[Elecraft] New Power Supply

[Bill Coleman]

On Feb 2, 2010, at 2:45 AM, Ruben Navarro Huedo wrote:

> I am using a Samlex SEC1223 with my K3.
> I have some "electric-noise" on low bands (80 and 40 meters) and i think 
> it comes from the power supply. I have been reading other friends with 
> the same problem.
> I want test another one.
> What is your recomendation?

When I got my K2 back in 2001, I was using a linear power supply. A few years ago, that power supply died when the power transformer primary opened. I bought a Samlex 1223 supply to replace it. I was impressed with the size of the unit, and it had no trouble supplying my K2/100 to full output, even on 160m. 

However, I was disturbed to find numerous spurs were audible on my antennas on 160m. The loudest of these was near 1845 kHz, about an S7 on the K2 S-meter. Note that the location and strength of the spurs varies with supply load.

First, you need to identify if it is the Samlex that is causing the noise. I did this by momentarily turning the supply off. The K2 (and K3) draw little current in receive, so you should have about 2 seconds of receive before the radio goes dead. If the noise moves away rapidly or disappears completely, it is likely the power supply. I would recommend you try to find the strongest noise source for the load you have while receiving. Note that these switching power supplies will shift the intensity and location of their birdies when the load changes.

The SEC-1223 does not have a filtered AC cord. I used a snap-on toroid core to determine that the AC line could use some filtering. If your AC receptacle is already filtered, you can go directly to the DC output lines.

I found that putting a snap-on ferrite core around the power cord reduced the spur to an S5 indication. A similar reduction could be had by placing a snap-on ferrite around the power output leads.

My first modification was to de-solder the white and black power cord jumpers inside the unit in order to slip on seven FT50-77 toroids. After this modification, the spur was reduced to an S5 indication.

Since that wasn't sufficient, I then made a modification similar to that designed by ZL2DF and published by N0SS: http://www.n0ss.net/samlex_1223_rfi_mods_from_zl2df.pdf

My unit was different from ZL2DF, in that it did not have the screw / compression power terminals. Instead, my unit used two binding posts that were soldered directly to spade terminals on the PC board. 

After pulling out the PC board, I removed the spade terminals. I replaced these with 16 gauge wire fed through eight type 43 ferrite beads for each lead. The wire is small, but it is the largest that would fit through the beads. the output terminals are bypassed across each other and to chassis ground with .1 50v multi-layer ceramic caps.

After this, the spur at 1845 kHz was just audible. At night, atmospheric noise covers it up.

This supply was originally designed to work with VHF/ UHF gear, and likely does well. With minor modifications, it is suitable for use with HF gear.


Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901