Hi everyone! I just built a Chrome Dome distortion, but having an issue. It works mostly, but somehow when I change the tone knob, it cuts the volume and sounds quite dull in the middle and if I turns it back and forth a bit, then the volume comes back and it sounds normal again. I already checked the tone knob and other components multiple times but couldn't figure out. If anyone can help, it will be really appreciated. Thank you guys!
Chrome Dome Distortion
- Thread starter blivjc
- Start date
Robert
Reverse Engineer
Another guy had an issue with the tone control having dead spots, changing to MPSA18 seemed to help.
The tone control always behaves a little oddly, even when everything is functioning perfectly. (Including my original)
The circuit is basically a Crybaby wah in front of a 4049 distortion.
At some point I would be interested in trying the circuit with a fasel wah inductor instead of the audio transformer...
The tone control always behaves a little oddly, even when everything is functioning perfectly. (Including my original)
The circuit is basically a Crybaby wah in front of a 4049 distortion.
At some point I would be interested in trying the circuit with a fasel wah inductor instead of the audio transformer...
Chuck D. Bones
Circuit Wizard
I am surprised that there would be a difference between those two transistors, they are pretty similar. I am not surprised that this circuit misbehaves. It is well-known that the original CryBaby circuit is sensitive to loading. For an extreme example, listen to how David Gilmour exploits that behavior in the middle section of the Pink Floyd tune "Echoes." IC1.3 presents a non-linear, reactive load to Q2. There should really be a resistor in series with C7 (maybe around 10K?).
FYI, you can change the filter Q by altering R9 and the tuning range by altering C5. Increasing R5 to increase the midrange presence is also a popular mod.
One more thing, CD4049s can be noisy. If your pedal has excessive hiss, try a different CD4049, preferably one from a different mfgr.
FYI, you can change the filter Q by altering R9 and the tuning range by altering C5. Increasing R5 to increase the midrange presence is also a popular mod.
One more thing, CD4049s can be noisy. If your pedal has excessive hiss, try a different CD4049, preferably one from a different mfgr.
Robert
Reverse Engineer
It's odd, I know, especially considering the original uses 2N5210... But this will be the fourth time swapping them to MPSA18 has cleared up weird tone control behavior.
Chuck D. Bones
Circuit Wizard
I suspect that Q2 or Q3 may be oscillating at high freq. If anyone can scope this pedal when it's misbehaving, that would be informative. Even if this circuit appears to be working normally with MPSA18s installed, it could be right on the verge of failure.
[rant mode on] By all indications the majority of guys who design these pedals fail to perform any sort of worst case analysis or deep-dive engineering testing. They build a bunch and if they sound good and the return rate is low enough, they think everything is ducky. [rant mode off]
[rant mode on] By all indications the majority of guys who design these pedals fail to perform any sort of worst case analysis or deep-dive engineering testing. They build a bunch and if they sound good and the return rate is low enough, they think everything is ducky. [rant mode off]
I realized that the sound comes back to touch with the finger on the central leg of the tone potentiometer ...
first and tried to weld this to gnd ... but the potentiometer changed to a kind of volume. provide resistance and voila a resistance of 120k between the central leg of the tone potentiometer to gnd it works. the thing is between 100k and 150k at 100k loses the tone function at 150k returns the volume cut ... honestly I am not electronic and I do not know why it works ... if someone can shed light on this matter it would be appreciated. Cheers
first and tried to weld this to gnd ... but the potentiometer changed to a kind of volume. provide resistance and voila a resistance of 120k between the central leg of the tone potentiometer to gnd it works. the thing is between 100k and 150k at 100k loses the tone function at 150k returns the volume cut ... honestly I am not electronic and I do not know why it works ... if someone can shed light on this matter it would be appreciated. Cheers
Chuck D. Bones
Circuit Wizard
Very strange. You must have a bad pot or a bad connection somewhere. All 3 legs of the TONE pot should read 0VDC with power on. Please confirm. Also, measure and report the volrage on Q1-C and Q2-E. Those voltages should not change when the TONE control is rotated.
hi guys, just came over from diy stompboxes, someone let me know this thread was here. im having the same issue, but ive got some voltages to see if that will help
q1 is e-170mv b-0.7v c-3.4v and its a 2n5088
q2 is e-190mv b-0.7v c-5.8v a 2n5210
q3 is e-4.7v b-5v c-14v a 2n5210
ill try and hold things steady to see if anything changes at q1 and q2 while i turn the tone pot and report
q1 is e-170mv b-0.7v c-3.4v and its a 2n5088
q2 is e-190mv b-0.7v c-5.8v a 2n5210
q3 is e-4.7v b-5v c-14v a 2n5210
ill try and hold things steady to see if anything changes at q1 and q2 while i turn the tone pot and report
so, changed Q2 and Q3 over to mpsa18, tied the middle leg of the tone pot to gnd through a 120k resistor, and still the same.
how ever, when i used a charge pump to supply 18v instead on 9, cleared it up! still some static noise when your finger makes contact with the pot in the 'dead' position, but goes away when you realease.
wierd!
how ever, when i used a charge pump to supply 18v instead on 9, cleared it up! still some static noise when your finger makes contact with the pot in the 'dead' position, but goes away when you realease.
wierd!
Mcknib
Well-known member
Posted this on your DIYSB post too Otomo
I've de enclosured mine seeing as you got my interest back in trying to find the problem
Initially I thought it was a cap problem or the 'variable cap' from reading RG's technology of the wah pedal
The inductor looks to the second transistor like its far side is grounded, through the 4.7uF capacitor. To the inductor, the capacitor kind of looks like it's grounded because its far side is connected to the emitter of Q2. Q2's emitter has a low output impedance and therefore looks like "ground" if you ignore the signal coming out of the emitter. At the junction of the inductor, capacitor, and 1.5K resistor, the voltage looks like the voltage that would happen across a parallel L/C circuit. However - the current through the capacitor is NOT determined by the voltage across the inductor/capacitor, it is also determined by the voltage driving its "ground" side, and that voltage is increased or decreased by the position of the wah pot. If the wah pot setting increases, the capacitor will let more signal current through because the voltage driving it at Q2's emitter is bigger, so the capacitor has to let in more signal current. If the wah pot setting decreases, the capacitor will let in less signal current. A "capacitor" may be thought of as a special instance of ohm's law by the amount of signal current it lets through. The change in the effective current through the capacitor makes the capacitor look bigger to the inductor and rest of the circuit than it really is! We have a variable capacitor!
With it being a crybaby in front of a 4049 distortion I compared the crybaby part to a crybaby schem noted the value changes and also the 82K and 4u7 had changed places but don't have a clue why, and thought maybe that's it Q3 is not getting enough signal when the tone / wah pot is at a certain resistance setting, I think I'm right in saying most wah pots don't use the whole range of the pot and this tone pot fault kinda mimics that with the dead spot being near the start and end of it
Anyway was messing with it today and purely by luck when I was taking voltages it sprung into life when my probe was on Q3 base and as long as I kept it there it was fine right through the dead spot
So I traced it back touching it with my now disconnected meter probe and got audio until the junction of R11 470K and C3 the 4u7
Anyway perhaps the guys with more technical knowledge than me (that'd be everyone) can figure out what's happening I thought maybe it was something to do with the meter circuit but I disconnected the probe and it still came alive when I touched it
I've de enclosured mine seeing as you got my interest back in trying to find the problem
Initially I thought it was a cap problem or the 'variable cap' from reading RG's technology of the wah pedal
The inductor looks to the second transistor like its far side is grounded, through the 4.7uF capacitor. To the inductor, the capacitor kind of looks like it's grounded because its far side is connected to the emitter of Q2. Q2's emitter has a low output impedance and therefore looks like "ground" if you ignore the signal coming out of the emitter. At the junction of the inductor, capacitor, and 1.5K resistor, the voltage looks like the voltage that would happen across a parallel L/C circuit. However - the current through the capacitor is NOT determined by the voltage across the inductor/capacitor, it is also determined by the voltage driving its "ground" side, and that voltage is increased or decreased by the position of the wah pot. If the wah pot setting increases, the capacitor will let more signal current through because the voltage driving it at Q2's emitter is bigger, so the capacitor has to let in more signal current. If the wah pot setting decreases, the capacitor will let in less signal current. A "capacitor" may be thought of as a special instance of ohm's law by the amount of signal current it lets through. The change in the effective current through the capacitor makes the capacitor look bigger to the inductor and rest of the circuit than it really is! We have a variable capacitor!
With it being a crybaby in front of a 4049 distortion I compared the crybaby part to a crybaby schem noted the value changes and also the 82K and 4u7 had changed places but don't have a clue why, and thought maybe that's it Q3 is not getting enough signal when the tone / wah pot is at a certain resistance setting, I think I'm right in saying most wah pots don't use the whole range of the pot and this tone pot fault kinda mimics that with the dead spot being near the start and end of it
Anyway was messing with it today and purely by luck when I was taking voltages it sprung into life when my probe was on Q3 base and as long as I kept it there it was fine right through the dead spot
So I traced it back touching it with my now disconnected meter probe and got audio until the junction of R11 470K and C3 the 4u7
Anyway perhaps the guys with more technical knowledge than me (that'd be everyone) can figure out what's happening I thought maybe it was something to do with the meter circuit but I disconnected the probe and it still came alive when I touched it
Chuck D. Bones
Circuit Wizard
It's oscillating at high freq. Your finger is damping the oscillations. Try adding a 1nF cap from pin 2 to 3 on the TONE pot. It's a band-aid, but should suffice in this case. Read my earlier posts. I think a better fix is to put a resistor in series with C7.
Mcknib
Well-known member
Thanks Chuck
I tried a 10K I'll try different values see if any work I've read and tried every suggestion unfortunately none have worked for me
It's not a massive problem more interesting to see if there's a fix I built it a while back the circuit is nice just that dead spot it's more annoying than anything else
I tried a 10K I'll try different values see if any work I've read and tried every suggestion unfortunately none have worked for me
It's not a massive problem more interesting to see if there's a fix I built it a while back the circuit is nice just that dead spot it's more annoying than anything else
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Chuck D. Bones
Circuit Wizard
You put it in the wrong place. You put it in series with the TONE pot. It needs to go in series with C7.
Lift one leg of C7, connect 10K between the pad on the board and the lifted leg of C7. Reconnect TONE pot pin 1 to the board.
Lift one leg of C7, connect 10K between the pad on the board and the lifted leg of C7. Reconnect TONE pot pin 1 to the board.
