Vintage MXR Distortion + Repair! Gray Bench Electronics 

GRAY BENCH, you should make a separate video about the slew rate distortion on the oscilloscope because the 70's 741 chips had different specs on the datasheets for the slew rate time and rise time. The new 741 chips are faster slew rate and rise time so you don't get that slew rate distortion. You can compared the older chips and new chips with a 2 channel Oscope to compare the slew rate distortion in a video lesson.

One thing to note is R3. It's showing a 4k7 resistor, but I have seen stock D+ pedals with 1k in this slot!! We should understand what that one change does and net effect of lowering it. According to the Wampler Distortion Plus MODS page one can find, Mike says changing the 4.7k to 1k and the 4n7 to .22uF increases gain but does not change the high frequency response! Doing another search for the video "How MXR Distortion + Reinvented Drive Circuits | Gain Appreciation", at 3:16 in, it shows you the formula for Gain = 1+(1M/(gain know resistance+resistor)), which is 1+(1000000/0+4700) = 214 = +46dB. Changing the 4k7 to 1k would result in = 1+(1000000/0+1000) = 1,000! So about a 5x increase in distortion. Not sure what that equates to dB, but > +46dB!!

Outstanding video, I wish I knew what you know. But very informative, and inspirational Joe, I have a couple of wonky pedals on my junk box , ill dig them out and try some simple remedies.

Always appreciate someone showing cohesive troubleshooting. The only thing I would have done differently, especially with old fragile traces like that, would be to put a socket in first. Then the owner can play with all that personality change stuff that others are discussing in the comments.

I've got a few of these pedals. Three of the 2 knob version and one of the four knob AC version (aka MXR Distortion II) Which I bought new in the early 1980s. I can attest to the foam problem. It's a pain in the ass. But this one really looks good on the outside. And you did a great job cleaning it up. Rock and Roll!

A masterpiece. Like an Indians jones archeological find and analysis

They came with foam wrapped around the board originally, but it rots, eventually it turns to corrosive goo so anyone with old MXR pedals should remove it. The AC powered ones you can get rid of the goo on the back plates with some acetone. I use fish paper to insulate the boards in the small ones.

Great! I was gifted a broken Script Logo Distortion + and was just getting ready to repair it. Perfect timing! Thanks 👍

I have a 78 block logo that had the exact same problem. I replaced the op with a newer Texas Instruments 741 until I finally found an older one that was an exact match to the original. They all sounded the same to my ears. The only reason I did it was to keep it looking stock as much as possible. Great video, thanks!

I just repaired my Distortion + from 1981. I installed a new op-amp and it's now working ok. The problem I had when removing the internal wiring and hardware from the casing was old, dry solder joints. Every move I made broke a wire and I was forever resoldering the connections. My 1981 version has an LED and a power connector. Not sure if the LED was a modification from a previous owner. Thanks for the video.

Brass gun cleaning brushes are great for cleaning the inside of jacks.

I love these videos man, keep up the good work.

Those solid tantalum capacitors make this effect performs exotically. You can replace 741 opAmp with CA3140 or LF351 or LM301A or LM318 or TL071 for different personalities.

Really enjoyed this, both entertaining and informative. I run a little mods and repairs shop in Scotland and regularly tune into your videos for more insights, keep up the good work!

Thanks! Great walk through troubleshooting process

Killing it! I’m starting my own effects company called Evil Eye FX after owning a guitar shop for the past few years. Your videos have really helped me learn the basics in pedal building and repair! Thanks man!

Another great video! I really dig your attention to detail and the simplicity of your videos. It really shows how much the content matters and your content goes to 11 on a scale from 1-10.

cool! glad you got it fixed Joe!

I think i just watched all your videos in the last couple days. My new fav channel. I really like your guitar chords you play. Im about to start a pedal build with NO knowledge of anything. Could you show us some of your guitar playing?

My first thought was a socket for the OP amp but honestly now looking at it that might be too tall to put the backplate on. Fantastic vid!

This video's a bit more "ELI5" which I enjoy. Thanks!

Cool video. My first pedal was a script logo MXR Distortion + that I bought new back in the mid-70s. Still my favorite OD pedal. Wish I still had it. Do you know if the newer ones are the same as the original ones?

my pedal is a distortion III, it spent a long time with the battery inside, which broke and stopped turning on the distortion. the two states (on and off) the led appears lit, while the switch when in the off state, the true by pass works, but when in the state that should be on, it does not output any sound. I will try to change the diode and capacitor to see if it comes back to life. I am sad...

The circuit looks very similar to the 1974 Ammeron distortion pedal without the circuitry connected to the offset pins and final buffer stage.

5:29 I had to roll over and have a smoke. Definitely brought some metal on those quarter inch jacks.

Another awesome video, did you get this off ebay? I may have bidded against you a month or so back haha! Very cool to see you go through the signal path with the scope, very helpful to understanding things.

Awesome! Were are you located? Can a ac plug be put in one?

Man, that jack has never been "cleaned" like that before.

I'm subscribed, FYI. I have a basic understanding of electronics, but I'm wondering if you have any videos that would help me further that. I enjoy building, modding and working on pedals, but would feel better if I knew more about what I'm doing. Thanks!

Hello, Joe. What are you doing with the scalpel after applying the de-soldering wicker? Scraping the pad?

Tape a penny to the top of the op amp to act as a heat sink.

Hi, great video. School explanation! I too would like to replace the foam. What material do you use to cover the pcb? Plastic, plexiglass or something else? What is this Boss-like cover for? Thank you

Are you going to keep the vintage “broken” op-amp IC?

Greetings from Buenos Aires, Argentina!! What's what you sprayed into the pots? Thanks! Your channel is really great

If you had to replace the battery connector in a vintage pedal like this, would you try to find a NOS one or whatever you had in stock?

Mid to late seventies and it's fifty plus years old? Don't go ageing us prematurely dude! ;-)