Barely Any Overdrive at Max - PedalPCB Chop Shop / Fairfield Circuitry Barbershop

[Macca]

Hi all,

I'm trying to troubleshoot a completed PedalPCB Chop Shop / Fairfield Circuitry Barbershop build. The build functions as intended everywhere except the DRIVE pot. I'm at a loss at what to check next. I read other Chop Shop build threads on here and everything seems to be addressed.

I have reflowed every solder joint and double-checked everything. All is good. I biased the two J201s properly to 6v with the SAG pot set to 9v. (I am using Aion FX's J201s soldered to adapter boards.)

There is little to no overdrive, except when DRIVE is fully clockwise. Even then, the amount is very low.

What can I check next to help me find what is wrong? Is there a chance I will need new J201s? Thanks in advance for your help.

 

[Nostradoomus]

Post plenty of clear pictures.

 

[Macca]

@Nostradoomus, will do when I get home! I thought I'd get a head start on the thread.

 

[Locrian99]

sag pot can play a big role in the amount of over drive. Is your sag pot fully clockwise?

 

[Macca]

sag pot can play a big role in the amount of over drive. Is your sag pot fully clockwise?

Thanks for your message. From reading other Chop Shop threads, I made sure to set SAG fully clockwise and confirm that lug 1 was reading 9v, which it was, before biasing.

 

[Locrian99]

I built a chop shop last week and thought the overdrive was weak, eealized my guitars volume was turned down to 6ish…

Its a small circuit. Verify each resistors color code, and each capacitors code. If its working but not quite right an incorrect value could be a likely cause, also its worth reflowing all the solder joints just in case youve got.a cold joint in there

 

[johnk_10]

i've built quite a few of them (etched my own PCBs for them) and found that the circuit is VERY particular on the choice of J201's used in it.

 

[Alan W]

sag pot can play a big role in the amount of over drive. Is your sag pot fully clockwise?

Maybe I made a mistake on mine, but my sag is at maximum sag (lowest voltage) when fully clockwise.

 

[Locrian99]

Maybe I made a mistake on mine, but my sag is at maximum sag (lowest voltage) when fully clockwise.

I think it might have changed when the new version came out.

 

[Macca]

i've built quite a few of them (etched my own PCBs for them) and found that the circuit is VERY particular on the choice of J201's used in it.

Can you elaborate on this? I know it's best practice to measure these things before installing but I don't have a tester.

 

[Macca]

Here are some photos, @Nostradoomus. I measured each resistor and capacitor before soldering. A couple of those solder points look dodgy but they've been reflowed. The pedal works, the J201s are biased to 6v with SAG set to 9v, but total amount of gain seems low.

I'm thinking @johnk_10 is on to something. I did no measuring or matching of the J201s. I don't recall any of the other Chop Shop threads recommending certain J201 values.

 

[Erik S]

That sag pot with no cover on it might cause you problems. Looks like when you tighten it down in the enclosure there’s a good chance of the back grounding out those pokey leads underneath.

I haven’t built one of these yet, but if I remember right, this is supposed to be kind of a low gain drive pedal right? How low gain are we talking? Have you compared what you’re getting to some demos and concluded what you’re getting is definitely wrong?

 

[Ale_jellybelly]

I have the v1 board but it has plenty of gain, gets almost fuzzy when cranking gain and sag

 

[FormulaFourteen]

I built a Chop Shop a few years ago and didn't really like it so I sold it but from what I recall the Drive control was really interactive with the Sag control.

Have you tried turning the Sag to both extremes (I forget whether max or min Sag was dirtier) and then playing with the Drive?

 

[Sturdag Lagernathy]

From Music6000:
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.

 

[LukeFRC]

From Music6000:
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.

this