SOLVED Neurocyton Q2 doesn't want to bias

[tang]

Hi all, I've put together most of the Neurocyton with a charge pump board and running into an issue where I can't seem to bias Q2. No matter where I set the trimmer, it always shows ~17.4v (at 18v, with 17.5v at VCC). The trimmer seems fine, if I check the resistance, I see a linear sweep from just over 0ohm to just under 100kohm. I also tried removing the J201 and soldering on a new one, but no success. Any tips on how I can troubleshoot this? I have no background in electrical engineering, so please assume I know very little!

Some extra details:

• When I play through it, it mostly makes no sound and becomes a (very quiet) sputter-y fuzz-like distortion when the gain knob is turned all the way up.
• I'm using PedalPCB's charge pump board with a TC7660S instead of a TC044SCPA (I believe they're compatible?)
• If I try to bias at 9v, the same thing happens on Q2 but at ~8v. Q1 and Q3's biases become too low, while Q4 seems to be fine.
• When I first soldered the board (before adding pots), I was seemingly able to bias all the pots at 9v

For convenience, here's the build docs: https://docs.pedalpcb.com/project/Neurocyton-PedalPCB.pdf

 

[JamieJ]

Where have you grounded your circuit?

 

[music6000]

Hi all, I've put together most of the Neurocyton with a charge pump board and running into an issue where I can't seem to bias Q2. No matter where I set the trimmer, it always shows ~17.4v (at 18v, with 17.5v at VCC). The trimmer seems fine, if I check the resistance, I see a linear sweep from just over 0ohm to just under 100kohm. I also tried removing the J201 and soldering on a new one, but no success. Any tips on how I can troubleshoot this? I have no background in electrical engineering, so please assume I know very little!

Some extra details:

• When I play through it, it mostly makes no sound and becomes a (very quiet) sputter-y fuzz-like distortion when the gain knob is turned all the way up.
• I'm using PedalPCB's charge pump board with a TC7660S instead of a TC044SCPA (I believe they're compatible?)
• If I try to bias at 9v, the same thing happens on Q2 but at ~8v. Q1 and Q3's biases become too low, while Q4 seems to be fine.
• When I first soldered the board (before adding pots), I was seemingly able to bias all the pots at 9v

For convenience, here's the build docs: https://docs.pedalpcb.com/project/Neurocyton-PedalPCB.pdf

Not much to go on with what Pic you have shown???
Big Question, Did you use plastic Dust Covers on the 3 Pots under the PCB Board???
If you did, Continue below. If you did not, you have your problem!
Here is a Section that you can give us a Continuity test on.
1. Test the matching coloured Circles for Continuity.
2. What Voltage are you getting on the Red Circles? :

 

[tang]

Thanks for the responses, I really appreciate it!

Where have you grounded your circuit?

My board(s) are hooked up like this. Given this, I think it means my circuit is grounded at the (-) pad on the charge pump?

Not much to go on with what Pic you have shown???
Big Question, Did you use plastic Dust Covers on the 3 Pots under the PCB Board???
If you did, Continue below. If you did not, you have your problem!
Here is a Section that you can give us a Continuity test on.
1. Test the matching coloured Circles for Continuity.
2. What Voltage are you getting on the Red Circles? : (snip)

Apologies about the pic, I'm not sure what to take a picture of to be useful!

I don't have dust covers, but the 3 pots under the PCB board are not wired directly to the board (they're connected by wires) and I have some electrical tape on their bottoms. The issue persists when I ensure the pots are hanging and not touching anything or each other. I'll add some dust covers to my next order, but is there any more troubleshooting I can do before then?

1. All the blue pads have <0.2ohm resistance between them, same with the red.
2. The voltage at the red pads are 17.56v @ 18v, 9.10v @ 9v (maybe the tolerance on the caps on the charge pump board make the values not exact?)

 

[giovanni]

The voltages are good, so the charge pump is probably fine. Did you check your component values on the main board?

 

[tang]

The voltages are good, so the charge pump is probably fine. Did you check your component values on the main board?

I've just double-checked the component values and they all seem good. I'll note that I used a 2.21M resistor for R3, as I couldn't find a 2.2M resistor; I'm unsure if that would cause issues.

I'll go through and double check all the connections and solder joints...

 

[music6000]

I've just double-checked the component values and they all seem good. I'll note that I used a 2.21M resistor for R3, as I couldn't find a 2.2M resistor; I'm unsure if that would cause issues.

I'll go through and double check all the connections and solder joints...

Can you check what number is written on the side of all your 3362 Trimmers?

 

[tang]

Can you check what number is written on the side of all your 3362 Trimmers?

It says "P 104" and below that and upside down, "133C0"

Big Question, Did you use plastic Dust Covers on the 3 Pots under the PCB Board???
If you did, Continue below. If you did not, you have your problem!

Revisiting this, I think there was something shorting earlier in the box. Since i've taken it all out and back in, the pedal seems to work, but the gain knob does little from 50-100% and sounds pretty fizzy at high gain.

Now, Q1, Q2, and Q4, will bias fine, but Q3 doesn't want to go low enough (shows ~11v to ~17.5v). I'm expecting it's just another short somewhere?

 

[giovanni]

Good news! Did you change anything or did you just take it out of the enclosure?

 

[music6000]

It says "P 104" and below that and upside down, "133C0"

Revisiting this, I think there was something shorting earlier in the box. Since i've taken it all out and back in, the pedal seems to work, but the gain knob does little from 50-100% and sounds pretty fizzy at high gain.

Now, Q1, Q2, and Q4, will bias fine, but Q3 doesn't want to go low enough (shows ~11v to ~17.5v). I'm expecting it's just another short somewhere?

Check your resistor values again at R9, R10 & R11!

 

[tang]

Good news! Did you change anything or did you just take it out of the enclosure?

I didn't change anything major; the enclosure is pretty tight, so I snipped excess leads poking through the I/O board and the charge pump board (which is just floating). I also re-seated everything... so I imagine its just poor assembly skills on my part?

Check your resistor values again at R9, R10 & R11!

R9, 10, and 11 are marked 680R (blue, gray, brown), 5K6 (green, blue, red), and 1M (brown, black, green).

Edit: I re-seated everything and put electrical tape between all possible shorts and was able to bias all the transistors. After biasing them all to 1/2 of VCC @18v, it still didn't sound quite right. When I switch the pedal over to 9v, VCC was ~9v but the voltages at the drains of the transistors were all over the place. After re-biasing the pedal at 9v, the pedal sounds great at both 9v and 18v.

Does what I've described make sense? Should I be biasing my J201's at 9v? Should it matter?

Thanks for all the help so far!

 

[giovanni]

If it works the way you biased it, no need to change anything!

 

[tang]

Sounds good to me, thanks all for your help!

I'm going to mark this as "solved", thanks again.