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Quietening the Samlex 1223 switching power supply – Keith Ericson, K0KE

– Posted in: 630 Meter Instructional Topics, 630 Meters

Keith, K0KE, submitted this item that some users may find useful when plagued by power supply noise:


“…I hooked up an FT950 to a Samlex 1223 switching power supply to use on 630m and I found serious birdies every 20kHz on the receiver below 500 kHz.  They peaked at 180kHz.  Peak signal was S7 with no preamp.  After finding that they weren’t there when running the rig on a linear supply I figured it was the switching supply.   I found this link with the answer.

http://w4kaz.com/images/fp1023/samlex_1223_rfi_mods_from_zl2df.pdf

It says to bypass the plus terminal to chassis  ground, the negative terminal to chassis ground, bypass across the two terminals,  add some ferrites and a few other additions.  Several different clamp on ferrites did nothing, bypassing across the terminals did nothing, bypassing the positive terminal to chassis did nothing BUT bypassing the negative terminal to chassis made a huge difference, the birdies went from S7 to in the noise.  The article recommends using .1ufd caps.    Not having any I scrounged up some .05ufd caps.  The first one helped, added the second, things improved some more, added a third cap for a total of .15ufd, all from the negative  terminal to chassis ground (a screw on the side) and considered it a success.   Don’t know if this will apply to other switching supplies but maybe it is a good starting place.”