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PostPosted: Mon Jun 27, 2022 2:30 pm 
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Joined: Fri Jun 24, 2022 4:19 pm
Posts: 4
Location: Seattle, WA
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:
  1. On the PCB I accidently mislabeled R14 & R17, they are swapped. Fixed that. I have some sound output now.
  2. 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).
  3. I accidentally ripped off the pad for the effect audio input, so that has been jumped from pin 2 of the level pot.
  4. I found and fixed a solder bridge under C13 that fixed a lot of the white noise/hissing sound I was getting.
  5. 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
  1. 4.23
  2. 4.23
  3. 2.1
  4. 0.01
  5. 4.15
  6. 4.23
  7. 4.23
  8. 8.45

4558
  1. 4.2
  2. 4.23
  3. 4.16
  4. 0.01
  5. 4.23
  6. 4.23
  7. 4.23
  8. 8.44

2N3904
  1. 6.33 Collector
  2. 0.66 Base
  3. 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
  1. R1 is connected to input and ground and input is not continuous with ground.
  2. C1 has no continuity between pins or between each pin and ground and is connected to R1 and R2.
  3. R2 has no continuity between ground and each pin and is connected to C1 and R3.
  4. R3 has no continuity between ground and each pin and is connected to R2 and IC1_A pin 3.
Voltage
  1. R1 is 0.01 and 0.01
  2. C1 is 0.01 and 2.1
  3. R2 is 2.1 and 2.1
  4. R3 is 4.2 and 2.1
  5. 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=dwdzCg

I 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.


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PostPosted: Tue Jun 28, 2022 9:56 am 
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Joined: Sun Jan 22, 2006 1:39 pm
Posts: 5639
Location: Moses Lake, WA
It's not uncommon to see a low bias voltage at pin 3 with a 1M bias resistor. We've noticed some problems with 2N3904 lately, so you can swap that transistor and you may still have problems. Maybe try a 2N5088. I'd assume your problem is with the "LPB" part of the circuit tacked on at the end. It's only there to boost the overall signal. You can simply omit or bypass that portion to verify that the majority of the circuit is working as expected.

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PostPosted: Tue Jun 28, 2022 3:53 pm 
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Joined: Fri Jun 24, 2022 4:19 pm
Posts: 4
Location: Seattle, WA
byoc wrote:
It's not uncommon to see a low bias voltage at pin 3 with a 1M bias resistor. We've noticed some problems with 2N3904 lately, so you can swap that transistor and you may still have problems. Maybe try a 2N5088. I'd assume your problem is with the "LPB" part of the circuit tacked on at the end. It's only there to boost the overall signal. You can simply omit or bypass that portion to verify that the majority of the circuit is working as expected.


Thanks for the help! Your inclination makes sense. I don't have any 2N5088 on hand, so I just ordered some from byoc. In the meantime, I've taken off the pots and the transistor and will install a socket for the transistor and continue to clean up solder joints. I'll try with a new 2N3904 (I did originally install mine backwards due to a bad datasheet I was using so it got a little beat up) and if that doesn't work, I'll try out a 2N5088 and report back.

Again I'm so grateful for this forum, site and community.


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PostPosted: Wed Jul 20, 2022 9:21 am 
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Joined: Fri Jun 24, 2022 4:19 pm
Posts: 4
Location: Seattle, WA
Well, I've finally found some free time to get back to getting this working. After getting everything cleaned up and put back together the board works and sounds correct. There is still the fade on issue, even with swapping out the 2N3904 with a 2N5088 :(

Here's a few more details on the issue; The sound only fades on for the clean signal. If I have the blend pot turned all the way dirty there's no issue. After probing with an audio probe, I found the clean signal only has this fade on issue after Q1. At the Base pin of Q1 there's no fade on issue. At the Collector pin of Q1 and after in the circuit is where the issue is heard.

I'd really love to get the LPB working but obviously it can be bypassed, and the pedal will work great. Any thoughts?


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PostPosted: Tue Jul 26, 2022 1:31 pm 
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Joined: Sun Jan 22, 2006 1:39 pm
Posts: 5639
Location: Moses Lake, WA
The fact that it is only the dry signal that has the problem and that it only occurs after Q1 is extremely puzzling. You could maybe try adding a capacitor between IC1b and lug 2 of the blend pot. You'd also need to add a bias resistor off of pin 5 to 1/2v since you've decoupled from vref.

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Please do not PM me. email is prefered. keith@buildyourownclone.com


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PostPosted: Tue Aug 02, 2022 4:59 pm 
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Joined: Fri Jun 24, 2022 4:19 pm
Posts: 4
Location: Seattle, WA
byoc wrote:
The fact that it is only the dry signal that has the problem and that it only occurs after Q1 is extremely puzzling. You could maybe try adding a capacitor between IC1b and lug 2 of the blend pot. You'd also need to add a bias resistor off of pin 5 to 1/2v since you've decoupled from vref.

So some good news and some bad news! The good news is I got the problem with the fade on all sorted out. The bad news is there was no problem with the pedal this whole time... :lol:

After deciding to breadboard just the problem area of the circuit the same problem was happening! Then I decided to breadboard just the LPB-1 circuit! Still happening! And it's like the most simple circuit so I was sure it was put together correctly! Which led me to look into outside factors... Then I found the big issue.

My effect tester box I was using this whole time was wired so that when the effect was bypassed it also disconnected the effect from ground. I did this intentionally thinking that it'd be a good idea to have the effect completely off when not using it. But I unintentionally made it so that when trying things like bypassing the effect, it has no power... thus the fade on. I'm actually quite surprised it had no problem with this when the blend was set to fully wet :roll: After rewiring the effect tester box there is no more problems! Now I can box this thing up and enjoy it!

Thank you so much everyone!


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PostPosted: Tue Aug 02, 2022 6:49 pm 
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Joined: Sun Nov 18, 2012 5:13 pm
Posts: 4621
Location: Soddy Daisy,TN
Nice, we’ve all been there. That’s my go to overdrive for bass.


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