Skip navigation

Feb14006

Some general info first.
Schematic link

Layout link

Parts list

On to some more mods!

1. Bandwidth reduction for less noise
• Increase C4 (20pf) to 50pf or 100pf
• Increase C7 (50pf) to 100pf or 120pf
You can also parallel two or more small value capacitors to achieve non-standard values like 40pf or 70pf. Adjust to your taste
2. Op-amp replacement for less noise
• Replace U2 (RC4558) on the schematic with an op amp with a lower noise spec.
• Jack Orman’s op amp noise calculator here  to compare various op amps. I like the TLC2272 as a general low noise replacement.
• Of course you could replace all the op amps with lower noise types to reduce the noise even further. I would install dip sockets so you can audition different types easily.

3. To  mod the pedal for DC power follow these steps:
A. Remove the AC power cord by unsoldering the AC power cable from the printed circuit board. Remove the grommet holding the cable. Squeeze the grommet from the outside untill you can pull it out.
B. Install a standard 2.1 mm power connector in the hole left by the power cable strain relief. This is a standard Boss style connector available here.
C. Connect the outside (+)  conductor of the new power connector to Input pin of the LM7815 regulator with a new wire.
D. Connect the center(-) conductor of the new power connector to the negative terminal of C19 (400 mfd).
Also as a general rule replace C19 while you are in there. The original 30 year old caps are a frequent failure point in these vintage MXR pedals.

 

4. Mod for reduced bass response:

  • Remove C2(1.0 mfd) and replace with a  .022 mfd film capacitor.
  • Remove R16 (75k) and replace with a 150K resistor.
Advertisements

23 Comments

  1. Concerning C4 and C7. It looks like you are in essence saying to roughly double the value on both of those caps to decrease some noise. Do you have a personal favorite value3 and why? When you say noise, I am thinking high end/white noise type hissssss? Is this correct? Also, can you tell what effect, if any, this has had on the eq or tone of the overdrive? Any commentary on that I would appreciate, thanks.

  2. @Matt

    I am using 50pf and 100pf for C4 and C7 respectively. I choose those values as they are standard available ones. Yes, noise refers to hiss in this application. The tonal effect is to roll off some high frequency content along with the noise. I am happy with the overal response of the pedal but different guitar pickup combinations can determine the final tone of the pedal. As you increase the drive on the pedal it becomes subjectively brighter due to the addtional upper harmonics. So if you use the pedal at higher gain settings you may want to increase C4/C7 even more. You can also use the filter control to tame the high frequency noise at all drive settings.

    • On mine, I have done the resonance mod, the ac adapter mod, and output boost mod, and changed the main OD chip. I run the drive/tone/output all about 2 o’clock and the resonance at zero. This is the sweet spot where everything sounds right. For lower gain setting, I can turn the tone up some, but I have found that to turn it down any less than about 2 o’clock, it removes too much higher mid cut along with the top-top end hiss. i have found that just cracking the resonance control, say from between 0-1, can sometimes add a certain slight push to tone, but turning it anything past that, even with the mod, would be something more of a special effect because of all the low end added. So, I usually just leave the resonance at zero. i would really like to see someone build this overdrive circuit stripped down to the actual overdrive circuit without the wierd switching, and without the resonance circuit, with improvement to the tone circuit for more tone and less noise/hiss. It seems like pedals have come a long way since the introduction of this one back in the 80’s and it could benefit from the advances. The only guy I know of that has tried to duplicate the distortion ii is “madebymike” but sadly, his version does not sound anything like the natural drive of the original and he is trying to duplicate the Smashing Pumpkins sound when it is used with a fuzz. I could care less about that. What I love most about the Dist II is the natural sound, range of gain from low to high, and the way it reacts so well to the guitar volume control. Very, very amplike. What I don’t like is the hiss, the resonance control, and the un-needed switching. Millinium bypass would be better. I use the pedal a lot live and get a lot of compliments, but i just feel like someone should redo this pedal right. I am actually surprised MXR hasn’t done anything with it. It’s just still way under the radar I guess. I also read where someone said since the resonance circuit was basically a parallel gain stage, you could actually just get rid of the eq part and make the resonance control another adjustable gain stage. I thought that was interesting too. Imagine a Dist II with 2 adjustable gain controls and a really usefull tone circuit. I’lll have to try the cap changes at some point and see if it works for me or not. If you ever hear of anyone making a refined version like I have described I’d love to know. Thanks for your help.

  3. Well, it looks like BYOC is now making a copy of sorts with some changes. Haven’t heard any sound clips yet. Looking at it, I can’t help but think that changing from tantalum to electrolytics and from carbon film to metal film resistors is not going to get the warm smooth sound of the original. I know for a fact that the pedal sounds better at the original voltage levels as well.

  4. Hi guys. NOOB question.
    I’m done with the DC adapter (boss type). With 9v it run well. But can i feed to 12v or 18v?
    Thank you

    • The pedal has an internal 15 VDC regulator chip. You need to feed it 18 VDC or higher for the regulator to work. Don’t go over 24 volts as C19 is rated at 25 volts.

        • David
        • Posted April 3, 2015 at 7:06 pm
        • Permalink

        Thanks for the tip ironman. I’ll try with a new power adapter.
        I want to change the C19 also. I can’t find the 400uF value on ebay. MIne is the big blue Sprague 300D 400uF 0-25VDC
        Can i change it with this one?: http://www.ebay.it/itm/181016972564?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649 or this one http://www.ebay.it/itm/371290805485?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
        Reduced bass response is a great mode. Congratulations!
        I have some doubts wth the “Bandwidth reduction for less noise” because i don’t want to lose some high frequency since use always the gain at 12 o’clock, not more. What do you think about this?
        Thank you!

        • ironman28
        • Posted April 4, 2015 at 12:36 pm
        • Permalink

        The value of C19 is not as important as the voltage rating. I use 330uf or 220uf @ 35vdc capacitors as replacements. Modern capacitors are smaller so you may find it easier to use a radial type as they are much more common. The bandwidth reduction is not required unless you find the noise objectionable at higher gain. I generally like to use roll-offs between 5KHZ and 6KHZ. Most guitar speakers exhibit similar high end roll-off but in the end it is a matter of taste.

  5. Another thought.. I need to replace the switch on/off because doesn’t work properly. Can i use the new Carling 112P switch with the same wiring of the original mxr switch?
    Thank you for your time.

    • I believe the original switch is a Carling 112P. Any quality SPDT should work as long as it clears the PCB cutout.

        • David
        • Posted April 4, 2015 at 8:05 pm
        • Permalink

        Ok thank you.
        Can i replace all op-amps with the TLC2272 except the A1 (on schematics)? It will affect the tone?

        • Matt
        • Posted April 4, 2015 at 11:24 pm
        • Permalink

        As far as the TLC2272, I would only recommend it, or any other IC you like, tried in one place, and would leave all the others as is with the original TL072 in place. If you have the back off and are looking at the board with the pots at the top, it would be the top most IC on the board kind of in the middle. It’s also right above the cap/resistor mod that trims the bass response of the resonace circuit. Stock, I believe it’s a 4558 . That’s THE one to swap IC’s in to find the flavor you like. So I’d recommend putting a socket there, and swap IC’s till you find the flavor you like. To me the TLC2272 sounds the most natural and amplike, blended the best with my amps clean tone, which is why I liked it most.

        • ironman28
        • Posted April 5, 2015 at 1:03 pm
        • Permalink

        I don’t typically “roll IC’s” looking for great tonal differences but the MC33178 has some great qualities and is used in several great sounding pedals.
        Give it a try in place of the 4558.

  6. Thank you guys. I’ll try the TLC2272 and the MC33178 soon.
    The original “A1” CD4016BE op amp should be replaced after 30 years of working?

    • Another question. What are the correct value for the pots? (i mean drive, reso, filter and volume) because mine are not original
      Thanks

        • ironman28
        • Posted April 6, 2015 at 7:47 pm
        • Permalink

        Drive, Resonance and Output are 50k. Filter is 500K

  7. Linear pots for filter and resonance and audio pots for output and drive?

    • Can anybody PLEASE send me an audio of this pedal?

        • ironman28
        • Posted June 17, 2015 at 4:26 pm
        • Permalink

        Here is a You Tube video of the stock pedal:

  8. Thanks! : ) i saw that demo of this pedal long ago. but i want to hear a little bit more of this pedal it’s a shame that there aren’t more demos of this pedal. if anybody in here has made a clone of this one and has an audio please post it in here i want to build a clon but i don’t know if the clon sounds like this :/

  9. Hi ironman. I have one question.
    There is a mod for increase the volume of the pedal for use it like a gain booster?
    Thanks

    • You can increase the value of R21 to boost the output level. If you raise the value too high you may find that the pedal starts to self oscillate at high gain levels. This is because the internal wiring is unshielded and coupling can occur between the input and output wiring. The best solution is to replace the unshielded leads from the input and output jacks and to and from the output pot with shielded wire. You can also try moving the wires around to eliminate any oscillations if they occur. I would start with a 15k to 20k in place of the R21 10k to start with and see if it gives you enough boost level.

  10. Hi ironman.
    A question on “Drive”.
    There is a mod for increase the distortion of the pedal?
    Thanks

    Damiano, from Italy


2 Trackbacks/Pingbacks

  1. By MXR Distortion II « nonLinear on 12 Aug 2012 at 7:09 pm

    […] Here are the mods along with the PCB parts layout to help you locate the components.. Like this:LikeBe the first to like this. […]

  2. […] the electrolytic coupling caps start to dry out and affect the tone. Here's a link to my mods: MXR Distortion II mods | nonLinear There are some other mods I do that help improve the tone without altering the fundamental sound […]

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: