Tone bender Mk3 - No sound unless I strum the guitar REALLY hard

[neiltheseal]

Hi everyone. I hope this is the right spot to post this

I just put together a Total bender from PCB Guitar mania. I have managed to get the Mk1 and Mk2 working but am having trouble with the Mk3. I'm hoping someone can help.

This is the silicon version and in the Mk3 transistors are 2n3904.

Currently I can get a very faint signal out of the Mk3 which seems to go to full volume and sound normal when I strum the guitar really hard. Below is a video.

https://www.youtube.com/shorts/Q1Ct6-CEC-w

I have asked around and the consensus seems to be that there is an issue with this PCB at least with the Silicon version. I have checked all of the parts are correct and re flowed solder etch and it all seems correct. Plus it sort of works.

Does anybody know what might be the issue here?

Build docs here https://pcbguitarmania.com/wp-content/uploads/2022/05/Total-Bender-Building-Docs-3232.pdf

MODIFICATIONS

- Someone in a facebook group suggested I put a 100nf capacitor in c3.4 where the build docs said to put a jumper. This has helped somewhat and now I get an audio signal (before that there was nothing)
- I put r3.7 (10k) in when the build docs say leave blank as this was in the original schematic.
- Removed c3.9 (470p) as this was not in the original schematic (although I just clipped the capacitor out and didn't remove the legs.
- Reversed the diode (d1) next to q3. I did this because comparing NPN and PNP schematics that they were reversed. My soldering here is bad I know.

For q3 voltage, flipping the diode, adding 10k resistor and removing the capcitor had an effect.
- Previous voltage was: collector is 8.48v, base is now -0.01v and emitter is 0v.
- Current voltage: Collector 4.27, base is 0.16 and emitter is 0

Here are some pics

https://imgur.com/gallery/PKu0iM4

Thanks in advance!

[mauman]

OK, let's give this a shot. Since 2/3 of the pedal is working, that's a good start, you've got power, in/out connectivity, and the 1:3 selector switch is working.  You've also got SOME sound out of this third section, it's just gating when your input signal is low level. 

Starting with the basics:
1. Are the transistors installed with the E, B and C in the proper holes?
2. Do you have this section's DIP switch set correctly for +9V power since these are NPN?
3. What are the voltages at each pin for each of the transistors?
4. Have you tried swapping out the transistors with spares?
5. If all that's good, I think I'd undo the mods and start with the straight build recommended by the PCB maker. When you get it working with the stock build, then you can add mods, and if one doesn't work, you'll know what did it.

Let us know!

[scotheath]

I had the exact same problem with a Tayda MK3 build. With the help on this forum I got it going.
https://www.madbeanpedals.com/forum/index.php?topic=33542.msg322007#msg322007
What's the volts at collector of Q1? needs to be 4.5v +-

[neiltheseal]

OK, let's give this a shot. Since 2/3 of the pedal is working, that's a good start, you've got power, in/out connectivity, and the 1:3 selector switch is working.  You've also got SOME sound out of this third section, it's just gating when your input signal is low level. 

Starting with the basics:
1. Are the transistors installed with the E, B and C in the proper holes?
2. Do you have this section's DIP switch set correctly for +9V power since these are NPN?
3. What are the voltages at each pin for each of the transistors?
4. Have you tried swapping out the transistors with spares?
5. If all that's good, I think I'd undo the mods and start with the straight build recommended by the PCB maker. When you get it working with the stock build, then you can add mods, and if one doesn't work, you'll know what did it.

Let us know!

Hi thanks for your response.

I followed your steps but still couldn't get it working as silicon. I did an audio probe be and the signal made it all the way to the base of q3.

I did try the pcb as the designer stated initially and it didn't work. The mods came about as suggestions to help get this working. And the mods helped. There was no sound initially.

For voltage :q3 collector is 4.27v, base is now 0.16v and emitter is 0v.

I tried my best to bias q3 with a pot but no luck really.

Some people suggest that perhaps the pcb was poorly designed...I'm not really qualified to comment but it seems to be the case.

BUT GOOD NEWS

I removed the parts and set up the pcb to be PNP. I tested it out with whatever PNP transistors I had lying Aryan's it worked. Not well but it worked.

I ended up buying matched germanium transistors and it works perfectly now.

This is a good outcome as I now can switch between germanium and silicon. The mi3 Germanium is now my favourite but it might be confirmation bias.

[neiltheseal]

I had the exact same problem with a Tayda MK3 build. With the help on this forum I got it going.
https://www.madbeanpedals.com/forum/index.php?topic=33542.msg322007#msg322007
What's the volts at collector of Q1? needs to be 4.5v +-

Hey thanks. I actually did find this post and followed the tips but no luck unfortunately. This post (and this forum) helped me get a tayda tone bender mk2 working.

I ended up changing it to germanium and it works perfectly now.