SOLVED Chop Shop gain not working

[SYLV9ST9R]

Hi guys, I’ll be looking at one of my first build, as the gain/drive pot stopped working on this Chop Shop.
Volume and Sag seem to be working fine, but an overdrive without the drive is a bit pointless.
As I’m still a noob when it comes to troubleshooting, I was just wondering if someone had any hindsight of things I should be looking at, or if I should just replace the potentiometer.
I’ll take it apart one night this week, but from visual inspection, the soldering seems fine.

 

[phi1]

I checked mine and i used trimmers for the drain resistors, and set the drains at 6v (sag at minimum). mine works well over the range of the sag knob. You don’t have to use a trimmer, you could just test resistors and solder in the ones that gets you closest to 6v on the drains.

Others have mentioned this circuit is finicky if not dialed in just right. So that may explain what you’re having.

 

[Chuck D. Bones]

Those JFETs seem to be running a little on the cold side. Spec Vp for J201 is -0.3V to -1.5V. These are running just under -0.2V. You could try increasing the drain resistors to 15K.

Based on one of the demos I watched on YouTube, the SAG control is wired backwards on this board.

 

[utmigs]

Thanks guys! I swapped the B10K and increased the drain resistors (to 15K, as Chuck D. Bones recommended), and everything works well. Drain voltage was reading right at 6.0V on Q1 and Q2. The pedal sounds great. I ordered some Trimmit adapters for the next build, as that would've really come in handy. I really appreciate the help!

 

[Chuck D. Bones]

Good to hear you got it sorted out. Where did you get your J201s?

 

[phi1]

He mentioned earlier he’d gotten them from pedalpcb, pre-soldered smd. Which is a little bit concerning as I was hoping the smd would be consistently within spec. Unless there is an error somewhere in the build causing it to bias differently than expected.

 

[Chuck D. Bones]

Some pedal circuits require either the JFET(s) or resistor(s) to be selected. Even in-spec JFETs are not guaranteed to work in every circuit. That's why so many of the pedals containing JFETs also contain trimmer resistors.

 

[music6000]

Some pedal circuits require either the JFET(s) or resistor(s) to be selected. Even in-spec JFETs are not guaranteed to work in every circuit. That's why so many of the pedals containing JFETs also contain trimmer resistors.

As Chuck noted, this pedal is very fussy with the J201's.
I built a Handwired version years ago & got in touch with the Builder & he told me they Hand Pick the J201's for this pedal.
I had external trimpots & couldn't dial it in until I measured them individually with a jfet matcher I built to get them in spec.

From the Builder, Thanks Carl - FC :
Let's start by sorting the JFETS. As we saw on the datasheet, J201's can have an Idss value from 200uA to 2mA. That is a huge range. For Q1, you want a J201 that has an Idss from 600-800 uA. For Q2, you want one with Idss about 400-600uA.
So you'll have to see what you have to work with. You might be unlucky and have all J201's with crazy values like 1500uA. In that case, the drain resistors will need to be changed.
In the Rev 1 Barbershop, we used to use a 2PN2222A, not a regular 2N2222. They are similar in electrical characteristics, but have the reverse footprint. If you're using a normal 2222, you'd have to reverse it. I'd start with that for sure.

 

[utmigs]

Good to hear you got it sorted out. Where did you get your J201s?

Yes, they were presoldered SMDs from PedalPCB. I’m ordering another Chop Shop PCB, and I plan on building that one with some InterFET J201s that I purchased from Mouser and trim pots in place of the drain resistors. Thanks again!

 

[phi1]

Sounds like a good plan. I’d still recommend socketing the jfets just in case your having issues biasing you could try to swap it for another. (It’s a good idea to solder at least one of the legs into the socket before boxing up to make sure they can’t fall out).

 

[Chuck D. Bones]

Agreed. Trimpots will get you just so far. Knowing Fairfield's selection criteria should make dialing in the bias straightforward.

 

[Alan W]

As Chuck noted, this pedal is very fussy with the J201's.
I built a Handwired version years ago & got in touch with the Builder & he told me they Hand Pick the J201's for this pedal.
I had external trimpots & couldn't dial it in until I measured them individually with a jfet matcher I built to get them in spec.

From the Builder, Thanks Carl - FC :
Let's start by sorting the JFETS. As we saw on the datasheet, J201's can have an Idss value from 200uA to 2mA. That is a huge range. For Q1, you want a J201 that has an Idss from 600-800 uA. For Q2, you want one with Idss about 400-600uA.
So you'll have to see what you have to work with. You might be unlucky and have all J201's with crazy values like 1500uA. In that case, the drain resistors will need to be changed.
In the Rev 1 Barbershop, we used to use a 2PN2222A, not a regular 2N2222. They are similar in electrical characteristics, but have the reverse footprint. If you're using a normal 2222, you'd have to reverse it. I'd start with that for sure.

I'm just getting this pedal completed, and have a general question about Idss—besides the measurement, in mA, there is a reference Vds involved, at least I assume so. My tester (DCA75) gives me the Idss at a Vds of 3.00 volts. The data sheet mentions 20V and there is a chart at 10V—I guess my question is, do I just use the Idss as tested (with a Vds of 3V), or do I need to convert the Idss to what it will be (assuming it changes) with a Vis of 6 V, which this pedal trims at?

 

[Chuck D. Bones]

Idss will be essentially the same for any Vds above 1 or 2 volts, so just use the Idss reported on your DCA75.