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I have a Panasonic landline (wireless) setup in my home. They have multiple charging stations, 2 remote bases, and one actual base station. They’ve worked reliably for at least 8 years, and I know Panasonic is excellent quality.

Recently, I’ve noticed one handset hisses during charging. It does this regardless of whether or not there are batteries in it, and after removing the back, I cannot tell where the noise is coming from. I tried holding down various components within it to see if I could narrow it down - to no avail. There are a couple big ICs and two PTH caps. The rest are SMD, and the PTH caps are held down by white adhesive.

Some digging online suggests that capacitors can hiss, piezoelectrically, but that seems limited to ceramic caps, not electrolytic ones. There are no vents and no bulging on either PTH cap, and besides the color & specs, they resemble the one pictured below. Any thoughts?

enter image description here

Edit: model is an older version of the “dect 6.0 plus” / “tga401” Edit:: no components are hot to the touch when noise is made.

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    \$\begingroup\$ Possibly dumb question, but is there any chance it's the speaker hissing and not an internal component? Seem possible that if you've got a bad filter or decoupling cap that noise from the charger circuit could be introduced into the speaker amplifier circuit. That handset has 2 speakers, so it just seems more likely to me that the issue is there vs a possible but unusual piezoelectric component phenomena. If you've already got the thing apart, try unplugging the speaker(s). \$\endgroup\$ – Phil C Apr 22 '18 at 5:01
  • \$\begingroup\$ @PhilC It's entirely possible. I'll get back to ya on that! I thought of this as well, but I didn't think it would be possible. I guess it is, based on what you're saying. I can try desoldering both speakers. \$\endgroup\$ – Bobdabiulder Apr 22 '18 at 12:50
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I once experienced hiss from pseudo random oscillations in the boost regulator ceramic coil being driven any a buck regulator. So it was PWM fixed f driving a PFM variable f boost regulator.

The technical term in my case was called chaos noise, a metastability condition that causes hiss. Actually it was more like running water. I fixed it by using a larger and lower ESR cap on the boost regulator input.

Your issue may be as you indicated, ceramic piezo-mechanical hiss which may coincide with thermal rise or outgassing from the electrolytic cap (CAUTION-toxic).

A plastic or small rubber hose to locate the sound is what I would use, unless you know someone with a stethoscope.

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  • \$\begingroup\$ The cap has no bulging, could it really be outgassing? \$\endgroup\$ – Bobdabiulder Apr 22 '18 at 12:51
  • \$\begingroup\$ Not likely as they ought be sealed unless defective which would be rare. The ferrite on the inductor is 1st guess from Chaos \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 22 '18 at 13:21
  • \$\begingroup\$ No inductors here. Again, caps look fine & have no vents, so idk \$\endgroup\$ – Bobdabiulder Apr 22 '18 at 15:19
  • \$\begingroup\$ I used a straw and a helper to figure out that the sound is indeed from the caps. They are 420uF and 450uF, 6.3v electrolytic caps. The sound they make is similar to this: link, if you turn the sound way down. \$\endgroup\$ – Bobdabiulder Apr 22 '18 at 15:42
  • \$\begingroup\$ Yes Chaos is design problem with random white noise effect.. You are the 1st to experience this on a cap, what brand was it so I know to avoid these Will a bigger cap with low ESR fit? 0.315" Dia (8.00mm) 0.709" (18.00mm) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 22 '18 at 16:56

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