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
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One more question about this pedal, related to drive/distortion knob (not volume/output). If I turn it all the way down, with all the other knobs turned all the way up, what should I expect if it was all working? Currently the volume is really low, much lower and less distorted than if the pedal was bypassed (the chain is simply guitar -> pedal -> amp).
Sorry for all the questions, i just wanted to add one more thing that I don’t understand. I went back to the audio probe and im hearing a full/loud signal on IC2 pins 1,2 and 3. Pins 5, 6, 7 barely have a signal; i can hear something but it’s really faint. I also probed R21 and the same very faint signal is there. Voltage on pins 1, 2, 6 and 7 is around 4.43V; voltage on pins 3, 5 and R21 (the leg connecting to the IC) is around 4.38V (I’m not sure if the small difference is of any significance).
The faint signal detected here seems problematic, but i have no idea why or what would cause it. Probing R22 and R23 (connected to the treble/high circuit) and the associated IC pins all have good signal.
The faint signal detected here seems problematic, but i have no idea why or what would cause it. Probing R22 and R23 (connected to the treble/high circuit) and the associated IC pins all have good signal.
jimilee
Well-known member
Are you sure you’re reading them right? 5,6 and 7 have your 1n4148 diodes. What is your pin 1 voltage on q2 and do you have audio on pin 2 and 3 of q2?
I’ll be glad to be wrong, but are you sure you’re not referring to IC1? From looking at the schematic that’s the one that has the diodes connected to pins 6 and 7.
To your question about q2, this is what I see/hear: pin 3 has 4.5V, pin 2 has 0.6V and pin 1 has 0V. I hear audio on pin 3, and just barely hear something on pin 2 (nothing on pin 1)
jimilee
Well-known member
Yep, you’re right, I was looking at 1.2. Maaan, I was all kind of twisted, I meant q3. Let’s start over. Do you have any audio on all 3 pins of the low pot.
For q3: pin1 shows 8.2V, pin2 is 7.5V and pin3 is 3.8V. Also yes I’m getting audio on all 3 pins of the low pot, however adjusting the pot doesn’t change the sound at all (unlike the high pot where adjusting the pot makes an obvious difference in sound)
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jimilee
Well-known member
Ok, I found this. Does it at all sound like this. The bass pot does very little if anything.
Harry Klippton
Well-known member
giovanni
Well-known member
As Chuck would say, did you start with basic visual inspection (of solder joints and component values) and continuity check? A lot of problems can be resolved that way, typically cold solder joints or shorts.
It sounds like all your pots are working fine (like they do change resistance), but double check if not.
Next, I think you already measured all voltages on ICs and transistors? If not, try that and summarize here?
If nothing apparently wrong comes up from the above, you can try audio probing at key nodes in the circuit, usually at the input and output of the various amplifying stages. You need to be able to read and follow the schematic to have success with that.
I hope that helps!
It sounds like all your pots are working fine (like they do change resistance), but double check if not.
Next, I think you already measured all voltages on ICs and transistors? If not, try that and summarize here?
If nothing apparently wrong comes up from the above, you can try audio probing at key nodes in the circuit, usually at the input and output of the various amplifying stages. You need to be able to read and follow the schematic to have success with that.
I hope that helps!
jimilee
Well-known member
Care to share any up close gut shots?
Thank you all for the help so far and for posting the videos. In the video from @Harry Klippton the bass knob does have a pretty clear effect on the tone and it's what I would've expected to hear. The other thing I mentioned and I think is confirmed by the video from @jimilee is that turning down the distortion all the way down should not decrease the output volume, which it does in my case.
I figure it's time to post a video of how it sounds, just to make sure I'm not mistaken in my expectations. I don't normally do this so please excuse the video quality. One thing to note is that I mistakenly installed the potentiometers backwards, so counter-clockwise is the max value.
I just uploaded this so if there's any issues with viewing it please let me know:
@giovanni I did a few visual checks already, and resoldered where needed and I don't see anything wrong in that respect (of course I could be wrong). I checked the voltages in the past and things looked good, but I'll do it again later and post back. I've used the audio probe in a few spots of the circuit, and I had a few weird findings that I noted in my previous post that I don't understand, if you wouldn't mind taking a read.
edit: one thing I forgot to mention is that the treble knob seems to have a big impact on the output level (seen on the oscilloscope too); I don’t know if this is expected or if it’s part of my problems
I figure it's time to post a video of how it sounds, just to make sure I'm not mistaken in my expectations. I don't normally do this so please excuse the video quality. One thing to note is that I mistakenly installed the potentiometers backwards, so counter-clockwise is the max value.
I just uploaded this so if there's any issues with viewing it please let me know:
@giovanni I did a few visual checks already, and resoldered where needed and I don't see anything wrong in that respect (of course I could be wrong). I checked the voltages in the past and things looked good, but I'll do it again later and post back. I've used the audio probe in a few spots of the circuit, and I had a few weird findings that I noted in my previous post that I don't understand, if you wouldn't mind taking a read.
edit: one thing I forgot to mention is that the treble knob seems to have a big impact on the output level (seen on the oscilloscope too); I don’t know if this is expected or if it’s part of my problems
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Harry Klippton
Well-known member
Check your pot values. You should have B10k for both the high and low controls. A10k for volume and a250k for distortion. Make sure they're all wired correctly since you have them off board. My guess is that you have something hooked up wrong or a wrong value somewhere
Checked the pots again, and they're all as you mentioned (at least as printed on them). I measured the low/high pots (as connected in circuit) and the resistance between the wiper and either side goes between 2-5k as the pot is turned. The 5k max I think makes sense as the two pots are wired in parallel, not sure why the min is 2k, but at least the value is changing.
The pots I think are connected properly (the middle wiper pin is always connected to the middle one on the PCB), but I'll go through everything again and make sure I have the right parts in the right place. Thanks for the help, I just wanted to make sure I'm not missing something obvious.
Thank you for pushing me to go back and check the basics. I did a continuity check and found that there was no connection between R25 and IC2 pins 6/7, and also no connection between C16 (negative side) and the low pot wiper. This would explain the bass pot issues; I fixed it temporarily by putting in a wire between those components and I’ll go back to fix it properly later when I get my pcb mounted pots.
I think it sounds mostly ok now but as I’m not familiar with an original I wanted to confirm a few things before I spend more time trying to fix something that’s potentially not broken:
- Is there supposed to be big change in sound when turning the high or low pots by just a few degrees from the max value? After that the change is much more gradual. @Harry Klippton I think that may be the case from looking at your video but would like to confirm
- the distortion pot: when turning it all the way down, is it supposed to reduce the volume by a massive amount as well, or do I have something potentially broken in build?
thank you again everyone for the help
benny_profane
Well-known member
The distortion control does next to nothing here. Ensure everything is as it should be with R10 and C5.
Harry Klippton
Well-known member
The high control particularly changes drastically just shy of full
This seems to be my last issue that I need to figure out then. In my case, just as the distortion pot is about to go to minimum the volume drops by a lot where the output is barely audible. I checked the connections and component values of R10 and C5, the pot itself, and that all looks good. I used an audio probe to check a few of the components around that part of the circuit, and I'm wondering if this makes sense to anyone. With the distortion pot on full I'm getting an audible signal which seems reasonable this early on in the circuit. When I turn the pot to min then the checking some of the components before and after the pot results in a barely audible signal. These are the components I checked: R11, R14, R15, R17, R10, R16, C5. I don't understand how/why turning this pot down would have such an impact on the components prior to it in the circuit. Not sure if this would point to something that's a little more obviously wrong to someone.
Thank you for confirming, that's exactly what I'm hearing. It looks like this part of the circuit is now working.
My last remaining issue is that turning down the distortion also seems to turn down the overall volume/output, which seems wrong. It may be working fine when the distortion is maxed out, but it's hard to tell as I don't have access to a working version of the pedal. Also, it kind of sucks to leave the pedal in a mostly working state.
I've checked all the component values, all the traces look good with everything connected as per the schematic. Would someone more familiar with circuit design be able to maybe narrow down which section of the circuit I could focus on as to where the problem may be? I write software, and debugging issues in the analog hardware world is quite a bit different.
I've checked all the component values, all the traces look good with everything connected as per the schematic. Would someone more familiar with circuit design be able to maybe narrow down which section of the circuit I could focus on as to where the problem may be? I write software, and debugging issues in the analog hardware world is quite a bit different.
I've been coming back to this build trying to see what's going wrong and I'm no closer to figuring it out. Before I just get another board and start again, I wanted to check again with others here if they have any idea on what could be going on.
The main issue I'm trying to solve is the distortion section where it doesn't seem to be distorting as expected. I've been able to confirm with an oscilloscope that the drive knob just changes the amplitude of the signal rather than creating a square wave. I built up the circuit in LTSpice just so I can have a reference and that's been helping. I narrowed things down to Q2 (2N3904) where the signal on pin 2 (base) looked about the same on the oscilloscope and the simulation (peak to peak voltage was around 60mV in both case I think). Pin 3 (collector) though is where things are massively different. In my circuit the signal stays roughly the same (small signal, with roughly the same peak to peak) while in simulation the signal is massively increased with peak to peak voltage being closer to 3V.
This is about all I can tell and I'm not really sure where to go from here. If someone has any ideas or suggestions I would really appreciate it.
The main issue I'm trying to solve is the distortion section where it doesn't seem to be distorting as expected. I've been able to confirm with an oscilloscope that the drive knob just changes the amplitude of the signal rather than creating a square wave. I built up the circuit in LTSpice just so I can have a reference and that's been helping. I narrowed things down to Q2 (2N3904) where the signal on pin 2 (base) looked about the same on the oscilloscope and the simulation (peak to peak voltage was around 60mV in both case I think). Pin 3 (collector) though is where things are massively different. In my circuit the signal stays roughly the same (small signal, with roughly the same peak to peak) while in simulation the signal is massively increased with peak to peak voltage being closer to 3V.
This is about all I can tell and I'm not really sure where to go from here. If someone has any ideas or suggestions I would really appreciate it.
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