Radio Frequency Interference (RFI) caused by a bad "Ltec" switching power supply filt

[wa2ise]

I was getting mild to bad RFI on the AM broadcast band, and lesser amounts on the amateur radio shortwave bands (hence the source of my user name here, it's my amateur radio callsign). After hunting it down by unplugging suspects, found it was my ethernet switch. Okay, took it out of service to figure out how to silence the RFI. Took the covers off, and after trying some additional powerline filtering (like those IEC metal bricks with internal coils and caps), noticed that the main power supply filter on the primary side of the switching power supply had a ruptured top! Well, need to replace that anyway. Maybe the RFI was from the power supply controller circuit working hard with almost raw unfiltered bridge rectified input DC? Sure enough, after finding a replacement cap (the new one is a Panasonic 105C, physically bigger than the "Ltec" brand (also claiming to be 105C) that went bad) the RFI was gone (below background level). Probably a good thing, before the rest of the power supply dies. When I removed the bad cap, one of the leads broke off and stayed with the circuit board.

Looks like RFI isn't always just annoying, it could flag a problem that something is about to die. When said RFI seemed to start up for no obvious reason (like I didn't buy some new electronic gadget recently).

[ben7]

Re: Radio Frequency Interference (RFI) caused by a bad "Ltec" switching power supply

I was getting mild to bad RFI on the AM broadcast band, and lesser amounts on the amateur radio shortwave bands (hence the source of my user name here, it's my amateur radio callsign). After hunting it down by unplugging suspects, found it was my ethernet switch. Okay, took it out of service to figure out how to silence the RFI. Took the covers off, and after trying some additional powerline filtering (like those IEC metal bricks with internal coils and caps), noticed that the main power supply filter on the primary side of the switching power supply had a ruptured top! Well, need to replace that anyway. Maybe the RFI was from the power supply controller circuit working hard with almost raw unfiltered bridge rectified input DC? Sure enough, after finding a replacement cap (the new one is a Panasonic 105C, physically bigger than the "Ltec" brand (also claiming to be 105C) that went bad) the RFI was gone (below background level). Probably a good thing, before the rest of the power supply dies. When I removed the bad cap, one of the leads broke off and stayed with the circuit board.

Looks like RFI isn't always just annoying, it could flag a problem that something is about to die. When said RFI seemed to start up for no obvious reason (like I didn't buy some new electronic gadget recently).

Yep.
A bad capacitor can cause the power supply circuitry to oscillate at other frequencies than it is supposed too. Also, the bad capacitor doesn't filter the voltage quite as well, and it can cause issues with the equipment being powered.

Glad to hear you successfully fixed it!