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.