From an earlier post, where you measured the voltages around the capsule during working and non working periods, there seemed nothing wrong. That would have been the area where I'd expect moisture to wreak havoc your circuit. The CMOS oscillator is also somewhat sensitive, but it seemed to work alright, given the capsule polarization voltage was present. So I'm not convinced that the R?de solution would work, but it's worth trying of course.
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You may have guessed it yourself already, but if you want to use an ultrasonic cleaner, then only clean the PCBA. Do not inmerse the capsule! Use a suitable? cleaning fluid. DIY recipies for cleaning fluids can be found on the web. Use demineralized water and thoroughly dry the the PCBA in an oven at 75C for several hours. If C8 does not have small standoffs, reflow one of its solder pads and lift one end a bit, so the fluids can reach that area. And at least as important, can also evaporate.
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Good luck!
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Jan
