black to the metallic case. Voltage?
    Idem with black to a ground track at the PCB.
    It's usually thicker, goes almost everywhere (maybe that's why it's sometimes called the "common" point or connection), certainly it goes to pin 4 of the IC.
    Idem with black to pin 4 , red to pin 7 of the IC.
    Idem with black to pin 4, red first to pin 3, then to pin 6.
    State all voltages.
    Still on 20VDC scale.
    Good luck.

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    Senior Member guitician's Avatar
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    Make sure you plug a mono jack into the input. I've seen people use a stereo plug for a mono plug and just left the ring unconnected. The input jack is a stereo jack and uses the ring connection to ground the battery and turn on the effect. The "ring" is the insulated second connection behind the "tip" of the plug.

    Another thing I thought of was...if you clip off the old LM741 and tack solder a socket on, it may be easier to fix. Or buy some of the silver glue they make now and avoid soldering all together.

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    actually the red wire doesnt go straight to the curcuit bord but to what looks like a diode attached inside where the AC jack is. the black wire goes to the inside where the input jack is. I read good voltage where those wires start but no reading at the other end of the wires. are the wires shot?

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    if i hold one end of the vfoltage tester on one end of the red wire and the other on the other end I get a full voltage reading. when I hold the tester on both ends of the black wire I get no reading whatsoever

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    I Think I found the problem. forget about my last 2 posts. The red wire connects to a clear diode-like thing. on the in side of the diode I get full electricity, on the other side there is nothing. what is the importance of this component? do I need to replace it or can I just snip it off and solder the wires together?

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    Old Timer J M Fahey's Avatar
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    It's probably a series diode to protect the pedal from a reverse battery or PSU.
    Replace it with a similar one.

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    Senior Member guitician's Avatar
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    Short across the glass diode with needle nose pliers for a moment and see if the pedal turns on, the LM741 may still be bad.

    After looking back at post #12 where you were getting .3-.6V, and it seems that the series diode is where that voltage is coming from. A shorted 741 chip is leaving nothing but the diode drop to measure.

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    Last edited by guitician; 09-16-2011 at 05:08 PM.

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