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Feb14006The MXR Model 142 was manufactured from approximately 1978 to 1984 when MXR was acquired by Dunlop. Production stopped around 1984 prior to the sale to Dunlop. The 142 was never reissued by Dunlop as other classic MXR pedals were. Although it is the same mustard color as the ubiquitous Distortion +, the circuit design is completely different and although they share the “distortion” moniker they sound nothing alike. The 142 was designed to emulate an over driven amp pushing a 4-12 cabinet.¬† The resonance control provides a signal with a 55hz centered hump or pass-band that could be added to the basic over driven sound. 55hz is the resonant frequency of the old Celestion G12-M ¬†speakers loaded in many Marshall cabinets.

Although the 142 is well designed most guitar players find it a bit bass heavy or woofy with certain guitar/amp combinations, particularly when using 4-12 cabinet in their rig. I came up with some basic modifications that alleviate the overbearing bass levels and allow the resonance control to be used with higher settings on the drive control. I also convert these pedals from an AC power cord to a DC power input connector. The power conversion allows the pedal to be powered by an external DC power supply of 18 to 24 volts. You can run this pedal on a 9 volt DC supply but the internal 15 volt regulator drops out at 9 VDC so the 9 volt supply should be regulated. Dont expect the pedal to sound the same on a 9 volt supply as it does from an 18VDC supply. BTW the internal voltage regulator (LM7815) requires an minimum of 18 volts to supply a regulated 15 volts to the circuit.

Here are the mods along with the PCB parts layout to help you locate the components..

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9 Comments

  1. Hello, I posted over at Indy Guitarist but I figured I would try here as well.

    I bought a used MXR Distortion II a couple of months ago and love it. I do find myself wishing however that there was just a bit less hiss/noise than what it gives at hi-gain settings.

    Would the DC input mod lower the pedal’s noise floor? What about replacing the stock power transformer with maybe something else (and still using the AC-in design)?

    I haven’t opened mine up, but if there are any electrolytic caps in there, would replacing them clean up the hiss? Any other mods that would do this trick?

    I personally am ok with the way the Resonance control affects the tone – I just typically don’t crank it above a certain level.

    I love this pedal so much… It really is, for me, the distortion/dirt that I’ve been searching for all my life!

    I just would like to bring it into its best voice…

    thanks for your help,
    Paul

  2. The noise components generated in the power supply are hum and buzz. Hum is the 60 cycle noise created by the power line. Hum can be picked up by the circuit through proximity to the AC line or the transformer. Removing the transformer from the circuit inside the box eliminates this source of noise. Buzz is generated by the power supply rectification of the the AC power. Typicaly buzz creates a 120Hz interference noise along with harmonic multiples of the rectified AC. Buzz can be eliminated by proper filtering and regulation of the powersupply output. The DII has a well filtered 15V regulator so buzz should not be a problem. Again removing the power supply components to out side the pedal should eliminate any buzz coupled by proximity.
    Hiss is generated by the active gain devices in the circuit, in this case the 4558 and TL072 integrated circuits used. Some noise can be created by high value resistors in the circuit. Modern metal film resistors can help to reduce noise when used in specific parts of the circuit.
    The bulk of the noise is from the integrated circuits themselves. You can try swapping out the IC’s for newer lower noise types. Some IC’s can actually become noisier over time. Look here: http://www.geofex.com/circuits/when_good_opamps_go_bad.htm
    Be aware that you may hear subtle changes between opamp types.
    Another way to reduce noise is to limit the bandwidth of the circuit. This means rolling off the high frequency response of the circuit. This should be done at several points in the circuit to create a cumulative reduction in the noise level. I have done this mod as it is relatively simple to implement. I will work up some more specific information on bandwidth reduction for you and post it.
    Thanks

  3. Thanks a tonne! Looking forward to the extra info and I’ll probably have a few more questions afterwards..

    cheers,
    Paul

  4. Any news…? ;)

  5. Hey,

    Just wonderin’ if you’ve had any more thoughts on this…

  6. Hi,
    if i just want to convert the pedal to a DC power input connector and ignore the “Bassy cure” what parts of the mod instructions i should ignore?

  7. Would you happen to have the whole component layout for the Distortion II? I came across an original PCB but it doesn’t have any component reference. Thanks.

  8. Moved Comment content to a new post

  9. please what would you replace the other opamps with? please


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