Vast Oceans distortion: problems in 4th stage of TL074

[hans h]

Hi fellow DIYers,

I made a distortion and was hoping to post it here for inspiration. However, I ran into trouble with the conversion from breadboard -> layout -> soldered pedal.
The breadboard version worked perfectly, but somehow the soldered pedal does not. I can trace signal up to the input of the fourth opamp stage (active bass control, denoted by a 'B' on the respective pot in the layout). However, no signal comes  out. I checked for continuity where there should be and noncontinuity where there shouldn't.

The voltage in the 4th stage is off:
Vin: 19.876
Vin2 (after diode and resistor): 18.58
Vref: 9.284

Pin 1 (top left); 18.009
Pin 2: 1.5
3: 9.2
4: 18.7
5: 8.65
6 to 14: around 9.2 to 9.3 V.

Some background on the pedal:
I wanted a  distortion pedal that could do high gain without sounding all muffled or nasty. Wanted a Mesa and a Marshallish frequency response.
This was done by changing the frequency boosted by the Bass control. As is, with bass and treble at noon and switch to Mesa the frequency response is highly similar to the Mesa tonestack
(with notch at ~400-500 Hz, also similar to Fender).
With controls at noon and switch to Marshall the tonestack is highly similar to a marshall stack with notch at ~800 hz.
The frequency response of the 2 opamp preamp section was tailored to taste (quite middy which is made-up for by the tonestack similar to most higher gain amps).
All the frequency stuff I looked at using Ltspice for this circuit and Duncan's Tonestack Calculator for the amps.

Mosfet clippers in the feedback loop for a soft ('natural') type of clipping.
Each stage can be switched to asymmetric clipping by putting a led or diode in the B-socket

This is the schematic and layout:




I hope you have an idea of what is going on. It is highly frustrating to be so close after all the work that went in!

Thanks a bunch,

Hans

[idy]

Pin 11 is not ground?

[hans h]

Whoops, pin 11 is most definitely ground and 0V. Thanks for the catch.

[ashcat_lt]

Something is dividing down the DC bias in the feedback loop of that stage.  I would guess the 1u cap is shorted, but if that was true, the voltage at pin 2 (and pin 1) should depend on the position of that knob.  It's got to be around in there somewhere though.  Look real close for solder bridges or missed trace cuts or something.

[hans h]

So if I understand correctly I should be looking at an additional path to ground somewhere. I'll try your suggestion of turning the bass knob first and if the voltages do not change I'll look elsewhere. Thanks for the suggestions!

[ashcat_lt]

It has to be in the feedback loop because everything else is reading as expected.  Pin 3 is biased about right, Pin 4 is doing what we'd expect if Pin 2 was as screwed up as it is.  I don't think that can be a direct short, otherwise Pin 2 would be 0V.

[hans h]

OK I'm at a bit of a loss here. Knifed between all the tracks in that section again. No improvement. The voltage does not change when turning the bass knob. The 1uf reads 1uf and the 10 k towards the 1uf is also fine.

What I also did was measure resistance between the opamp pins and ground with AND without connecting the battery. This is what I got:

Pin 1: open circuit (0r when connected)
Pin 2: 19m (22k when connected, value independent of bass setting)
Pin 3: 16m (0r when connected)

Can I learn anything from the above? What is the significance of the 22k reading?

[ElectricDruid]

There's only five parts in that circuit, so we should just go through them one by one.

You said you've tested the 10K and the 1u and they're both ok. Good.

So what about the pot? With the power off and the op-amp removed from the socket, do you get the pot resistance between pins 1 and 2 on the socket? If you test from pin 1 of the socket to the pot wiper, do you get sensible readings as you move the pot?

Actually, looking at your stripboard layout, I can't see where pin1 of the pot connects to pin1 of the op-amp, or where the 33n and 22n connect to pin1 of the pot. What's going on there?

[hans h]

Well since it is my own layout it is not absolutely certain that it is flawless

Thanks for catching that! The cut shouldn't be there and I need a jumper between that row and the out pin. I'll first try it with a crocodile cable to see whether the voltage issues are also fixed that way.

One of the caps is connected to pin 1 of the pot (black dots are connections). When closing the switch the other cap is connected as well.

Once more, thanks Tom. As you may have noted this is the circuit I was working on when I asked about baxandall tone stacks. The tone stack is something entirely different now. The treble circuit is a modified tube screamer tone knob (without the connection to +in) and the bass circuit is modified from that.

[hans h]

It's working now! Really happy with the pedal, thanks.

[ElectricDruid]

Glad you got it going, Hans! Nice work!