Wireless router D-Link DIR-635 making high pitched noise
My D-Link DIR-635 emits a high pitched tone from it's circuit board which is very, very annoying. It makes the noise regardless of what power supply is being used with it and it's definitely coming from the circuit board.
The circuit is running from 5V. If I connect the router to a regulated PSU the noise is there as well, and if I feed it with less than five volts the pitch changes. The pitch also changes with blinking of the status LED, so I suspect it's an onboard-PSU issue.
I have already varnished all coils that are on the board with superglue, but it didn't change a thing. I desoldered some of the capacitors to see if the noise came from them, but it didn't help or give any new clues. For me it's impossible to locate the origin of the noise on the board just by listening, I think the pcb conducts the sound too well for that.
Does anybody have any ideas how to tackle the problem? Any known suspects on the board to look out for? What other pats can make a noise that high (downmodulation maybe)?
Edit: There are two of those cheapish crystal oscillators on the board, one 12.000 and one 25.000 (I'm guessing that's MHz?). Could they have gone "bad" and cause disturbance in the audible frequency range?
Since it changes with input voltage it is very likely a switching regulator that takes the input power and makes what is really needed. Take it out of the case and press things with you finger as there are no high voltages until you figure out which part makes the noise. My guess would be a small transformer coil, but it could be a capacitor.
Thanks for your input! I have already taken it out the case and poked around with my finger, but nothing notable happens. I will probably remove the caps on by one (there are maybe seven or so) and see if it makes a difference. There are four inductors on the board, and I already soaked them with superglue, so nothing should be moving there.
I hope I find the source, it's really annoying...
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