SAMLEX SEC1223 switching supply emitting intense RF sweeping carrier at 271kHz

[wb0gaz]

I've had a SAMLEX SEC 1223 switching 12V (23A rated) dc power supply as my main ham station power supply for some years; it's always worked well. Last night while operating on the air, I noticed a string of sweeping carriers appearing every 20 kHz or so; by sweeping, I mean that it is a carrier that sweeps across a few kHz, about 10 times per second, with this pattern repeating across the RF spectrum. I eventually tracked it down to this power supply, with the strongest signal at 271 kHz (as in it's easily detected with a hand-held Yaesu VR500 with no antenna attached at all, pins it's S meter with any antenna attached to the Yaesu even with the DC output disconnected, and is audible over a radius of several hundred feet with a trivial antenna on the VR500, when the supply's output is attached to the station DC wiring). I've taken the supply out of service.

I don't suspect the design of the Samlex SEC1223 - I bought it new, it has worked very well over the years with never any indication of noise generation (this or any other type of noise.) The problem appeared somewhere in the last couple of weeks (first noticed last night) and was not cured by power cycling the power supply.

I've not done switching power supply repair or troubleshooting in the past, however, this is either a fix or discard situation, and I'd like to attempt fix as it would be a good learning experience and the schematic was found online (and is attached, should appear at the end of this posting; if not it can be found online.) I do have experience working around AC-powered equipment from a safety perspective.

Any getting-started advice re this sort of issue?

Thanks very much!

[edpalmer42]

Does your unit have the RFI mod?  Maybe it's failed.

Apparently, the SEC1223 has a bad reputation for being noisy and various users added an RFI filter on the output.  I also saw a note that Samlex added a similar filter around 2017.

I've also attached a redrawn schematic that I found.

[wb0gaz]

Thanks, this unit is far older than 2017; I'll look inside.

Thanks for the additional data.

Dave

[TheMG]

If the power supply has recently gotten a lot noisier than it used to be, I would thoroughly  check all the electrolytic caps. Caps going bad in an SMPS, particularly the output filter caps, can cause a dramatic increase in EMI.