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I have a guitar effects pedal that has become microphonic, meaning if you tap the enclosure the sound can be heard in the amplifier. I've traced the issue to a single ceramic capacitor on the board. The cap value is 0.01uF and I'd like to replace it. The pedal isn't expensive so I'm not too worried about damaging it by attempting a repair. The problem is that it's a tiny SMD component, I think a 0603 package. Should I replace it with a different type of capacitor or is it a fluke that this ceramic one is defective?

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  • \$\begingroup\$ I assume you suspect the capacitor because jiggling it causes the audio effect. Do only taps get through, like causing a click, or will it actually pick up and broadcast other loud noises like vocals? If it's just clicking this could be disconnection and reconnection due to vibration and not actually a microphonic effect. If so you may just have a broken trace somewhere and it's even possible the capacitor isn't defective and just needs to be resoldered/reconnected. Caps do wear out though. \$\endgroup\$
    – K H
    Mar 12 at 2:00
  • \$\begingroup\$ @KH Initially I noticed this because I could hear the metal foot switch, and I mean not even when engaging the switch but just touching or turning it. I then opened the enclosure and did a bunch of chopstick tapping tests to test if the sounds where louder or softer around different components. When I tapped on this single cap, the sound is significantly louder. \$\endgroup\$
    – Fiver
    Mar 12 at 2:04
  • \$\begingroup\$ I think you should try something other than a tap test. Put the speaker in another room and get someone to listen to it while you scream at the guitar pedal for not working properly. Let us know if the screaming comes through the speaker. \$\endgroup\$
    – K H
    Mar 12 at 2:26
  • \$\begingroup\$ You want me to yell at my poor, sick pedal? ;) I can also hear just scraping the stick near the component, and the sounds are not just noise but sound like the source of the noise (metal from the switch, scraping the board, tapping the component). But, for science, I'll do it! \$\endgroup\$
    – Fiver
    Mar 12 at 2:34
  • \$\begingroup\$ As long as you're absolutely sure it's actually reproducing sound as opposed to making a similar sound due to vibration that's an adequate test. Buzzes, hisses, clicks, pops, thumps, scraping noises can all be simulated by bad connection and vibration, but a more complex vibration without touching the device like voice would be unmistakable. I'm not there though. The vibrations you've been testing with may well be adequate. \$\endgroup\$
    – K H
    Mar 12 at 2:41
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You can try replacing it with an NP0 dielectric ceramic capacitor, but it may be hard or impossible to find in 0603.

Edit: Actually that's wrong, Eg. Murata GRM1885C1E103JA01D in 0603 and many other options.

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  • \$\begingroup\$ Thanks, the board seems to have plenty of room given the tiny components on it, just didn't know if an 0805 would still make solid connections. I can also find film caps in 0805, would those be appropriate? \$\endgroup\$
    – Fiver
    Mar 12 at 2:09
  • \$\begingroup\$ I suggest the NP0 as it's the same size. SMT film caps are fine electrically, but they're not very forgiving of being overheated or stressed when hand soldering. \$\endgroup\$ Mar 12 at 2:14
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    \$\begingroup\$ Got it, I'll order a few of them and report back. +1 \$\endgroup\$
    – Fiver
    Mar 12 at 2:20
  • \$\begingroup\$ No, 0,01uF is easally available in 0603. Using NP0/C0G dieletric is important as these are not acting as a piezotransducer/small speaker. Other ceramics are. \$\endgroup\$
    – Arcatus
    Mar 12 at 7:55
  • \$\begingroup\$ @Arcatus Yes, you're right. Edited. \$\endgroup\$ Mar 12 at 8:08

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