SOLVED Corduroy Fuzz - a problem again with the brighness pot

H

harmaes

Well-known member
I need your help with something:

I’ve played the Corduroy some more and found it too dark so I removed the 25kB pot and replaced it with a 50kB. It’s brighter now but the brightness pot doesn’t do anything anymore! This was one of the issues when I first build this circuit, this is my second PCB and trial.

Removing the pot was sensitive because it’s a tight space and it overheats quickly.
So after removing and replacing the pot I measured the connections and it connects to C19, and mass so it should work AFAIKT?

If I read the schematic correctly this 10n connected to the brightness pot to mass looks like a normal guitar tone pot but then reversed? For a neck pickup 10n or 15n is often used.

Chuck mentioned that the bias of Q4 influences the brightness pot as well.
- Could the bias be set to a level that the brightness pot doesn’t work anymore? I can’t imagine it but just asking.
- Not sure what else to measure from the brightness pot? Anything make a false contact maybe, maybe lugs 1 and 2 connected together thru heating?

Thanks for your help!
 
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Some more information:

I just measured the brightness pot again and it works normally when rotating one way it connects to mass, the other way it connects to the 10n cap. So the brightness pot seems to be operational in the circuit, only thing is that the 50kB value results in a non functioning pot. The advantage though is that the brightness of the circuit has improved a lot but maybe I could've better removed C18 the 2n2 cap?
I can try replacing the 50kB with a 25kB again and removing the 2n2. I don't really like doing this because the PCB is fragile and I haven't socketed it.
I did re-bias all drains to 4,5v and the circuit is sounding great with better brightness on the neck pickup and on roll-off.

So any ideas what could be happening here with the brightness pot not working anymore?
I remember from the first build is that I even tried to hook the brightness pot externally into the circuit which also didn't work.

Here's the link to the documentation:
 
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Can someone explain the resistor and capacitors around the brightness control and how they work?
EDIT: I found a tonestack calculator (Duncan's), it has a web and windows client. Which basic tonestack model in the app is closest to this brightness control?

From what I can tell in my circuit with the brightness pot not working it seems like it's set on its max setting which is 50k resistance to ground resulting in a brighter sound?
The tone stack has all C17-C19 and R26-R27 involved in this right?

I will share some pics but there's no sign of overheating or parts in the wrong place. Circuit is working, fuzz sounds great, brightness on max.

So if I would like to add an additional brightness pot to increase the brightness even more and have left darker and right brighter on the pot, can I add something like that BEHIND the brightness or volume pot?

Let me know if I can provide additional details? Would love to solve this issue!
 
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BuddytheReow said:
Here's a calculator for C19 and the BRIGHT pot. If you want more brightness then lower the value of C19. When you max out the BRIGHT pot you will have full resistance (50k in your case).

Guitar Pedals: R-C Filter Calculator

Calculate frequency cutoff for guitar effects pedals with vacuum tubes and solid state electronics. FREE calculator.
www.muzique.com www.muzique.com
Click to expand...
Any idea why the pot doesn’t work anymore? Maybe any tips on how to measure or probe?
 
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check to make sure the pot is good. Use you DMM to check the resistance between lugs 1 and 2, then 2 and 3 as you turn the knob. They should change as you turn it. If that works then check your solder joints on the pot
 
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BuddytheReow said:
check to make sure the pot is good. Use you DMM to check the resistance between lugs 1 and 2, then 2 and 3 as you turn the knob. They should change as you turn it. If that works then check your solder joints on the pot
Click to expand...
I checked the connectings from the pot to C19 and mass and these were good. I will measure the resistance between the lugs.
The reason the pot might not be working if there's a direct connection between one of the sets of lugs? 1/2 or 2/3?

It's a tight space to desolder the brightness pot and I want to avoid to much desoldering/removing parts.
The 50k resistance vs original 25k resistance shouldn't make the bright pot not working right?
 
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Just check resistance between lugs 1 and 2. 2 and 3 are tied together per the schematic.

Changing the pot value just changes the corner frequency of the control.
 
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I’ve measure the brightness pot and it’s working normally from 0k to just below 50k resistance between lugs 1 and 2. The resistance is also measurable between lugs 2 and 3.

I’ve measure all connections from Q4 until the volume into output and everything is connected the right way including all connections to ground.
So I’m quite flabbergasted why it’s not working?

What is the corner frequency with a 50k pot?
It sounds brighter now but a higher pot will give a lower corner frequency compared to the 25k pot. Maybe the frequency is too low to be audible?

And following the schematic and the above mentioned calculator, do you enter 15000 and 0.01 as value to find the corner frequency which is 1061.6? Or do you add the resistance of the pot as well?
 
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Which learning sources do you recommend about tone stacks, how to understand schematics and how to calculate the frequencies affected by specific parts of a circuit? Books, online material and video training?
Thanks for your help.
 
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Ok, I rewired the previous 25kB pot again in the circuit but the pot doesn't influence the brightness anymore.
Somehow there's something broken on the PCB but I can't measure it? Everything is connected and working normally except the brightness pot which isn't influencing anything anymore tonally.

@PedalPCB: Do you know if there is some magic going on on your v1.3 PCB with the brightness pot? It's connected to the C19 resistor and ground and all the other connections to volume and the previous resistors and C18 is connected in the circuit? Before I removed the 25kB brightness pot the pot did influence the brightness. It's really bugging me that this is not working anymore?
 
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I now remember why my tagboard builds had a connection between the middle and outer pot lugs.
The only purpose is to be able to define the amount of the cap that goes to ground. Life can be simple. :)
Great to have this issue solved.
 
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