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At the last two performances, I've started hearing loud random pops when the stage gets warm and I'm running my signal chain.

I've swapped out pedals and changed cables and it appears to only occur when I'm using a GK MK amp and one particular pedal, an EHX C9. How do I diagnose the popping? It's not easily repeatable and appears to go away when I change the setup only to return again.

Is it in the amp and the pedal is causing the problem to reveal itself or is it in the pedal? What could cause the pops? A bad capacitor?

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    I've had this. Popping can be caused by, a bad capacitor within the guitar, bad earthing on the guitar and the strings get too static, faulty pedal and the most common cause of popping is old valves within your amplifier. If your amp has valves just look at them and if one valve is glowing where it shouldn't be then replace it. I've had the same problem on a Warwick bass with a push pull mod that coil-split the pickups which would also make bopping noises. Other than that the only other thing I that I can think of is if you have active pickups and need to replace a battery. Good luck :) – jazzboy Jun 7 '16 at 1:07
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    Narrowing it down to one pedal and one amp is a good start. If you are reasonably sure it is temperature-dependent, a hair dryer is a good tool for doing some controlled experiments. With a reasonable amount of common sense, you won't fry anything permanently. But the cause is often mechanical rather than electronic, for example a poor connection somewhere in the signal chain, which may inside the pedal or amp rather than a fault with the leads. One of the pedal controls might have a "noisy" contact that only shows up over one part of its range, for example. – user19146 Jun 7 '16 at 1:56
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    "mechanical" can also mean vibration. You might try turning the amp on in a darkened room and rap it with a rubber mallet (etc) and look for arcing. Arcing usually will increase the air gap (by vaporizing metal) and lead to more arcing more often. The "warm stage" may mean expanding metal connections, which lead to air gaps. – Yorik Jun 7 '16 at 18:11
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When I have this sort of problem, I open up each component individually, inspect the circuit board or hand wiring with a magnifying glass, looking for defective solder connections, broken circuit board traces, and dirt. I repair anything I find to be defective, and clean up the dirt. I swab the input and output jacks with a q-tip and solvent and usually find a surprising amount of dirt and corrosion there. Next I check connecting cables and often find they need replacement. These things take a lot of abuse and poor quality cables cease to function well rather quickly.

If the problem persists, then I set up my workbench to do a signal trace. I put a signal generator into the input of the component and using the probe on my oscilloscope, I can monitor circuit function through each step. When I find the problem circuit, then I can test the individual components,(capacitors, resistors, solid state devices, tubes, etc.) at this point the problem has always revealed itself and I can make the repair. I should mention that I rarely have to do a signal trace because the problem is usually fixed when I perform the tasks in the first paragraph.

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