SOLVED Fornicus - bias issue

joeyd

joeyd

Well-known member
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.

1000003785.jpg
The trimmer is fully clockwise.

Any help is appreciated!
 
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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?
 
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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).
 
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joeyd said:
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).
Click to expand...
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.
 
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Fama said:
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.
Click to expand...
Did you swap out the LEDs in yours to get it working properly? I think I may have worded my initial post poorly - D2 looks like it's getting voltage and the LED flickers as voltage passes. I have no idea what kind of voltage is moving through (is it a guitar signal, is it just voltage from VCC?), but it looks like it's happening.

My voltages on Q5 and Q6 look very wrong (referencing a deadendfx build doc for their "copulator"). I've got 8.45V on the collector of Q5 and 8.45V on the gate of Q6. R20 and R21 are measuring voltages as if they're not in the circuit but I can't find a short. I'm getting weird readings on the TL072 too. Pin8 should have continuity with the VCC, right?

Thank you for bearing with me so far.
 
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Take a look around this research

www.marksescon.com

Kintsugi, a JFET drive with dynamic sag circuit. — Mark Sescon, RN, BSN, CCRN

Last year I built my first Smallsound/Bigsound Mini based on a PCB of my own creation . Over the course of the months that followed, I started becoming more curious about the F- Overdrive , which is a JFET based overdrive/fuzz/distortion that features a dynamically modulated starve circuit that is
www.marksescon.com www.marksescon.com

I built one from the PPCB board and also couldn't get it sounding quite right. I ended up following some of the research from Mark's site
 
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almondcity said:
Take a look around this research

www.marksescon.com

Kintsugi, a JFET drive with dynamic sag circuit. — Mark Sescon, RN, BSN, CCRN

Last year I built my first Smallsound/Bigsound Mini based on a PCB of my own creation . Over the course of the months that followed, I started becoming more curious about the F- Overdrive , which is a JFET based overdrive/fuzz/distortion that features a dynamically modulated starve circuit that is
www.marksescon.com www.marksescon.com

I built one from the PPCB board and also couldn't get it sounding quite right. I ended up following some of the research from Mark's site
Click to expand...
This did the trick! Specifically this page: https://www.marksescon.com/blog/202...g-circuit-in-the-smallsoundbigsound-overdrive

There's a note on the schematic of the Dynamic sag (the first schematic) that recommends that the forward voltages of D1 and D2 should match. I swapped out the LED that I had in D1 (formerly a 5mm yellow - I didn't have any 3mm yellow when I put it together) for a 3mm yellow (measured 1.91V to the 1.90V in D2) and the sag portion of the circuit started working. Also worth mentioning that the trimmer ended up dimed and the 1uF cap on the weight switch isn't doing a whole lot (I might replace it with a 4.7uF).

The distortion transforms from solid JFET drive to absolute filth. 10/10. I had my guitar tuned to drop B when I tested it. I tried out a couple sludge riffs and it was the aural equivalent to bathing in bong water.

I'm still somewhat confused because other people seem to get the circuit working just fine with mismatches in forward voltages. Another thread had someone putting a much higher forward voltage in D1 (vs D2) and it worked. I think it's within the realm of possibility that I touched/moved something unrelated while I was replacing the LED that broke a short.
 
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joeyd said:
This did the trick! Specifically this page: https://www.marksescon.com/blog/202...g-circuit-in-the-smallsoundbigsound-overdrive

There's a note on the schematic of the Dynamic sag (the first schematic) that recommends that the forward voltages of D1 and D2 should match. I swapped out the LED that I had in D1 (formerly a 5mm yellow - I didn't have any 3mm yellow when I put it together) for a 3mm yellow (measured 1.91V to the 1.90V in D2) and the sag portion of the circuit started working. Also worth mentioning that the trimmer ended up dimed and the 1uF cap on the weight switch isn't doing a whole lot (I might replace it with a 4.7uF).

The distortion transforms from solid JFET drive to absolute filth. 10/10. I had my guitar tuned to drop B when I tested it. I tried out a couple sludge riffs and it was the aural equivalent to bathing in bong water.

I'm still somewhat confused because other people seem to get the circuit working just fine with mismatches in forward voltages. Another thread had someone putting a much higher forward voltage in D1 (vs D2) and it worked. I think it's within the realm of possibility that I touched/moved something unrelated while I was replacing the LED that broke a short.
Click to expand...
It does sound like there might have indeed been a short (or cold solder joint?) earlier that got fixed, but the LED thing was covered in the thread I linked earlier I believe. I also had similar results to that thread, where similar (or small difference) between the two LED's forward voltages gave me a small effect, with a bigger difference making it much more pronounced.

The heavy switch not making much of a difference and trimmer needing to be dimed point towards that I think, but could also be something else wrong there.

I still need to fiddle with mine since I got some new LED's since I only had very similar ones, or a big difference, and not necessarily the sweet spot.
 
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