MXR Distortion+ 1979

[DrT]

I purchased a pair of MXR Distortion+ pedals from 1979.  The seller thought but we're nonfunctioning, but happily one arrived working well.  The other pedal will send a bypassed signal, but won't pass a distorted signal.  It will pick up AM or FM signal when engaged.  All the resistors are working, the pots are out of specification (low resistance), and I cannot find any shorts.  My next round of testing was around the op amp.  I tested the voltages against the working pedal and the V- and Vout appear to be incorrect (0v and 7.5v, respectfully). I assumed that the op amp was the issue, but I don't have any drop in replacements for the 741.  Instead, I have some JCR4558 dual amp chips.  I tried breadboarding in one of the amps but the outcome was the same.  Any ideas on other tests I could do or components I should replace based on this information?  The schematic is attached for reference.

[GibsonGM]

Hi Dr. T, Welcome.

Well, I think with the Dist+, which is very simple, you could get away with actually trying to check each connection.

V- is zero volts, that is ok. It's not a bipolar supply....

V+ seems low, are you using a battery? What is its voltage?

You should have nearly 4.5V at the non-inverting input (pin 3).

If it was mine, I'd pull the 741 and re-check the voltages with no chip there (power, gnd connection, input 4.5V).  Be sure the resistors are good, and the caps, then put in a new 741.    If there are no shorts, that should be safe to do.   If someone plugged in too high a power supply, or a reversed one, that could have boned the 741.

Feel free to use an audio probe to be sure the signal stops at pin 3...if you were very lucky, maybe it goes farther and you can deduce something from that. 

Let us know what you find!

[DrT]

Thanks for the post GibsonGM.  As I went to take a look at some of those values, I noticed a short in the leads coming from my breadboard.  I adjusted the leads and get that lovely distorted sound.  I'll be ordering some 741s and replace the op amp.  I might try some of the other components as well to see if it cleans up the signal a bit.

[GibsonGM]

Hey, good news!  Sometimes that happens.   Yes, do check out the other things.   The electrolytic caps are getting old..after about a decade, they start to dry up, so you might change them.    Wire frequently corrodes, maybe because we store things in garages etc.   Give it a going-over.

If you solder in the new opamp, go fast (like, a "3-count" of tip contat when soldering, then move to an opposite pin.  Allow things to cool)  Don't want the IC to get hot!  Safest route is to use an 8-pin DIP IC socket! 

[DrT]

Yeah, I order a six pack of ic and sockets.  Not the first chip I've replaced.  The caps do look a bit suspicious, but I'm going back and forth on buying a large set as I am planning on moving to a smaller place in the next couple of months and don't need the extra stuff to pack.

[EBK]

You could instead get yourself some SIL machine pin header strips and have any size socket you'll need for much cheaper.

[diffeq]

You could instead get yourself some SIL machine pin header strips and have any size socket you'll need for much cheaper.

Besids, collet sockets are also more reliable.

[GibsonGM]

Those never seem to do what I want them to do.  More labor.  I like the cheap-azz ones you get at suppliers like Radio Shack     They are 'blade type', not holes...

[EBK]

Those never seem to do what I want them to do. 

I'm almost too frightened to ask what you want from them.   

The blade type definitely require less insertion force and are more forgiving to misalignment, but some will say that comes at a reliability cost.  I imagine they will be just fine fine for single use though and will never be the first cause of a failure.

[ElectricDruid]

Picking up radio stations sounds more like an *open* connection to me, rather than a short, so I'd be looking to see that everything is ok and no solder joints have cracked over the years. Also that "optional" 8-10pF cap across the gain resistor looks like a good idea rather than a option.

Good luck!

Tom

[GibsonGM]

Those never seem to do what I want them to do. 

I'm almost too frightened to ask what you want from them.   

The blade type definitely require less insertion force and are more forgiving to misalignment, but some will say that comes at a reliability cost.  I imagine they will be just fine fine for single use though and will never be the first cause of a failure.

"But, but...you gotta cut 'em, and line 'em up and junk!"   Not available locally where I live...but we STILL have a Rat Shack!  I can get the 8, 14 or 16 pin DIPs pretty easily.  Just a preference....I've never had blade type fail, but I almost never remove them once they're in,  tho.   Some experiment more than I with diff. opamps etc...

[EBK]

"But, but...you gotta cut 'em, and line 'em up and junk!"   

I know you're just having fun, but I do want to address those those two points anyway because it took me a while to get those right myself (maybe it will help someone else). 

For cutting, I've found it useful to score them on one side with a blade or chisel, then the other, then again on the first side, and again on the other, until they snap on their own.  It makes nice clean cuts right between the pins instead of the mess you get if you try to cut them in one go.

For lining them up, you've got at least one useless* DIP in your parts bin, right?  Insert that into the socket rows and they will remain held at the correct angles for soldering.

*If you solder well (and patiently), it won't damage the chip you use for alignment, in which case you may ask whether socketing is strictly necessary, but I digress. ....

[thermionix]

Not all blade type DIP sockets are created equally.  Some work great, but the ones Tayda sells are junk.  Good enough to use as a mount, but not really for swapping, they don't handle extraction well.  Their machine pin sockets aren't the best either.  I have some of their SIP strip stuff and it's too loose to grip an LED lead, much less a 1/4W resistor.  Seems some kind of spring contact is needed to maintain positive contact with a variety of lead diameters.