CDXL Response

DGWVI

Well-known member
I've got another idea. I recall troubleshooting a Mutron III a while back. Turned out one of the opamps was oscillating at high freq. If this board was on my bench, I'd put a 'scope on the opamp outputs. Since you don't have one, let's see if some band-aids will fix things. Tack solder a 470pF cap across R7 and another one across R5. See if that makes any difference.

Tried this. While it got rid of the fizz and wobbles, it killed the sweep. Basically just sounded like a wah stuck close to the heel down.

There really should be a small resistor in series with C4, something around 100R would work. I've seen this time and again. Whenever D1 or D2 conduct, C4 and C5 load the opamp's output and that is a recipe for oscillation. It's sloppy circuit design and completely avoidable with one resistor. Before adding a resistor in series with C4, just remove C4 and see if you can tune the filter back & forth with the LEVEL control. If the sound is good with C4 removed, then put it back in with 100R (or something close to that) in series. You will have to get creative with how you accomplish that, but desperate times call for desperate measures.

I'll try this next. Does it matter if I replace C4 with a non-polarized cap?
 

Chuck D. Bones

Well-known member
Chuck, Should the 100r be in series with - side of C4 disconnected to right side of 1N4148's OR
+ side of C4 disconnected to 2m2 & Pin 1 of Opamp

Either side, doesn't matter. I can't promise that this will solve the problem, but it's a good idea regardless. If it does solve the problem, then we got smarter. At least about this circuit.
 

Chuck D. Bones

Well-known member
Tried this. While it got rid of the fizz and wobbles, it killed the sweep. Basically just sounded like a wah stuck close to the heel down.



I'll try this next. Does it matter if I replace C4 with a non-polarized cap?

Polar or non-polar, either are good. If you install a polar cap, observe the correct orientation.
 

Chuck D. Bones

Well-known member
Tried this. While it got rid of the fizz and wobbles, it killed the sweep. Basically just sounded like a wah stuck close to the heel down.

That should not be the case and indicates another symptom. Are you sure you didn't get a solder bridge at IC1 pin 1 or 2?

As for the stuck waa sound, turning LEVEL should change the filter tuning. Does it?
 

DGWVI

Well-known member
That should not be the case and indicates another symptom. Are you sure you didn't get a solder bridge at IC1 pin 1 or 2?

As for the stuck waa sound, turning LEVEL should change the filter tuning. Does it?
Yes, the level changes the filter. With those caps installed, there was no (or, at least, seemingly no) sweep. With them removed, I get the fizzy, bubbly sweep as described by the OP, with no sweep if the Level pot is dimed.
 

benny_profane

Well-known member
Ah drag. I was hoping that this had been figured out! I still haven’t been able to get to my board, but I guess we’re back to finding new solutions. I’m out of my depth—I wish I could provide more constructive input. Thanks for your assistance thus far, @Chuck D. Bones
 

Chuck D. Bones

Well-known member
Then something else is wrong with the peak detector (IC1.1, the diodes, etc.). Putting that cap on IC1.1 should not make it worse.
 

music6000

Well-known member
Chuck, Do you think by putting temporary external Trimmers (Potentiometers) in place of resistors on the PCB would yield a better result in fine tuning this Build.
This is what I do where circuits have been finalized with Fixed resistors measured from an external Trimmer (Potentiometer).
 

benny_profane

Well-known member
Perhaps the 220k resistors should be a place to look? Also, that level taper probably should be linear. But, we’re a bit away from fine tuning this guy in that regard.
 

music6000

Well-known member
Perhaps the 220k resistors should be a place to look? Also, that level taper probably should be linear. But, we’re a bit away from fine tuning this guy in that regard.
I plug an Original 1977 DOD 440 in last night & the Range knob only has a small window from CCW to just over halfway, after that I didn't notice any difference, The Sensitivity knob is in the same working zone as well & its at midway that it brightens up a bit like a tone control.
It has a TL022 in it.
 

Chuck D. Bones

Well-known member
Music6000, does your question about adding trimmers pertain to enhancing a working pedal or fixing one that doesn't work?

How is more than 1/2 of the pot rotation a small window? We can certainly adjust the pot ranges, but bear in mind that a good range for one guitar may not be a good range for another.
 

music6000

Well-known member
Music6000, does your question about adding trimmers pertain to enhancing a working pedal or fixing one that doesn't work?

Adding temporary external pots to act as trimmers in a semi non working CDXL pedal in the critical areas that might better match the Xvive VTL5C4/2 which I believe might be the Issue.
It's not the first time that Xvive reissues are causing problems with these old type circuits.

I also have a Tone Factor 442 (DOD 440 Clone) that works much the same as I described the Original.
 
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