I just finished my first PedalPCB build, and it was the Promethium, and I’m running into an issue that hoping someone can point me in a direction to troubleshoot. I find that the bass knob doesn’t affect the sound at all, and the output signal just sounds weak (not in terms of volume, more like punch). I posted a shot the build maybe someone will see something obviously wrong (please excuse the wiring, that’s all temporary until I get an enclosure).
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SOLVED Promethium Build
- Thread starter punchy712
- Start date
Harry Klippton
Well-known member
C16 is important to the bass sound of this pedal. Did you orient the polarity correctly of that tantalum cap? Also make sure none of the leads from all those caps standing off the board aren't touching
Thanks for confirming that C16 is the important one for bass. I’ll be honest I didn’t measure the cap before installing it and confirm its polarity. Measuring it now on the board, the value is correct but I get the same reading measuring it either way (polarity wise). I’ll probably have to take it off the board to confirm.
Thanks for checking over the rest. Nothing is touching for the other caps. Definitely some lessons learned on what kinds of caps to use for certain spots to make everything physically fit nicely.
Thanks for checking over the rest. Nothing is touching for the other caps. Definitely some lessons learned on what kinds of caps to use for certain spots to make everything physically fit nicely.
Any hints as to how I could go about figuring out why the signal just sounds ”weak”? Hard to explain what I mean, but it just doesn’t sound all that different compared to the original signal. The distortion and level pots do affect the signal but not overly so. Would the distortion section of the circuit be responsible for giving the signal some “punch”? Wondering if there’s any specific test points in the circuit I could use to check the signal with an audio probe.
DGWVI
Well-known member
Can you confirm the value of R9?
From the pic, it looks like you have a 2.2M there, when it should be 22 ohm
benny_profane
Well-known member
That sub is fine. Those diodes form a noise gate of sorts. The low Fv is what’s important.
Thank you for confirming the diode swap.
I was going through the schematic to measure the voltage of the components that connect to VCC and VREF. The first few in the signal path measure around 8.8V, however when I get to R17, one side is 3.7V and the other is 4.3V. It’s very possible that I’m reading the schematic wrong, but shouldn’t one side of R17 be equal to something close to 8-9V?
I was going through the schematic to measure the voltage of the components that connect to VCC and VREF. The first few in the signal path measure around 8.8V, however when I get to R17, one side is 3.7V and the other is 4.3V. It’s very possible that I’m reading the schematic wrong, but shouldn’t one side of R17 be equal to something close to 8-9V?
jimilee
Well-known member
Nope, it’s connected to vref, so half of that.
jimilee
Well-known member
What opamps are you using, I’ve found it’s a little temperamental. The TL072s gave me the best results.
I ordered most of my parts from Mouser and I couldn’t find TL072s so I used TL072CPs instead. Any experience with these ones? I should check the data sheets again to make sure they’re compatible
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Thanks for posting your experience. They were marked obsolete at Mouser and they didn’t have any so I picked up some CPs and BCPs thinking they’d be equivalent. Looks like I’ll have to source the real thing unless others have any other suggestions for substitutions.
benny_profane
Well-known member
TL072 is a product series with a number of package variants.
The TL072CP is a DIP with a operational range from 0 to 70°C. The TL072IP is a DIP with an operational range from -40 to 85°C. They are functionally equivalent.
The TL072CP is a DIP with a operational range from 0 to 70°C. The TL072IP is a DIP with an operational range from -40 to 85°C. They are functionally equivalent.
jimilee
Well-known member
I’ve tried a couple and the 72 sounds the best and it operates like it’s supposed to.
Reviving this thread to see if I can get some further help. I set this aside to finish my Parentheses and Shika fuzz pedals and those were successful at least.
I reflowed and cleaned up the best I could and I can't see any issues in that respect. I'd like to focus on the fact that the bass knob doesn't do anything at all. I'm hoping that if I fix that then maybe the rest of the pedal will work. I had a friend come over to help me with and he brought an oscilloscope too. We fed a 440Hz signal into the pedal, and the probed the output and then a few points in the circuit where I thought it should have an effect when turning the bass knob, but we saw absolutely no change on the scope (the amplitude maybe changed a few millimetres but nothing significant at all). This is what we probed: R28, C16, R21, C13 and saw nothing on the scope. We probed similar components on the treble side and the effect on the scope was very obvious. I also checked the low potentiometer and the resistance between the wiper and each other side changed between 0 and 5kohms, which I believe is expected.
Any help at all would be really appreciated.
small edit to the above; when I mentioned that we saw nothing on the scope at the probe points I just mean that we saw just the input signal, unaffected by the bass/low knob being turned
I reflowed and cleaned up the best I could and I can't see any issues in that respect. I'd like to focus on the fact that the bass knob doesn't do anything at all. I'm hoping that if I fix that then maybe the rest of the pedal will work. I had a friend come over to help me with and he brought an oscilloscope too. We fed a 440Hz signal into the pedal, and the probed the output and then a few points in the circuit where I thought it should have an effect when turning the bass knob, but we saw absolutely no change on the scope (the amplitude maybe changed a few millimetres but nothing significant at all). This is what we probed: R28, C16, R21, C13 and saw nothing on the scope. We probed similar components on the treble side and the effect on the scope was very obvious. I also checked the low potentiometer and the resistance between the wiper and each other side changed between 0 and 5kohms, which I believe is expected.
Any help at all would be really appreciated.
small edit to the above; when I mentioned that we saw nothing on the scope at the probe points I just mean that we saw just the input signal, unaffected by the bass/low knob being turned
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