Help with MXR Distortion+ Debugging/Repair

[effectsbay]

Hello All

I had a friend drop off an older MXR Distortion+ to see if I could help. I was assuming that there was a problem with a jack, etc.. you know.. something simple and common. Plugging it in, it became apparent that something more is going on with it. I decided to take on the challenge and dig deeper. Figured it was a good way to learn and get better at this.

The symptom is the distortion sound.. it's super supperty and fuzzy. The more the distortion is cranked, the sustain goes away and cuts out. Sounds nasty, nasely and more fuzzy than distortion. Dialing down the distortion almost all the way, it's almost useable.

What I know (or think I know).
Wiring connections seem good. Nothing loose, solder joints look good.

The board, looks crazy simple with minimal compontents. I did an initial test on the diodes with the multimeter, and they appear to be registering values. My first assumption was the diode was bad/blown. Nothing obvious with the resistors or caps (no discoloring, loose solder joints, etc). There is an IC - 741CN... which I'm assuming is for the buffer portion of the circuit? Again, that looks good too.

Any tips / suggestions on the next steps to go? My gut tells me it's diode since the distortion is farting so badly.

Let me know if you'd like me to post some photos.

Thanks!
hank

[Mark Hammer]

I assume it's this board, or something VERY similar?  The tantalum caps are not likely to dry out over time, the way that electrolytics do.  Quite frankly, the only thing I can think of that might do what you describe is that somewhere along the way the wrong power supply might have been plugged in, and done some damage to either the 741 chip, or those tantalums.  They're all easily replaced.

[GibsonGM]

+1 ^^

What are your voltage readings on the 741 pins, with no signal applied?  And, you could test the tantalums via ohmmeter...

[effectsbay]

I assume it's this board, or something VERY similar?  The tantalum caps are not likely to dry out over time, the way that electrolytics do.  Quite frankly, the only thing I can think of that might do what you describe is that somewhere along the way the wrong power supply might have been plugged in, and done some damage to either the 741 chip, or those tantalums.  They're all easily replaced.

Here is the board:



I could see the power supply issue. He had an plug adapter, which he may have tried a few to see what worked, and fried something.

Thanks!
hank

[effectsbay]

+1 ^^

What are your voltage readings on the 741 pins, with no signal applied?  And, you could test the tantalums via ohmmeter...

Thanks for your suggestions.. great learning moment here.

I put my capacitor tester on the tantalums and they're reading 1uF each, the writing says v105, and a quick google is showing that those are 1uF. Some of the schematics vary though. You suggested ohm meter on the caps. I've never tried that, so I set my multimeter to 2M ohm and the first cap (lower left) increments to '1' and stays there... as if my leads weren't touching anything. The other tantalum peaks at .982 and stays there.

Sorry for my lack of knowledge.. but I'm not familiar with what you asked about voltage readings on the 741 pins. Not sure how to do that test. Can you explain further?

Thanks!
hank

[Brisance]

Connect your common lead to ground and touch all pins with the red one, take note of readings.

[effectsbay]

Connect your common lead to ground and touch all pins with the red one, take note of readings.

Thanks!

[effectsbay]

+1 ^^

What are your voltage readings on the 741 pins, with no signal applied?  And, you could test the tantalums via ohmmeter...

With the IC pointing upward and going from left side down to the right side down.

left side
1 = meter set at 200m, the read out is 2.7
2 = meter set at 20, the read out is 3.04
3 = meter set at 20, the read out is 1.47
4 = 0

right side
5 = 0
6 = meter set at 20, the read out is 7.24
7 = meter set at 20, the read out is 3.30
8  = meter set at 200m, the read out is 2.7

Thanks!
hank

[Brisance]

Hard to decipher which pins mean which by your description, but as ICs have pin numbering standardized, maybe you can post proper pin numbers, referring to this diagram:

[GibsonGM]

Here's a couple of ways to test caps with a meter:  http://www.learningaboutelectronics.com/Articles/How-to-test-a-capacitor

The idea being that you'll see a high resistance (VERY high, 10's of Megs often) which begins to decrease as the cap charges.  It's not about any 'number', it's just about seeing that trend.  Watch polarity on the tantalums.  A dead cap will either do nothing when you connect it, or show very high resistance and stay there.  Don't know what you have for a meter - mine has a cap function so don't have to do this often....smaller caps might not show much, unfortunately.  You could add a series resistor to slow down the (fast) rate of charge they'll have.

Honestly, the easiest way to debug this is to use an audio probe, IMO:  http://www.diystompboxes.com/pedals/debug.html

No fuss, no muss, just follow the signal path til it stops...

[Brisance]

On the audio probe topic, I used to have an output of a sinewave from my computer, from a soft synth in a DAW, then had the "audio probe" connect to the Line in of the soundcard and routed back to daw, there I had a soft oscilloscope enabled which made a crude scope.

[effectsbay]

Hard to decipher which pins mean which by your description, but as ICs have pin numbering standardized, maybe you can post proper pin numbers, referring to this diagram:

So the diagram above it would be...

1 = meter set at 200m, the read out is 2.7
2 = meter set at 20, the read out is 3.04
3 = meter set at 20, the read out is 1.47
4 = 0

5 = meter set at 200m, the read out is 2.7
6 = meter set at 20, the read out is 3.30
7 = meter set at 20, the read out is 7.24
8 = 0

Thanks!
hank

[effectsbay]

Here's a couple of ways to test caps with a meter:  http://www.learningaboutelectronics.com/Articles/How-to-test-a-capacitor

The idea being that you'll see a high resistance (VERY high, 10's of Megs often) which begins to decrease as the cap charges.  It's not about any 'number', it's just about seeing that trend.  Watch polarity on the tantalums.  A dead cap will either do nothing when you connect it, or show very high resistance and stay there.  Don't know what you have for a meter - mine has a cap function so don't have to do this often....smaller caps might not show much, unfortunately.  You could add a series resistor to slow down the (fast) rate of charge they'll have.

Honestly, the easiest way to debug this is to use an audio probe, IMO:  http://www.diystompboxes.com/pedals/debug.html

No fuss, no muss, just follow the signal path til it stops...

Thanks.. I did a test with the tantalums..

"I set my multimeter to 2M ohm and the first cap (lower left) increments to '1' and stays there... as if my leads weren't touching anything. The other tantalum peaks at .982 and stays there."

The audio probe is pretty awesome.. but I do have signal going through (doesn't stop).. just sounds horrible.. like a farty fuzz pedal. Nothing like how the Distortion+ is suppose to sound.

Thanks!
hank

[Brisance]

Is the battery dead, 7.2 V at the supply rail seems awfully low? Might be something other pulling it down as well That might explain the awful sound as well with the dead battery headroom loss.

[effectsbay]

Is the battery dead, 7.2 V at the supply rail seems awfully low? Might be something other pulling it down as well That might explain the awful sound as well with the dead battery headroom loss.

I plugged it in via adapter and re-probed...

1 = meter set at 200m, the read out is 4.4
2 = meter set at 20, the read out is 14.98
3 = meter set at 20, the read out is 3.52
4 = 0

5 = meter set at 200m, the read out is 4.4
6 = meter set at 20, the read out is 17.71
7 = meter set at 200, the read out is 14.5
8 = 0

The pedal sounds the same with battery or using the plug.

[bluebunny]

Do you not have a 9V power supply?  Anyway, those values look quite off.  Pin 2 is too high (almost impossibly high, given the apparent power supply at pin 7).  It should be about the same as pin 3 (and pin 6).  And pin 6 is another fairly impossible value, being a few volts more than the power supply.  BTW, for your voltage readings, set the meter to the 20V.  Pin 4 is correct! 

[GibsonGM]

You'll get it hank, hang in there!  Marc is correct, WTH....you should NOT be getting a reading higher than your battery!    What is your power supply?   I suggest a 9V battery with alligator clips/jumpers.    If you're using a wall wart, it clearly isn't regulated   A battery will give you clean power and eliminate the need to think about THAT in this problem...

Power supply issues might introduce a few other problems.   If the caps aren't rated higher than the DC that was put in them, they could be dead.    So you measure for DC on the input and output jack, too....fastest way to see what's up with a cap is to see if it's blocking the DC that's generally on one side of it.     

While re-measuring the 741, Just measure input and output jacks for DC, black probe on ground and touch red probe to the jack tips.   Should be darn near zero (may fluctuate a few mV, no big deal....you'll know if it's a real reading!).

[effectsbay]

You'll get it hank, hang in there!  Marc is correct, WTH....you should NOT be getting a reading higher than your battery!    What is your power supply?   I suggest a 9V battery with alligator clips/jumpers.    If you're using a wall wart, it clearly isn't regulated   A battery will give you clean power and eliminate the need to think about THAT in this problem...

Power supply issues might introduce a few other problems.   If the caps aren't rated higher than the DC that was put in them, they could be dead.    So you measure for DC on the input and output jack, too....fastest way to see what's up with a cap is to see if it's blocking the DC that's generally on one side of it.     

While re-measuring the 741, Just measure input and output jacks for DC, black probe on ground and touch red probe to the jack tips.   Should be darn near zero (may fluctuate a few mV, no big deal....you'll know if it's a real reading!).

Dudes.. I shouldn't assume the power supply he provided was working correctly. It's a Danelectro DA-1 (9.6v), but hooking up to the multimeter.... it's showing 15.28V!!!



So I think I now know what went south, but need to isolate the components. I purchased the tantalums and the IC, but want to learn from this.. so I want to ID the exact component(s) failures. So I'll keep going.

I hooked up a one-spot with the adapter (verified with the multimeter - reading a solid 9.43v) Here is the new voltage ratings on the pin outs... (all with meter set at 20V)

1 = 0
2 = 4.20
3 = 1.90
4 = 0
5 = 0
6 = 3.74
7 = 9.32
8 = 0

Measuring DC on the jacks.. read out is 0 at 20V. If I set the multimeter to 200m, I do get a .1 or .2 out the output tip

Thanks everyone.. valuable learning!
hank

[effectsbay]

1 = 0
2 = 4.20
3 = 1.90
4 = 0
5 = 0
6 = 3.74
7 = 9.32
8 = 0

Looking at the PDF here: http://www.ti.com/lit/ds/symlink/lm741.pdf

Pin 7 is looking good at 9.32, how can I tell what is the appropriate voltage numbers for 2, 3 and 6? Also, pin 5 is negative voltage (I think).. shouldn't that have a value greater than 0?

Thanks!
hank

[GibsonGM]

Pin 3 seems low to me. Should be close to the pin 2 value (about 1/2 supply).   I'm guessing either something is wrong with the 1M feedback resistor that runs from pin 6 to 3, the .047u cap there, or the chip is blown (or all of the above).    Pin 6 admittedly is kinda close to what you'd expect...check the feedback loop.  Me, I'd just change the 741 as it's so low-cost, but I'd be sure there are no shorts with that cap or resistor, and that the drive pot is ok.  Bet he just blew the chip.

Pin 2 is near 1/2 the supply voltage ("1/2 Vcc") because the 1M that runs from the divider to that pin biases the opamp to run single supply...sets its 'resting point' to about 1/2 the supply so the chip can amplify the positive AND NEGATIVE portions of the incoming signal.   If you didn't do that, you'd only be able to amplify the part of the signal a bit above zero...we call that a rectifier  Sounds like poo....sounds a little like what you described!  *hint*

Pins 1 and 5 are 'offset null' adjustment points, not important to what we're doing here.