Hello! Can I first just say, thank you all for all you do on this forum! It's an incredible wealth of information.
A little background and explanation of the thread subject... I'm not new to DIY electronics, however I've mostly done IoT projects with Arduino and the like, never anything as extensive as a PCB layout and assembly from a schematic. I decided to jump into the deep end and take the Bass Overdrive schematic, layout my own board, drill my own enclosure, and do it all! I thought "hey I need a bass overdrive pedal, why not do that?" Well, I now regret not starting with something simpler, but you live and you learn.
Here's what I've done, I took the schematic, laid out a PCB, ordered all the components, populated the board (with quite a few errors at first) and started the long long journey of troubleshooting. Here's what I've already fixed and learned from:
- On the PCB I accidently mislabeled R14 & R17, they are swapped. Fixed that. I have some sound output now.
- I accidently ordered an MN taper pot for the blend control because I was enticed by the center click. I rewired that off board to rectify mismatching pinout. Now the blend works correctly (although probably not as intended with the normal log pot).
- I accidentally ripped off the pad for the effect audio input, so that has been jumped from pin 2 of the level pot.
- I found and fixed a solder bridge under C13 that fixed a lot of the white noise/hissing sound I was getting.
- I reflowed all of my solder joints and tried to fix any cold joints, underfilled joints, or overfilled joints.
I'm still having a problem with my build that I'm trying to diagnose and was hoping someone could point me in the right direction. I'm still getting too much static noise when the effect is engaged, that gets much worse as the level or gain are increased. I've been testing the effect with a testing rig so it's not in an enclosure shielding it from RF interference when I test, but this noise is still more than I'd expect. Also, when I switch the effect on I get silence for ~0.25 second then the overdrive effect fades in gradually over another ~0.5 second.
I took measurements of my opamps and transistor and found that the TL072 has an unexpected low voltage at pin 3 (around 2.1 volts). All the other measurements are within the expected range.
TL072- 4.23
- 4.23
- 2.1
- 0.01
- 4.15
- 4.23
- 4.23
- 8.45
4558- 4.2
- 4.23
- 4.16
- 0.01
- 4.23
- 4.23
- 4.23
- 8.44
2N3904- 6.33 Collector
- 0.66 Base
- 0.07 Emitter
The low voltage at pin 3 of the TL072 makes me think that the input signal is being boosted too much at the input buffer, resulting in noisy output after the gain stages? Also that voltage must be going somewhere so that might explain the fade on when engaged?
So I poked around that part of the circuit with the multimeter:
Continuity- R1 is connected to input and ground and input is not continuous with ground.
- C1 has no continuity between pins or between each pin and ground and is connected to R1 and R2.
- R2 has no continuity between ground and each pin and is connected to C1 and R3.
- R3 has no continuity between ground and each pin and is connected to R2 and IC1_A pin 3.
Voltage- R1 is 0.01 and 0.01
- C1 is 0.01 and 2.1
- R2 is 2.1 and 2.1
- R3 is 4.2 and 2.1
- IC1_A pin 3 is 2.1
Here are some images of the assembled board, the schematic, and Eagle .sch and .brd files.
https://1drv.ms/u/s!AgoaZE5BhY1vgZwOJG_ ... g?e=dwdzCgI know this PCB is trashed and my soldering skills are subpar, but I'd love to get this pedal 100% working because I have 4 more PCBs from the run that I'd love to assemble and give to friends/family if the effect works of course.
Any thoughts or pointers? Thanks in advance! Also if this is an inappropriate place to post this question I totally understand, please just let me know where it should go if it belongs on BYOC at all.