Mod of Samlex SEC 1223CE PSU

In my summer holidays June 2009 I experienced when using my Samlex SEC 1223CE*) PSU and FT-857D to my daily QSO with friends at 3724 kHz several irritating 'humming' S 7 carriers appeared every 30 kHz in the 80 and 160 m bands :-(

Distance between PSU and antenna 3 meters.

The carriers disappeared shortly after I switched off the power before the receiver went quiet - it means the carriers were generated in the PSU - switch mode noise!

When I returned home the PSU was opened - a 2.6 mm hole was drilled in the back plate between the output terminals 4 mm from the back plate top - M3 thread was cut and a solder tag mounted under a M3 screw + spring washer.

Then two suitable polyester foil capacitors 1.5 uF/100V were connected from each output terminal to the solder tag.

After remounting top cover - I had a "new" noiseless PSU with no "false" carriers!

20 minutes easy working!

The two foil capacitors reduce Common Mode radiation from the PSU output.

Please find few pics hereafter telling the happy story:

Unit type, remark the CE sign!

Before modification

After modification

Details near the output connectors

A very suitable foil condenser for the purpose

To view the spectrum I use a homemade sniffer based on a split core plus a thin coax from my scrap box connected to my Spectrum Analyzer. Snap the split core around the DC or power cable to view the generated spectrum.

To hear or view the noise level connect the sniffer to your receiver and enjoy the difference before and after the modification.



Spectrum before modification

Spectrum after modification

The two spectrums shows the modification has a very good effect on the radiated common mode noise from the PSU.
The measured switch mode noise level is below -115 dBm from 1 MHz to 30 MHz.

IMPORTANT: If you connect your SMPS to mains via a 3 wire mains lead (or the chassis to ground) the internal mains filter will reduce radio noise emission from the unit. Furthermore AC level of the chassis will be zero - no more electric shocks!

*) The "CE" models have much better 'radio noise' specifications compared to the "FCC" approved models and furthermore all electrolytic capacitors are of 105C type except C6 and C7 (470F/250V). Therefore I strongly recommend: buy the CE model next time!

Good luck

Addendum: A 'new' second hand Samlex SEC 1223CE is modified, please enjoy the following pics:

Type label, remark the CE sign!

Output spectrum before modification

Output spectrum after modification

Revised April 13, 2011