Chop Shop v2 - sag knob cutting off signal and ineffective drive knob

[benevolent-fur]

Hi all,

looks like I'm destined to never get one of these pedals right the first time! I thought I'd just finished the Chop Shop v2, but am having the following issues when trying to use it:

1. The drive knob appears to not be doing anything - there is grit, but no variance when the knob is turned
2. The sag knob has lots of grit and max volume when turned fully CCW, but from about 4 o'clock onwards when turning it CW the volume decreases so much that it's basically gone. There is still a signal when I look on the DAW but it's not audible.

Thankfully the volume knob functions properly.

I've tried to bias the pedal to the best of my limited knowledge, but am unsure if that could also be an issue.

I've tried running the pedal at both 9V and 18V and there is no change.

I've tried both single coils and humbuckers in case the issue was the strength of the pickups.

Any ideas what I've stuffed up here based on the attached picture?

Cheers

 

[Erik S]

What did you set the trimmers to?

Here’s what I had in my notes:

Set Sag knob for max voltage at pin 1 of the pot (square pad) and adjust trimmers for 6v at the test points.

 

[hamerfan]

What Erik said. No experience with this circuit, but the extreme settings on the jfet trims don‘t look right.

 

[benevolent-fur]

Thank for the suggestions. I had initially biased the trims as you suggested, but just went back and started biasing from scratch again in case I missed something. As you can see in the pictures below. The trim pot of the right biases normally. However the trimmer of the left side has some odd behaviour - it shows very little change in voltage regardless of how extremely the trim pot is adjusted.

Hopefully I've actually set everything up correctly with the multi-meter as I haven't had to properly bias a pedal before so it's all very new to me.

Does this spark any ideas as to what might be the issue?

 

[Erik S]

Your meter set up looks correct. Did the part about setting the Sag pot for max voltage on pin 1 before doing the biasing make sense?

If you’ve already done that, I’d double check that all your part values are correct. If you don’t have any errors there, read whatever you can find on the forum about these. Jfet drives often need some tuning or careful part selection to get them working right.

 

[OD is Glorious]

this is an exceptional circuit. Plug it in, get a flathead and strum, start turinng trimmers until it sounds good to you.

 

[benevolent-fur]

Thanks Eric. I took that to mean turning to sag knob as far as it would go CCW, but I've possibly misunderstood. Could you put it in layman's terms for me if I've got it wrong. I've uploaded a picture of the knob settings I had on the front at the time of biasing.

I was pretty slow and meticulous with parts values this time around as on my first build I made the mistake of not understanding that diodes had polarity and wanted to avoid the drama that was de-soldering! But I'll go back through and check again.

 

[benevolent-fur]

this is an exceptional circuit. Plug it in, get a flathead and strum, start turinng trimmers until it sounds good to you.

Yes I'm looking forward to it! Unfortunately I just can't get the drive to adjust at all. And the sag knob is having the odd behaviour I originally mentioned. I'm hoping it's just some little error I've made and wiser eyes than mine can pick it up.

 

[Erik S]

Thanks Eric. I took that to mean turning to sag knob as far as it would go CCW, but I've possibly misunderstood. Could you put it in layman's terms for me if I've got it wrong.

Same set up with your multi meter, just put the red probe on the square pad of the sag potentiometer (pin1) and check the voltage there. Same way you’re doing with the trimmer test points. Adjust the sag control until you get the highest voltage reading you can get there, then proceed to adjust the trims the way you’ve been doing.

 

[benevolent-fur]

Thanks for clarifying Erik. To get maximum voltage on the square pad of the sag pot I had to have the sag fully clockwise. However, again only the left trim pot's voltage was able to be reasonably adjusted. The right trim pot voltage was limited to between 8.21V and 8.61V depending on whether if was turned either fully CCW or fully CW respectively.