I recently breadboarded a clone of an MXR Distortion+, using the schematic attached. I made few modification, basically because I didn't have the exact components/values. Here's the main differences between the schematic and my build:
- LM741 -> I used a TL071
- 4.7K resistor before the 1M gain pot -> I used a 5K
- 1uF tantalum capacitor -> I used a 1uF electrolytic 50V
- clipping diodes -> I used a couple of red LEDs
As far as the rest of the components, I followed the schematic. I just added the click-preventing 1M resistor right after the input, but omitted the 10pF capacitor in series with the 1M resistor between IC's negative pin and output pin (I didn't have one handy).
Now the problem is when I power up the circuit with a 9V battery, it sounds good for a few seconds, then the sound quickly degrades into a sort of fuzz, then a further degradation make the guitar sounds like it's... well, farting
After that, it's basically muted.I double checked the schematic and tested the resistors with a multimeter, just to be sure I haven't mess up with colors, and it all looks good.
What's weird is that if I replace the 1uF electrolytic capacitor that it's connected between the three 1M resistors and ground with a 3.3uF, the sound degrades a little more slowly. If I use a 100uF, then I can play through the pedal for 5 minutes before I start to hear some sort of degradation. If I remove the capacitor and manually short it to discharge it, then put it back into its place, it all sounds good again for another 5 minutes. I then started playing around with different kind of clipping diodes, and discovered the tonal difference between them. So it looks like the circuit is somewhat working from a listener point of view.
Could the few components I replaced the origin of this degradation? Now, I ordered few 741 and a few tantalum caps to match exactly the schematic, but meanwhile is there anything else that I should debug?
To complete the information, here is the IC pins readings. I measured the ground in two places:
1) the 1M resistor right after the input:
- pin 1 -> 0.13V
- pin 2 -> 3.75V
- pin 3 -> 2.11V
- pin 4 -> 0.00V
- pin 5 -> 0.06V
- pin 6 -> 7.34
- pin 7 -> 8.00V
- pin 8 -> 0.00V
2) the 100uF (in place of the 1uF) electrolytic cap mentioned above
- pin 1 -> 0.04V
- pin 2 -> 3.76V
- pin 3 -> 2.00V
- pin 4 -> 0.00V
- pin 5 -> 0.13V
- pin 6 -> 7.41V
- pin 7 -> oscillates between ~ 7.60 and 8.00V
- pin 8 -> 0.00V
Also, forgive any mistake you may find while reading: English is not my mother tongue.
Many thanks in advance!
