POP goes the wha

[ronh]

A friend asked me to help a fellow guitar player with his problem concerning his wha. Its a Crybaby gcb-95 wah. I have reached the end of troubleshooting ideas, so I am asking for help. The problem is the wha makes a loud pop when switching to effect mode. Simple right, not so fast.
This wha was converted to true bypass by someone else. So I checked the wiring. Simple true bypass with a jumper across two terminals. The buffer is still being used. The wiring checks out. Other mods done, Fasel inductor, resistor across inductor increased to 39K and midrange resistor increased to 2.2K. Wha sounds great except for the pop, only when engaging the effect. No pop when going back to bypass mode.

What have I done. Changed the true bypass wiring on the switch to short the input when in bypass. Still pops. Added 1 meg resistor on input jack, still pops. Measured all voltages, no dc on the switch or pot. I changed out the switch, why not. Other things I have checked. Re-soldered some joints. I powered the wah from three different sources, my pedal board supply, separate adapter and battery.

On my supply, power up in bypass mode, engage and pop. Go back to bypass and engage, no pop. Wait 90 seconds before repeating and pop.
Same on the adapter. On battery, a pop only on first engage. Does not pop again until power down, up and engage, one time again.
i measured the 9V supply on the board while switching and no change in voltage. FYI, meter is Fluke 8060A. It looks like a power problem but I cant find it. So I am asking for help. Open to all offers. Thanks for stopping by.

[pdf64]

Try a new switch?
If it’s first in the chain, the ‘toe down’ settings may sound much better if the buffer is removed, gets rid of the screech.

[Leo_Gnardo]

If this circuit is like most that are powered from a single ended supply, +9V for instance, there's bound to be a DC blocking cap at the signal output. This cap can develop a DC charge when it's not connected to anything else - bypass mode. Then when you switch the effect in, that DC charge shows up as a pop in the audio. You can attach a pull-down resistor at the output end of the cap, between the cap and ground. May have to experiment to find a resistor value that does not reduce the signal level, but also "pulls down" the stray charge adequately. I'd start with something around 100K - 220K, see how that works.

This sort of thing wasn't much addressed by early effects designers. Musicians learned to live with the racket. Now they're much more discerning. No problem there, especially if the problem can be fixed with a cheap resistor.

[Mick Bailey]

+1.

A 1M resistor on the output of the wah circuit to ground should fix the problem. Note - this is not across the output socket - it has to be permanently connected to ground after the last cap. Popping is very common with true bypass switching unless there's a drain resistor.

[pdf64]

But the wah pot track VR1 already references the output coupling cap to 0V common https://www.electrosmash.com/crybaby-gcb-95

[ronh]

Thanks for replying. I tried a 1M resistor to ground on the input. Then I tried a 1M in series with input. Reduced the pop as well as the wha sound level. I will try smaller resistors to find a happy medium. Will try a low value resistor in series with the output. Then bypass the buffer and see what happens. If one of these things reduces the pop to a lower level, I will consider it fixed. I changed out the switch because easy to do. I suppose one could consider the "explosion effect" a feature.
Thanks again.

[Helmholtz]

Thanks for replying. I tried a 1M resistor to ground on the input. Then I tried a 1M in series with input. Reduced the pop as well as the wha sound level.

While not very likely, you could have a leaky input cap.

Does the wah have other mods, e.g. an added LED?

[Mick Bailey]

But the wah pot track VR1 already references the output coupling cap to 0V common

Ah, yes. I should have checked the schematic before posting.

[Mick Bailey]

If you switch the wah in quick succession does it keep popping, or do you need to wait until it does this? If it doesn't pop then I suspect a capacitor charging issue somewhere. The hand wired pedals with true bypass have a 10M resistor from the input cap to ground to prevent this, though as you've wired it with a shorted-bypass input I would expect this to take care of any charge buildup.

[ronh]

This problem is now solved. After more troubleshooting I removed the the filter cap in the power supply reg and filter. This looks like a cap problem so nothing to lose if this doesn't fix. Problem now gone. I removed the resistor, zener and the filter cap since the owner use a "brick" power supply.
No more pop, why? I dont know but am glad its fixed and so is the owner. Thanks for the help.