SOLVED Madbean Greenbean - FET concern

[Synchrony Pedalworks]

Just made the Greenbean (TS inspired clone). I was rocking before boxing and wondered if I have a problem. With the clipping toggle to the FET, I am getting no clipping. It is essentially unity sound. The pots all work and effect the signal appropriately. All power measurements match those given by Madbean.

I made a few substitutions due to availability. The BAT41 was changed to a 1N34a. Transistors are MPSA18.

With this in mind, is the FET supposed to clip (overdrive) the signal? Is the 1n34a or 2n7000 bad or is something else amiss?

 

[Synchrony Pedalworks]

I don’t know if this is important, but measured the voltage with the leads on the 1n34a. The voltage was 0.04. I am not sure if that info will help determine if the diode is healthy. Thanks again

 

[BryGuy]

Used 2n7000 in mine which sound nice and yes you should get clipping. Your just using the diode inside the transistor as a different flavor. Did you check the pinout of the transistors you used? Don’t know off the top of my head if they are the same or not.

 

[Synchrony Pedalworks]

I have double checked the pin out and verified continuity within that section of the schematic. Therefore, I think that I have parts in place correctly. Would this mean a part is fried (mosfet or diode)? How can you determine the bad part?

 

[BryGuy]

Seems unlikely that all three components are fried. D5 and Q3 are in series and clip one have of the signal while Q4 alone clips the other half for asymmetric clipping. If you’re confident that your switch is good I would pull Q4 to start. Maybe socket that side and test if you can get that half to clip.

Still seems odd. Even if everything for the FET side of the switch were non functional your diodes would start to clip the signal. LEDs clip at a fairly high threshold so think more transparent/crunch vs distortion. Just double checking also your using an ON/OFF/ON switch? That’s how you get three different clipping options.

 

[BryGuy]

After looking up the MPSA18 it looks that is a BJT NPN transistors which is definitely not the same as a FET. Honestly I’m not knowledgeable enough on different transistors types to tell you it will or won’t work but my gut is that’s your problem.

 

[Synchrony Pedalworks]

After looking up the MPSA18 it looks that is a BJT NPN transistors which is definitely not the same as a FET. Honestly I’m not knowledgeable enough on different transistors types to tell you it will or won’t work but my gut is that’s your problem.

I should clarify. The silicon transistors are the MPSA18, but the FETs are 2N7000s. Thanks for the help.

 

[PJS]

2n7000s and a single 1n34a is exactly what I used for clipping in my ADHD pedal. The arrangement is a bit different (clipping to ground rather than op amp feedback loop) but it sjhould still work.

 

[delayed]

Just a couple things from the images. I don't see much solider in the very top black diode. I see it in the bottom pic though. Might reflow? Second, is the switch physically shorting anything? It looks low in the picture but that just could be the angle of the picture?

 

[Synchrony Pedalworks]

As an update and for advice:
To date, I have check the following:
1. Continuity between the toggle (and toggle lugs), IC (pins 1 and 2), diodes, 2N7000 and LEDs. All connections were correct.
2. Voltages have all been verified from IC, 2N7000s, and MPSA18's as correct (based on Madbean voltages given).
3. Audio-probed the circuit, including the clipping section, and I have sound in the appropriate places.
4. Reflowed the clipping circuit
5. Removed LEDs to determine if sound was only going through the LEDs when toggled to run through the FETs. With LEDs removed, I still obtain clipping with the toggle in LED and diode (1n914) settings (curious finding). The clean sound is still present with the FET toggle setting. In other words, no change.

I am going to try the following to further detect the problem.
1. Remove Q4 (2n7000). If no change, then proceed with Q3 and D5 (1n34a).
2. Remove D3 and D4 (1n914).
3. Lastly, remove the toggle switch.

What do you all think? Good plan of attack or am I wasting my time. Thanks again for the help. Cheers.

 

[BryGuy]

Removed LEDs to determine if sound was only going through the LEDs when toggled to run through the FETs. With LEDs removed, I still obtain clipping with the toggle in LED and diode (1n914) settings (curious finding).

Yeah that doesn’t sound right. Maybe triple check continuity with the toggle switch in all three positions.

 

[Synchrony Pedalworks]

I think I have figured it out. Thanks for all of your help!

It looks like Q4 (2n7000) was bad. I guess I'll be making another order to Stompbox parts! I got the original 2n7000 from Tayda, so I don't know if it was bad from the factory or I destroyed it with my soldering skills.

To figure it out, I removed the LEDs, but the problem was still there. Next, I removed Q4 and I was able to get clipping and a volume boost. I placed sockets for Q4. When Q4 was placed, the problem returned.

 

[BryGuy]

Glad you got it sorted. Now you have a story to go with the pedal too!

 

[Feral Feline]

Congrats on sussing the problem.

MOSFETs are susceptible to damage from static electricity, so maybe your Q4 2N7000 got zapped.
Have you got a grounding bracelet or similar?

 

[Synchrony Pedalworks]

Congrats on sussing the problem.

MOSFETs are susceptible to damage from static electricity, so maybe your Q4 2N7000 got zapped.
Have you got a grounding bracelet or similar?

No. I do not have a grounding bracelet. However, this problem has suggested the need for one. Thanks for the help!

 

[BryGuy]

I don’t have a grounding bracelet but I obsessively touch anything metal around when handling any type of semiconductor.