Fornicus - bias issue

[joeyd]

I'm trying to track down what's going on in my fornicus. The threshold pot and the weight and sag switches aren't doing anything perceivable (I feel like a 10uf cap should be very obvious). I know signal is passing (I audio-probed everything to check), I've verified the components are correct, and I reflowed all the joints. The other half of the circuit seems to be functioning correctly.

There are two things that look suspect to me. The orange LED is always on (it looks like it's connected to the vref so maybe that's right?). I probably installed a closed spst switch because the red LED seems to always be on in the circuit vs. when the "boom" switch is engaged. At any rate, the red LED doesn't seem to be effecting the audio.


The trimmer is fully clockwise.

Any help is appreciated!

 

[Fama]

I hope someone can clarify things, but according to https://forum.pedalpcb.com/threads/fornicus-leds.20394/#post-274391 and from what I remember of my own troubleshooting, you want a decent difference in VREF between D1 and D2 - the part I'm not sure about is which way you want it. I think you want D1 to be higher than D2, so that when you play lightly, D2 conducts, but if you play hard, D1 conducts and it then sags?

That's the part I'm not sure about and my brain is still groggy so I can't figure it out right now. But either way, I suspect that might be the issue you're having, especially if it seems to work otherwise.

I assume you've tested with the trimmer the other way around too?

 

[joeyd]

I've tested with the trimmer in different positions and no luck. I'm going to test for shorts at the SPDT since I'm not hearing any difference between the 10uf and 1uf caps and possibly try swapping Q5. I think I've got a 5089 from Tayda in Q5. Their transistors always feel like a bit of a roll of the dice. I'm assuming I'm still getting a good distortion sound because the j201's are doing their job properly.

It would be great if it was just a TL072 issue like linked thread. I've got a duophase that's not working and I would love to blame a batch of bad TL072's that I've put in it and this thing (but I think that's wishful thinking - I should probably test some voltages too).

 

[Fama]

I've tested with the trimmer in different positions and no luck. I'm going to test for shorts at the SPDT since I'm not hearing any difference between the 10uf and 1uf caps and possibly try swapping Q5. I think I've got a 5089 from Tayda in Q5. Their transistors always feel like a bit of a roll of the dice. I'm assuming I'm still getting a good distortion sound because the j201's are doing their job properly.

It would be great if it was just a TL072 issue like linked thread. I've got a duophase that's not working and I would love to blame a batch of bad TL072's that I've put in it and this thing (but I think that's wishful thinking - I should probably test some voltages too).

Study the schematic carefully. The lower part seems to work fine, the issue is with the upper part - the 10uF and 1uF caps do not make any difference if the LED issue prevents any signal from getting through in the first place due to the LEDs.

The whole upper part is meant to be the sag function where the signal gets blown out when you play hard. So even if the upper part of the circuit is not working at all, you will still get a nice distortion sound. Chances are it's the LEDs and not the transistor or anything like that, because it matches the symptoms and I had the same issue, but of course I can't know for sure.

I haven't had any issues with Tayda parts tbh.