No sound coming out of my vintage MXR Distortion +. Need some help please.

[DirtyDeeds]

Hi there!
I recently acquired this vintage (not sure how to identify the exact year 🤔) MXR distortion + and it is not working... the red led turns on OK but there is no sound coming out of my amp, no noise, nothing... there is sound of my guitar when I turn it off so I don't think the switch is the issue really. I have very few knowledge in electronic so I can't really identify the source of the problem... I posted a few photos below so maybe someone can already identify something wrong with the circuit? I precise that I tried the pedal with different cables, 9v battery, adapter, and even replaced it with other pedals (one at the time) and all of them were were working at the exception of the Distortion +.
I have a solder iron and know how to use it if needs it to replace parts or else.
The foam inside is very damaged and gooey, I plan remove it tomorrow and clean the parts with lighter fluid. So I could then take and post more photos of the other parts if needed.
All your ideas and advices to help me identify the source of this issue is more than welcome and appreciated. 🙏

[idy]

Welcome to the forum!

Most likely things are: loose wire (cold solder joints look ok, but aren't) broken foot switch, dodgy jack.

The thing is to get a meter and check continuity through jacks and foot switch, then measure voltages on the IC. After that most helpful may be a signal probe (old guitar cable with an alligator clip on the ground wire and a capacitor on the signal lead) and follow the sound.

[CheapPedalCollector]

1st things first, get rid of all the foam, it will corrode everything it touches as it degrades into goo. Looks like it has already started to do this, and you can clean it very carefully with a very hard grade toothbrush and some 98% rubbing alcohol (wear eye protection and nitrite gloves). Check that the jacks are not severely corroded, this is the most common thing I see with MXR pedals. The tantalums *can* go bad, but not too often.

Build yourself a simple audio probe and start checking the circuit for where the signal stops, it's very simple and you should be able to diagnose it fairly quickly.

[Kevin Mitchell]

RIP opamp.
Lookin' pretty toasted there.

Replace with a socket. LM741 (or UA741)  are still available from various placed.

Repair done-dirt-cheap.

[Mark Hammer]

1) Welcome.  Make yourself at home.  Check out the Layouts Gallery above, and do yourself a favour by reading the FAQ and debugging links above.

2) It's a later issue Distortion+, likely early 80s.  First issue were script logo, and status LEDs and external power jacks were later additions.  Not a diss.  Rather, it gives you latitude to mod the hell out of it, because it holds no "collectible" value.

3) Recommendations for how to clean the decomposed foam are sound.  You'd probably get a better scrubbing of everything by removing the board, switch, pots, and jacks from the enclosure, leaving a bare box to clean thoroughly.  Not so sure lighter fluid is optimal.  You want to be able to see all the legending everywhere, so alcohol might be better.  At the very least, a good scrubbing with a DRY firm toothbrush will get most stuff off.

4) There is a good chance the pots will be scratchy from decomposed particles rolling around inside.  You CAN clean pots more thoroughly by gently prying the tabs of the back cover up to remove the back cover.  DON'T force those tabs too much because they can only take a bit of bending before they snap.

5) I am of the view that MXR used tantalum caps because there wasn't much room in the enclosure, given the size of the jacks, pots, and footswitch, so they needed caps that could be bent over to lay flat.  At that time, tantalums - though much pricier - were often a lot smaller than their electrolytic counterparts, for the same value and voltage.  Bent over, they were even more compact.

6) Even if restored to near new condition, you may be disappointed by the relatively low volume the unit has.  This has two culprits.  One is the use of germanium clipping diodes.  They provide more sizzle and distortion for the amount of gain applied, but they put a hard and lower cap on the maximum output level.  Swapping them with silicon diodes will noticeably increase volume level.  The second culprit my be the volume pot value.  I don't know when the value changed but earlier D+ units had a 10k volume pot.  Collectively, we've found much greater maximum output level using 50k or even 100k volume pots (audio taper), and MXR did change to a higher pot value in years after the script issue.

7) While many other "modern" op-amps will work in the D+ circuit, the circuit is what I like to call a "double clipper".  The gain applied pushes the 741 past its headroom capabilities, such that it is already clipping even without any diodes.  In the early years of this forum, builders would ask "Why can't I get anything clean from my Distortion+ ?".  That's why.  The diodes add a second layer of clipping over and above what the limitations of the 741 provide.  Feel free to install a socket where the chip goes, and try out other internally-compensated single op-amps like a TL071, CA3140, or NE5534, for example.  A 741 is the most "authentic", but if you can provide yourself an opportunity to learn a little more about chip-choices, why not?

Why is there no sound?  Chances are slim that it is the footswitch.  Those Carling switches are pretty robust.  You'll want to verify that +9V is reaching pin 7 of the chip, whether using battery or external power.  You'll likely want to change the power jack for a more standard 2.1mm barrel type, which most contemporary pedals adopt.  These come in two varieties: one where the retaining nut is screwed on from the inside of the box, and one where it is screwed on from the outside.  The first type takes up more internal space and is a royal pain to install, so use the second type.  This will require enlarging the hole with a drill or reamer.  yet one more reason for taking everything out of the enclosure.

[PRR]

> how to identify the exact year

The '741 may be date-coded.

[CheapPedalCollector]

Date codes on the pots, 137 (CTS) then year and week in 2 digits each. It's only a ballpark though, they bought in bulk.

My 75 Dist + has a 50k volume pot. I've had a couple dozen of them, never seen one with a 10K, it would be well below unity gain with that. 100K is a nice mod for lots more output to drive an amp with.

Early paper label ones didn't have the two 1nf caps rolling off the highs. They added that later because of picking up radio signals I think rather than tonal choice.

Some units have 1n276 diodes which are in blue casings with a black stripe.

If you socket the 741 get a low profile socket or it won't fit back in the case very well. A lower noise op-amp will sound the same. Script ones used a NS ua741.

I cut up thick plastic sheets in the same size/shape as the circuit board to use as an insulator under them so it doesn't short against things.

Easy mod for ones with the LED is to change it to millennium bypass so you can have true bypass (they tone suck a lot) and still have the LED.

Another mod is to change the 1/8th power jack to a switched type and remove the germanium diode to give yourself a small voltage boost from the battery so it will sound closer to a script one.

The ceramics can absorb moisture too, I've fixed a couple that had excessively noisy caps that sputtered and made all kinds of racket.

[GibsonGM]

Is that the Chia version of the Dist+?    Nice find, and welcome.

Only thing I'd add is, in case you don't know - be very careful about wiring up a new power jack and that jazz...you don't want to accidentally reverse polarity and undo your hard work!  Test using a 9v battery til it's running.  Maybe check out the 'Boss power configuration'.   And, some reverse polarity protection might also be a good idea when you get it going.  It's a simple circuit, but cool and worthy of tacking in a reversed diode, IMO.

  Dist + and an eq pedal is a lot of fun!  I use one all the time.

[CheapPedalCollector]

Yes, they use negative ground (+ tip) so maybe that's actually what happened to it...

[Mark Hammer]

As convenient as they could be in other ways, one of the problems with 1/8" phone jacks was that people would forget to plug the supply into the pedal before plugging the wallwart into the wall or extension cord.  The nice thing about 2.1mm plugs/jacks is that you can stick the plug into the pedal either before OR after you connect the wallwart to the house AC.  The downside was that, going back 40 years, some such wallwarts were tip-positive and some were tip-negative and not enough of us were aware of the difference or how to recognize it.  My own view is that's what led manufacturers to insist on using THEIR power supplies, just to avoid the nuisance of repairs stemming from someone using the right power with the wrong plug orientation.

[ElectricDruid]

> how to identify the exact year

The '741 may be date-coded.

Correction: The '741 may have been date-coded!!



I agree with what other have said, but particularly Kevin. Take it apart, clean all the foam and goo off it, and then replace that op-amp with a socket and new amp and see what happens. Perhaps it's not as bad as it looks on the photo, but on the photo it looks TERRIBLE!

[Mark Hammer]

replace that op-amp with a socket and new amp and see what happens. Perhaps it's not as bad as it looks on the photo, but on the photo it looks TERRIBLE!

Tom, do you realize just how many players have tried in vain to achieve "the brown sound"?  Maybe Dirtydeeds should leave the chip where and how it is.  It just MIGHT be the magic!!

[DirtyDeeds]

Wow I really appreciate all the replies, tips and advices thank you so much! 🙏

At the end I replaced the op amp (one of it's leg was off) and it works perfectly fine!! I also removed the decomposing foam inside and cleaned most of what I could without removing all the parts. The pedal works perfectly fine now!!! 🤘
Thanks a lot everyone you've been really helpful.

[GibsonGM]

Great. May have been that the degraded insulation allowed (the opamp?) a part to contact the enclosure and fry.   Enjoy it! 

[Mark Hammer]

Congrats DD!  An auspicious beginning to your membership here.