# Chrome Dome Distortion



## blivjc (Jul 7, 2019)

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!


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## Robert (Jul 7, 2019)

A few people have had better results with MPSA18 instead of 2N5210 for Q2 and Q3.


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## blivjc (Jul 7, 2019)

Thank you for the tip! So was that an issue with transistors?


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## Robert (Jul 8, 2019)

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...


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## blivjc (Jul 8, 2019)

I checked Q2 and Q3 separately. Q3 is the one that causing the issue in my case. With 2N5210 in Q2 and MPSA18 in Q3, it still works. It sounds like that the sonic difference is almost unnoticeable. Thanks you very much again!


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## Chuck D. Bones (Jul 22, 2019)

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.


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## Robert (Jul 22, 2019)

Chuck D. Bones said:


> I am surprised that there would be a difference between those two transistors, they are pretty similar.



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.


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## blivjc (Jul 22, 2019)

Thanks for the info, guys! I really appreciate it.


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## Chuck D. Bones (Jul 22, 2019)

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]_


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## liani (Jan 31, 2020)

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


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## Chuck D. Bones (Jan 31, 2020)

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.


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## otomo (Apr 30, 2020)

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


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## otomo (Apr 30, 2020)

so Q1-C does not change voltage during sweep, and Q2-E doesnt either.

and i have zero volts at all three legs of the tone pot.


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## otomo (May 1, 2020)

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!


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## Mcknib (May 2, 2020)

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


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## Chuck D. Bones (May 2, 2020)

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.


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## Mcknib (May 3, 2020)

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


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## Chuck D. Bones (May 3, 2020)

Are you saying the 10K didn't work?


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## Mcknib (May 3, 2020)

Yes soldered it in series between the junction of tone pot lug 1 and C4 to C7


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## Chuck D. Bones (May 3, 2020)

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.


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## blivjc (Jul 7, 2019)

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!


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## Mcknib (May 3, 2020)

I'll retry Chuck I soldered it to the back of the board between tone pot 1 and C7 leg that had continuity with the pot lug but didn't lift out C7s leg

I'm at work for 10 hours so might be a while but I'll report back


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## Chuck D. Bones (May 3, 2020)

Ok, I hope my instructions made sense.


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## Mcknib (May 3, 2020)

Sure do Chuck 

I'll try this then a red fasel just to see what it does I expect it'll just act like a wah when turned 

I do have an old Gould OS255 15mhz oscilloscope which I've never used I just usually audio probe problems so don't have a clue with the o scope but I'll fire it up and maybe start learning and hopefully get some useful info


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## Mcknib (May 4, 2020)

Chuck

I can confirm that your solution works thank you very much for your advice and knowledge 

Unfortunately mine has developed another fault nothing major or related to the 10K fix but it hindered me in my testing, my volume kept cutting out and from the old wiggle test I've got a dodgy connection on lug 1 it wouldn't alter volume or dump signal to ground, once I held it in place several times and rotated the tone pot through its range the dead spot had gone

So when I get a minute I'll sort the volume 

Thanks again Chuck I know this has appeared on another couple of forums so I'll pass on your fix with credit to you


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## otomo (May 12, 2020)

great to hear chucks fixed worked, ive yet to try the 1nf on the tone pot, but ill dig the board out again and try with the c7, pretty sure that cleared mine up, but yea, ill try the 1nf as well!


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## Chuck D. Bones (May 12, 2020)

I like the 10K resistor fix better than the 1nF capacitor fix because it addresses the problem directly.  No guarantees that the 1nF fix will even work, it's a band-aid at best, just something to try.


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