Help with MXR Distortion+ Debugging/Repair

[effectsbay]

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.

Looking at the schematic.. are you meaning the pin from 6 to 2 (not 3)? I'm also assuming you can't technically test resistor when it's in the circuit correct?

When I test that resistor on the board.. it's reading .876 (not 1M), but I'm wondering if it being soldered in the board it's not accurate.

I measured ohms on the .047u cap to see if it was dead. Set multimeter to 2M ohms, and it's reading .595. The article are about bad caps with ohms.. says 0ohm or several megs is bad, so I'm guessing that this is good?

Thanks again for all the help!
hank

[Brisance]

Thanks everyone.. valuable learning!

Since that seems to be one of the objective, and being one of the divinest! (self education really is the best goal in doing anything) How about, while debugging it with us, trying to figure out, how that effect works exactly? It's a good start too, one of the simplest.

[bluebunny]

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!!!

That's OK.  As Mike suggested, it's probably an unregulated supply: without any load, it puts out a higher voltage than it says on the tin.  But unlikely to have caused any damage (unless the tants were particularly low-voltage specimens).  But as Mike also says, for such a cheap part it won't hurt to swap out the 741 (and perhaps put in a socket too), and the tants are relatively cheap too.  Nothing much else is damageable, unless it was plugged into the mains.   

[Brisance]

and since both tantalums are a meager 1u as well, why not replace with more reliable ceramics?

[GibsonGM]

+1    If you have any poly's around, I'd just put them in if you care to take the time.  Ceramics may be more 'grainy' sounding...  After we get it working would probably be best, tho, ha ha.

The best way to test a cap, IMO, is to look at the schematic and see what they do.  Dist +, they'll all have DC on one side.     Power it up....measure for DC at the input and output jacks, and at the drive pot that connects to the .047u.     You said no DC already - caps are good with about 99% confidence.   They are there to block DC that's used for the opamp to work, and to pass AC signals (as well as filter them, giving us nice tone shaping).     If they're blocking DC they are probably fine (unless open, ha ha!  But you hear some signal, so...).      

Yes, when parts are in-circuit, they often look odd when testing...the .8xx meg is fine.   Charge is bleeding to the cap there most likely. I wanted to see if it was OPEN (burned) or SHORTED (burned, fused, like a wire).  

A somewhat blown IC, showing those voltages, will amplify 1/2 the signal, resulting in the VERY distorted sound you noted (1/2 wave RECTIFIED).   Sounds like only 1/2 the signal is being processed....sounds like clipping, sounds like crap        New 741, re-test, and see what you get.   If you do your soldering gently and well, it should be up and running after that!

[PRR]

> how can I tell what is the appropriate voltage

"Generally", battery-power pedal op-amps will show---

One power-pin at battery voltage (or a volt down if there is a power filter)
Other power pin at zero
Output at "half" of supply voltage (so the audio can wiggle both ways equally)
Inputs maybe at half-voltage, or maybe sagged way down by meter loading.

Stray (unknown) pins maybe should NOT be probed. The '741 has "offset" pins which adjust DC precision which we do not need. The circuit behind these pins may be very delicate. Even more delicate is the "Iabc" pin on the '3080 OTA (gain-controlled amplifier) which is incrediby easy to blow.

So look-up the pin-out, find V+ V- and OUT, look for reasonable voltages (9V, zero, 4.5V). If it has power and the OUT is about halfway between the power pins, it is probably healthy and happy.

[effectsbay]

Ok.. update on this. I built an audio probe.. and damn... these are soooo useful. I was easily able to identify that IC was the problem. Here is the vid of the probe with the Distortion+

https://www.youtube.com/watch?v=1lJaFZMd9ro

I replaced the IC and included 8pin socket to swap it out easier down the road, and the pedal sounds great. Thanks for everyone's help on this!!!

hank

[GibsonGM]

Good work!

Know what works really well with that audio probe?  R.G.'s "quick and dirty oscillator", which you slap together and use to give yourself an input signal to trace!  No more need to strum a guitar while you work.
  http://www.geofex.com/FX_images/q&dosc.pdf