Redoing a Pedal

YourGuitarist

YourGuitarist

Active member
Bout to vent...

I've been troubleshooting a pt2399 delay with a big muff tonestack. Custom build.

The blend does not go 100% wet despite it being a pot to pan between dry and wet signal. The pot measures continuity across rotation. There is a volume control directly coupled after.

Then the tone control acts like a volume control which goes to 0 on the treble side. I thought this could've been a shorted cap to GND but alas it is not. Pot measures continuity across range.

Perhaps most confusingly is that while the main delay can be muted via the tone control, I can still hear the repeats.

I've checked the DC voltages which are all fine.

I recycled the pots and some caps from other builds before.

I'm confounded to say the least. I'm wondering is there a point where it's better to start over from scratch?
 
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To add to what Cybercow said:

Are you sure the circuit itself is fine, or could there be an issue there? If it's a custom PCB, are you sure the layout is fine?

In general, troubleshooting a pedal is sort of a gamble, you spend X amount of time on it and you might hit the answer quickly, or it might take a long time. If it's a component issue or mistake in building it, starting over from scratch could fix the issue. But if it's an issue with the circuit or PCB layout, it's more time and money wasted (although then you can be pretty sure it's not a component issue at least).
 
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I would start with a fresh look in the morning and if nothing jumps out at you, I would remove the tone control setup and check if the rest of the behavior is the same (wet/dry being off, repeats still there in weird situations)

Then remove the mix pot and directly wire the wet and then the dry signals individually and how it behaves.

Try to notice other behavior when you make these changes and see if you can pick out a pattern or a part of the circuit the faults are all related to

To me it's worth going down these paths because then I won't stay awake at night wishing I'd solved it to the end. I'd rather destroy a circuit troubleshooting it than just throw it away without trying
 
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Cybercow said:
Schematic? Good clear photos of the build?
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WHile this is a hobby project, I work MI so I have non competes against sharing the circuits I make 😔.

Ik it's frustrating not having the circuit, I try to explain without pictures as best I can

The blend pot is 10k followed by 100k volume.

I will bypass the pots. Hadn't thought of that. Also think doing a sweep at each stage would be good.
 

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It is a custom layout and schematic. It's NOT high gain nor high impedance so I'm apprehensive that bleed is a crosstalk issue from layout. But, I can't see a bleed path in schematic.
 
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YourGuitarist said:
WHile this is a hobby project, I work MI so I have non competes against sharing the circuits I make 😔.

Ik it's frustrating not having the circuit, I try to explain without pictures as best I can

The blend pot is 10k followed by 100k volume.

I will bypass the pots. Hadn't thought of that. Also think doing a sweep at each stage would be good.
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Can you at least schematically show us how the Blend is followed by the Volume? It's possible the issue is there.
 
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I measured wiper to each end at both CCW and CW for the blend pot. The level pot loads down blend so that 10k5 become 9k5 at max wet. SO upping my level pot to a 250k or 500k could help.
 
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in other delay circuits, the level control doesn't follow the blend control directly, rather it's implemented differently - there is either a level control for the delay only before it joins back with the dry path or there is a mix control in lieu of a master vol control.

I would remove the a100k level control and see if your blend improves.
 
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YourGuitarist said:
WHile this is a hobby project, I work MI so I have non competes against sharing the circuits I make 😔.

Ik it's frustrating not having the circuit, I try to explain without pictures as best I can

The blend pot is 10k followed by 100k volume.

I will bypass the pots. Hadn't thought of that. Also think doing a sweep at each stage would be good.
Click to expand...
So you do this professionally and you are asking for help on a hobbyist forum? 🤔
 
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Paradox916 said:
So you do this professionally and you are asking for help on a hobbyist forum? 🤔
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I am fresh out of college. Less than a year deep in work. There are people on this forum who also work stem and for much longer than myself.

Humility goes a long way. It's arrogant to assume I'm the smartest in the room.

Furthermore if I'm building circuits in my free time on top of the demands of work, that's proactive and taking initiative. Laying foundations takes time, a handful of college courses and a 9-5 hardly sharpens the iron.

No need to stir the pot.
 
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I have built the Drebbel Bass preamp, which has a big muff tone control and also a separate mid control. With the mid control turned all the way down on that the tone control does function as a volume control, so that tone setup does seem to be a bit interactive in that way. I have built 6 Drebbels and they all work the same way, so I think that is part of the design, not a construction error. You may need to check out what is in the circuit around he tone control that could be changing its behaviour.
 
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PJS said:
I have built the Drebbel Bass preamp, which has a big muff tone control and also a separate mid control. With the mid control turned all the way down on that the tone control does function as a volume control, so that tone setup does seem to be a bit interactive in that way. I have built 6 Drebbels and they all work the same way, so I think that is part of the design, not a construction error. You may need to check out what is in the circuit around he tone control that could be changing its behaviour.
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Yeah I have the big muff wiper feed direct into a non inverting buffer. It worked on the last rev. SO did the blend and volume pots.

I removed the blend pot to see if the volume control was acting as an antennae, somewhat is the case. After lowering some gains and the output bleed resistor to kill the crosstalk, it mostly went away.

Dropped blend pot back in dry bleed came back. That suggests pot is problem, but pot measures fine offboard.

Double checked supply filter caps to see if signal was coupling through the rails/vmid, not the case.

I'm starting to think I damaged the board substrate.

Wondered if the low passing for the PT2399 was too aggressive and choking the treble side of the big muff but ran audio precision sweeps, not the case.
 
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Cybercow said:
Yeah, seeing that confirms my suspicions. IMO: the volume pot would be better served after the opamp, in place of R19.
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Sorry to push back but the volume/blend has worked for me on previous builds.

It's true that volume loads down the blend but that's why volume pot impedance is a magnitude higher. I could increase further to reduce interactivity though.

If I follow the opamp with volume then my output impedance is higher and variable, suppose with a 10k vol pot that's less of an issue. Battle of compromises, it is.
 
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