SOLVED Chop Shop voltage only get up to 3v with trim pots maxed

[Fish]

I’ve been working on this chop shop on and off for a little while now and when I boxed it up there was very little gain even after messing with the trim pots. It sounded like a clean boost.

I replaced the J201’s with mmbfJ201’s and it definitely solved the gain issue but it was weak sounding(Im not sure if this od is this light of an od or not but it feels off).

Now im on the bias stage and using the test pads Im getting readings of 3.1 with the trim pots maxed out (chop shop from what I have read is looking for 6v).

I have the correct 50k trimmers
I DID NOT ground the input when testing (not sure if that was a big deal or not?)

I tried to bias with the sag down CCW and all i got was 3.1v max and got minimal gain.. I biased with the sag maxxed CW and was able to get 9.1v but when trimmed to the suggested 6v i had zero gain again.

Any suggestions?

I was thinking switching the 50k trimpots to 100k? But i have no idea if that will increased the range on dialing in the voltage.

Im still pretty new and now trying to dive into the “Why” part of this whole thing

 

[rwl]

For what it's worth, I'm seeing the same problem. With gain and volume maxed, and sag fully CCW, and grounding my DMM against the DC jack's ground, I only get 4-4.8v, and the volume is very low at max, barely unity. My J201s are SMDs adapted for through-hole, bought from Aion, so I think they're OK.

I checked all the component measurements and reflowed solder with no luck.

If I bias the traditional way (sag CCW, gain CW) to around .5-.6v, it sounds pretty good. Not quite as crunchy as demos I saw, but maybe that comes down to guitar/amp differences? I have to wonder whether the "correct" voltage is different for V2. The V2 build doc is from December 2023, and I saw no references to appropriate biasing voltage being 6V after the V2 release. None of the build reports show the switch, so it's the older version.

Another thing I noticed is that everyone on the older builds says to bias with the sag knob CCW and gain CW. But the V2 manual for the Barbershop says "Full voltage is applied when the control is set fully clockwise." So I wonder if the knob's function was reversed between V1 and V2? We're supposed to bias without any voltage drop, which would suggest the sag knob should be clockwise (and so should gain). This would match my hypothesis that with sag fully up the max voltage you can even get with the trimpots will be limited, while more voltage would be available with sag down. So for now I've biased to 5.5V and the sag knob seems to have an effect of darkening the tone and making it less "sizzly," and the volume is good.

 

[Locrian99]

Hmm. Ive built several of these with no issues. As i recall the sag control works opposite of how i would expect. To me in my head the sag control fully ccw should be max voltage. But as i remember its opposite. The sag control as i remember (dont have one handy to double check. Didnt do a ton. I sort of found my spot and never moved it. And its nearly maxed out.

So i guess my thought would be are biasing it with the sag pot fully ccw or cw.

 

[temol]

Do not rely on the Sag knob position. Measure voltage in the circuit - you want max possible voltage on pin 1 of the Sag.

 

[Fish]

For what it's worth, I'm seeing the same problem. With gain and volume maxed, and sag fully CCW, and grounding my DMM against the DC jack's ground, I only get 4-4.8v, and the volume is very low at max, barely unity. My J201s are SMDs adapted for through-hole, bought from Aion, so I think they're OK.

I checked all the component measurements and reflowed solder with no luck.

If I bias the traditional way (sag CCW, gain CW) to around .5-.6v, it sounds pretty good. Not quite as crunchy as demos I saw, but maybe that comes down to guitar/amp differences? I have to wonder whether the "correct" voltage is different for V2. The V2 build doc is from December 2023, and I saw no references to appropriate biasing voltage being 6V after the V2 release. None of the build reports show the switch, so it's the older version.

Another thing I noticed is that everyone on the older builds says to bias with the sag knob CCW and gain CW. But the V2 manual for the Barbershop says "Full voltage is applied when the control is set fully clockwise." So I wonder if the knob's function was reversed between V1 and V2? We're supposed to bias without any voltage drop, which would suggest the sag knob should be clockwise (and so should gain). This would match my hypothesis that with sag fully up the max voltage you can even get with the trimpots will be limited, while more voltage would be available with sag down. So for now I've biased to 5.5V and the sag knob seems to have an effect of darkening the tone and making it less "sizzly," and the volume is good.

I actually biased it based on the sag knob all the way up, CW and the pedal had no drive i was able to bias it to 6v however it sounded off. I feel like i need more range with the trimmer The best sound i can get is with the trimmers dimmed but its still weak. I need more range. Could changing the trimmers work or maybe the resistors going to the jfets?

 

[Locrian99]

I actually biased it based on the sag knob all the way up, CW and the pedal had no drive i was able to bias it to 6v however it sounded off. I feel like i need more range with the trimmer The best sound i can get is with the trimmers dimmed but its still weak. I need more range. Could changing the trimmers work or maybe the resistors going to the jfets?

Idk if it would make a whole lot of difference but i think i always bias mine to 6.6v.

And it is a very light OD

 

[Fish]

It’s definitely a light overdrive, I still think there is more in it than im getting. I think changing resistor values may help, I kind of doubting swapping out the trim pots Im at a loss but I think this thing will sound good if i can get that bias voltage up

 

[Fish]

Do not rely on the Sag knob position. Measure voltage in the circuit - you want max possible voltage on pin 1 of the Sag.

View attachment 82898

Is there a way to change a resistor value to increase the voltage?

 

[temol]

Is there a way to change a resistor value to increase the voltage?

First let's make sure you're doing everything correctly.

1. Set the maximum voltage on the pin 1 of the Sag knob. Sag should be probably at zero (max CCW) but it's better to measure the voltage than stare at the knob. From now on - do not adjust Sag. You want max voltage.. like there's no Sag controll at all.
2. Set the trimpot @ middle position and measure the voltage on a test point. Now - turning the trimpot clockwise should increase the voltage. And of course - turning left (CCW) should decrease the voltage. Still talking about voltage on a test point.
Take a look a the schematic - when rotating the trimpot to the right (CW) you decrease value of the resistance (so you increase the voltage). At maximum position to the right (resistance is zero ohms now) you should read close to the supply voltage. And with the trimpot at minimum setting (max CCW) yoo should read below 1V. With 50k trimpot.
3. Set 6V on a test points.

 

[Fish]

First let's make sure you're doing everything correctly.

1. Set the maximum voltage on the pin 1 of the Sag knob. Sag should be probably at zero (max CCW) but it's better to measure the voltage than stare at the knob. From now on - do not adjust Sag. You want max voltage.. like there's no Sag controll at all.
2. Set the trimpot @ middle position and measure the voltage on a test point. Now - turning the trimpot clockwise should increase the voltage. And of course - turning left (CCW) should decrease the voltage. Still talking about voltage on a test point.
Take a look a the schematic - when rotating the trimpot to the right (CW) you decrease value of the resistance (so you increase the voltage). At maximum position to the right (resistance is zero ohms now) you should read close to the supply voltage. And with the trimpot at minimum setting (max CCW) yoo should read below 1V. With 50k trimpot.
3. Set 6V on a test points.

I will go through the process again i newver checked for voltage at the pin, but i did turn the pot ccw and from there my max voltage was 3V.

I tried it with the sag turned CW and i could get the Voltage all the way up to 9V with the trim at max, however when I set it to 6V there ways virtually no drive.

 

[temol]

But you know you have to set 6V on a test points, right? And only when Sag does not limit the voltage?

 

[rwl]

@temol I tried your process on the V2 board and it worked - the only difference is that the sag knob must be CW. As @Locrian99 said, it's counterintuitive that the so-called sag knob is actually the inverse: turning CW decreases sag but increases voltage (as measured at pin 1 of the sag knob on the v2 version of the board).

The pedal is working well now for me biased around 6v, and I boxed it. Volume boost is strong (about the same as a typical overdrive) and the OD is fairly light; there's not much clipping. It sounds great with other pedals running into it like fuzz or a KoT- it mostly preserves their tone but makes it a bit crisper and fuller. I was expecting a heavier drive sound but I think it's a more unique and useful pedal for my board without that.

 

[Fish]

Yes, what I should have said was with the sag turned CW the test pad had a wide range up to 9V at the test point, but set at 6v there was very little gain almost sounded lime a boost. When sag was turned CCW the highest voltage I could get at the test pad was 3.1V, wich was maxed out but it did have a decent overdrive. Thanks for your help, hope what i'm saying is not confusing

 

[Fish]

@temol I tried your process on the V2 board and it worked - the only difference is that the sag knob must be CW. As @Locrian99 said, it's counterintuitive that the so-called sag knob is actually the inverse: turning CW decreases sag but increases voltage (as measured at pin 1 of the sag knob on the v2 version of the board).

The pedal is working well now for me biased around 6v, and I boxed it. Volume boost is strong (about the same as a typical overdrive) and the OD is fairly light; there's not much clipping. It sounds great with other pedals running into it like fuzz or a KoT- it mostly preserves their tone but makes it a bit crisper and fuller. I was expecting a heavier drive sound but I think it's a more unique and useful pedal for my board without that.

Ok so with the v2 it needs to be biased with the sag CW giving the pedal max voltage, which makes sense the only problem is it doesn’t have much gain at all that way. Im gonna refill with sder everything tonight maybe theres a problem there not sure whats going on

 

[Fish]

But you know you have to set 6V on a test points, right? And only when Sag does not limit the voltage?

I wasn’t sure what the sag knob was supposed to be doing when biasing, but if its at full voltage then it is CW. Everything I’ve read said turn it CCW, but that may be for the old version. I guess it is biased correct when I plugged it in today with fresh ears and with the gain maxed there is a decent drive/tone. The sag knob on this version is def backwards you have to turn the sag CW all the way for max voltage. Then I checked the test pads and it was at 6V. I might have to build another one to see if there are any errors, but maybe just not my cup of tea.

 

[temol]

I wasn’t sure what the sag knob was supposed to be doing when biasing,

nothing..absolutely nothing. You can even not install Sag pot before setting the bias. Jumper pads 1 and 2 of the Sag pot, set the bias, install the pot.
Looking at the schematic of the v1 and v2 there's no difference between Sag pot wiring, unless there is an earlier version of the schematic.

 

[Fish]

@temol I tried your process on the V2 board and it worked - the only difference is that the sag knob must be CW. As @Locrian99 said, it's counterintuitive that the so-called sag knob is actually the inverse: turning CW decreases sag but increases voltage (as measured at pin 1 of the sag knob on the v2 version of the board).

The pedal is working well now for me biased around 6v, and I boxed it. Volume boost is strong (about the same as a typical overdrive) and the OD is fairly light; there's not much clipping. It sounds great with other pedals running into it like fuzz or a KoT- it mostly preserves their tone but makes it a bit crisper and fuller. I was expecting a heavier drive sound but I think it's a more unique and useful pedal for my board without that.

I got the same results and to bias to 6v i needed to open the sag up all the way CW. So i guess the knob works as if your adding voltage to it. It’s definitely light overdrive. I heard the buddy blues demo and im sure hes pushing a driven amp now that i hear the pedal, plus the dude is a beast. anyway As i put it down last night i was underwhelmed, but i think my ears where just done for the night because i plugged it into day and after turning some knobs started to get some cool sounds. Even felt like I dialed up a cool john scofeild kinda tone. It’s a cool pedal, not sure if it’s proper or not lol so should I mark this thread solved, because I really don’t know, but I got exactly what you described as well.

 

[Fish]

nothing..absolutely nothing. You can even not install Sag pot before setting the bias. Jumper pads 1 and 2 of the Sag pot, set the bias, install the pot.
Looking at the schematic of the v1 and v2 there's no difference between Sag pot wiring, unless there is an earlier version of the schematic.

I already had it boxed up when i went to bias. I do think the sag knob goes up in voltage as you turn it up CW. Because as I was on the test pads my max range for voltage was CCW 3.1 and CW was 9.1. I don’t know man this is my first time really digging into anything like this, I biased 2 son of bens before and they came out nice. I did the first one by ear because i didn’t have a multi meter yet and when I check the vas pretty close dead ass to the suggested volts. But this was a cool build. Thanks for your help