Trying to repair a non-working Fiber to Ethernet Converter. When connected to power, the unit made a hissing/sizzling/scratching sound and the all LEDs glowed dimly. I isolated a small surface-mount choke on the input power side, filtering 12VDC input from wall-wart to input of main 3.3V voltage regulator. (Schematic below.) Removed this choke and replaced with a straight wire shunt. The unit powers up and works perfectly.
I don't have an inductance meter, but the choke measures about 0.1 Ohms, so it's not burnt out.
The voltage regulator is a LM2591HVT-3.3 (a 150kHz Buck switching regulator). I see a clear 150kHz signal at the input after replacing the choke with shunt. Before replacing, there was a very noisy signal at this point, almost impossible to see with my scope.
1) What actually failed inside this choke? Why does it "sizzle" when powered?
2) What possibly caused this failure?
3) Is it OK to replace this choke with a shunt? What problems may arise from this modification?
Output of power circuitry is now stable at ~3.4VDC and unit powers up normally.