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.
Three questions:
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.
Thanks!
Schematic:
simulate this circuit – Schematic created using CircuitLab
Best Answer
There are many options.
First of all, as said in the comments it's possible it wasn't the choke, but the caps that have gone bad. It's possible it was the choke after all.
The choke is there to get out the buck converter's switching noise spikes, so that:
The second may not be an issue for you, the adapter will probably filter that out before it gets to the AC and possibly it will not couple from the DC wire onto anything else.
If it was the choke its inductance may have increased due to a mechanical default in the core. But you should also make sure the caps aren't dry by looking at the ripple if you put in a small extra resistance in the supply path.
Or just replace them anyway with a couple fresh ones.