Cobalt drive - Gain pot not working

[zeropluszero]

I built a cobalt drive.

The 250K dual pot was purchased from pedalpcb, and has some weird behavior.
I'm aware of the J201 biasing issue with this pedal, I'm hoping for some other info before i get to that.
So far, the build is stock. I had played around with R6 for biasing, but have put it back to a 4.7K resistor while I look at the main issue.
Measuring the dual pot in the circuit, if I measure between 3 and 1 pins without power, I get 0 ohm and 250K, on both 3 and 1 pins.
if I connect power, the inner pins drop to about 3.9k ohm when fully clockwise.
I can then lower this back down to 0 by going anticlockwise.

if I play guitar, i get signal between about 7 oclock and 10oclock, after this it gets gated and then splutters out.
after a few minutes the range of 10oclock drops to 9 and 8 and eventually stops passing signal, almost like something was getting overheated or damaged and eventually cuts out.
I've reflowed all solder joints, and I will take some photos of the pcb and resistors tomorrow morning in the light.
Any ideas?
I don't have a spare single opamp unfortunately

 

[zeropluszero]

Dammit I already see it...the measurements of the inner I thought I was taking were the blue inner and white outer. I'll try again tomorrow

 

[zeropluszero]

soldered in some new measurement wires, the blue outer wires seem to work fine, 0 to 250K ohm
the inner ones measure 0 and at about 9 oclock drop into overload.





so I'm guessing the pot is busted.

 

[Sturdag Lagernathy]

Are you taking resistance measurements with the power on?
The power should be off for resistance.
It may not change your current readings, but it sure will give erroneous readings sometimes.
The meter sends a known voltage through the leads, then measures the current, which gives resistance- stray voltage will affect this reading.

 

[zeropluszero]

Thanks for that. I'll measure again tomorrow.

 

[zeropluszero]

Ok so yeh, if i measure without power, the gain pot appears to be working as its supposed to.
have a new tl071 in the mail...

 

[zeropluszero]

new tl071 in, no change. bummer.

 

[pbrommer]

do you have an audio probe?

 

[Erik S]

Glad you figured out the pot was okay!

I built one of these over the weekend and I’m pretty happy with it. I hope you get yours going.

At one point I kicked on a fuzz going into the cobalt, and hitting it with a huge signal did get it to sag and cut out a little, so I’m wondering if what you’re describing is a possible normal behavior of those jfets depending on what’s going into them and how they’re set up.

If you haven’t read this, there’s a good explanation of getting the voltage right.

Thread 'This Week on the Breadboard: Blues Driver BD-2 & BD-2w - part 4'

Jul 19, 2021

The Cobalt Blues Driver - Boneyard Edition

I've shown you all of the pieces of the Boneyard Blues Driver in Part 3. Here in Part 4 I put it all together and make some final updates. All ref des in Part 4 refer to the Cobalt Build Docs. If you can't hit a part value dead-on, anything within ±20% is acceptable.

First Stage
The first stage is everything from the board's IN pad up to and including diodes D1-D4. What I wanted to accomplish here was to raise the input impedance up to match the factory pedal, use JFETs I had in my stash, balance the JFET currents, sub an A100K dual...

• Chuck D. Bones
• Replies: 38
• Forum: Chucks Boneyard

• 
• 
• 

Long story short, Chuck recommends trying to get the voltage at these three points I marked in red to be extremely close to each other.


The bottom of the 10k resistor is your reference voltage and you’re trying to set the two 3906s to equal it by adjusting the values of r6 and r7.

If it were me, I’d always start with triple checking all the part values on your board. Get the magnifying glass out and check all the color bands on those tiny 1/8w resistors.

If that checks out, maybe just start with measuring those three points to see if they’re in the same ballpark.

A quick point on measuring voltage differences (just in case it’s helpful seeing as you posted science-dog earlier)…

In voltage mode, a meter reads the voltage difference between the two leads.

If you want to know the voltage present at a single point in your circuit, red on the point, black on ground, and the meter tells you the difference between ground and the point you’re measuring.

But if you want to know the difference between TWO points on your board that both have voltage, the meter will do the math for you. If one point is at 3.0v and the other is at 3.06v, the meter will read the .06v difference.

That’s what’s going on in that procedure I linked to.

Apologies if that was obvious or unnecessary, but definitely wasn’t the first thing I learned how to do with a meter.

Good luck, and let us know how you make out.

 

[zeropluszero]

No to the audio probe, this is looking like the project that is going to make me make one I think

I'll take some measurements and get back to you Erik.

Does the behaviour I'm describing, where when the pedal turns on I can get a bit of sound from 7-10oclock gain, but then after a few minutes I can't get anything at all point to a particular problem? I haven't seen anyone complaining about anything like this before.
My first guess is something overheating but haven't noted any part getting hot.

 

[Erik S]

My first guess is something overheating but haven't noted any part getting hot.

Have you gone through and verified all your part values are correct?

I don’t have any guesses specific to this circuit, but a 4.7k resistor somewhere you were supposed to use a 470k is always my first guess when a pedal is passing sound but acting weird.

 

[zeropluszero]

Ok, checked over all my resistors and capacitors, and they're all correct to build doc.

I measured bottom of R101 at 4.1v, and the side of Q5 ant 4.3v, the right of Q6 gave me a reading around 8.1V
Guess I should first pull out Q6 and resolder or replace it?

Check stash - of course no spare 3906

 

[Erik S]

Are you sure you’ve got a 4.7k in r7?

Pretty tough to see those tiny color bands on the 1/8w resistors in a photo, but try posting some pics if you want some more eyes on them.

 

[zeropluszero]

yeh 4.7K
brown, brown, black, purple, yellow - 1% resistors.

-If I put in a 5.6K resistor to R6, I get 4.08v on R101, 4.17v on Q5, and gain on minimum - 6.3v on Q6.
a 6.8K gives me 6.6 on Q6

 

[zeropluszero]

OK, I may have got it working.
Here's what I did.
I resoldered (for the third time) all the transistors.
I checked the six SMD j201 I had in my TC1. I noted the results.

Noting that #2 and 3 had different values than the rest for Idss, Vg. I took these out, and put the other 4 in, I know this isn't the same as matching the Vp as per Chucks guide, but I thought it was worth a shot.
The R6 resistor, I soldered in the 6.8K

 

[zeropluszero]

Ok, it's not complete...I have gain working until about 1230, then it starts getting very spluttery and gated, some signal still passes all the way clockwise though just super spluttered. Definitely improved.