Chop Shop works but no clipping / drive

[MaxIokua]

Hello, I'm new on here, and new to pedal building (just doing kits), so hopefully I'm asking this in the right place...
I just built a chop shop, calibrated the J201s to ~6VDC, and tried it out. It works, but there is such little drive (actual clipping) that is sounds like a clean boost and nothing else. The signal doesn't break up at all. Volume at 50%, Sag at 0 (fully CCW), and drive at 100% (fully CW).

Any ideas on what to look for would be greatly appreciated. I understand that J201s are not all created equal, could they be that far out of spec as not to function even mildly as intended? Or am I missing something. The parts came from musikding so I would expect that the J201s are decent quality. I have already performed general troubleshooting duty (ie, reflowing solder, rechecking components, wire connections, power supply etc)
Thanks!

 

[scheffehcs]

Hard to tell, might want to double check the pinout on the 2n2222. Looks like it could be backwards.

 

[scheffehcs]

Hard to tell, might want to double check the pinout on the 2n2222. Looks like it could be backwards.

Never mind, looks correct to me after checking

 

[MaxIokua]

That was my first suspect also, so I double-checked.

 

[brightlight]

My money's on the socketed j201s not connecting properly

 

[BuddytheReow]

Dumb question, but is your guitars volume knob maxed out? I’ve done that.

 

[Robert]

Just for the sake of science, what happens if you adjust the trimmers by ear rather than setting them to a fixed value?

Can you dial them in for more gain outside of the target 6V range?

 

[MaxIokua]

Dumb question, but is your guitars volume knob maxed out? I’ve done that.

Yes, no worries there

 

[MaxIokua]

My money's on the socketed j201s not connecting properly

I'm going to check this.

 

[MaxIokua]

Just for the sake of science, what happens if you adjust the trimmers by ear rather than setting them to a fixed value?

Can you dial them in for more gain outside of the target 6V range?

I can get them higher than 6V, I will give this a shot this morning.

 

[Robert]

Not necessarily higher voltage, I'm just curious if the gain increases at any other setting... higher or lower.

 

[MaxIokua]

OK, here's what I found. Wiggling the J201s in their sockets made no difference, so they seems to be making decent contact.
I adjusted the trimmers by ear, putting them both at their perceived loudest point. This does cause the pedal to break up. However the sag knob has to stay maxed otherwise it gates and sounds anemic. When I measured the voltages of the J201s, they were both around 2 millivolts...!
At this setting the pedal clips nicely with gain maxed out (mild clipping) but the pedal never quite cleans up even with drive at minimum. It does lessen the clipping, so the drive pot is doing its job. I am still not sure the pedal is doing what it's supposed to, but I think I can get it to a usable place by increasing the voltages slightly, so it cleans up a bit more at lower drive setting.

Thanks for the suggestions! Interesting that such little voltage is what increases the gain. Sciencing strikes again!

 

[temol]

J201 in TO92 package = possible problem. Often it's not J201 but other JFET with higher Vp.
Measuring Vp may be a good idea.

 

[MaxIokua]

Oops! Made a mistake with my measurements, Black lead on meter wasn't in the right place. The actual voltages at max output were around ~2V. Now that I've dialed it in by ear, these are my final results:

 

[MaxIokua]

J201 in TO92 package = possible problem. Often it's not J201 but other JFET with higher Vp.
Measuring Vp may be a good idea.

Thanks for the tip, but how do I measure Vp? And what numbers am I looking for to tell me whether these are J201s or not? Sorry for the noob question.

 

[temol]

From the original J201 I would expect a Vp value in the range of -0.6V to -1.0V.

How to measure Vp.

Half my J201s are bad?

Long story short, it looks like half my through-hole J201s are bad. Short story long: I was troubleshooting my in-progress Low Tide build, as it had farty, distorted output, which pointed to one of the JFETs. I assumed I had done a poor job soldering the surface mount 2Sk208, but my audio probe...

forum.pedalpcb.com

The fact that you can set 4V, 6V or any other voltage does not matter much. The same can be done here for J201, 2N5457, 2N5458 - thanks to the fact that you have trimmers that allow you to change the resistance on the drain side. What is more important is what transistor (what parameters) was used.

 

[MaxIokua]

Thank you!

 

[spi]

When you originally set the trims to 6V, was the sag knob all the way down (ccw)?

 

[MaxIokua]

When you originally set the trims to 6V, was the sag knob all the way down (ccw)?

Yes, all the way CCW. I have since set the trimmers by ear and would say the pedal is more or less working as it should, although it still has much less gain when maxed than I expected.

 

[music6000]

I believe that you have fake J201's, Fairchild don't stamp the F or j201 like that.
They are probably jfets, not J201's