Sunday, February 24, 2013

Samlex SEC 1223

Unit #2 - before modifications.
I guess you can say I'm pleased with the Samlex  SEC 1223 power supply. After all, I own three of them.

Back in 1988, I bought a CB-grade 12 volt linear power supply capable of 25 Amps output. It needed a little work to function correctly, but it powered my venerable TS-430S and later the K2/100 for 19 years. It finally went west one day when the power transformer opened in the primary winding.

Soon after, I was hunting for a replacement at a hamfest. I found a Samlex SEC 1223 that fellow had replaced with an Astron SS-25M. Used, the price was right, at about $50. The unit is impressively small, and it had no trouble powering the TS-430S, K2/100 or K3/100 to full output on every band.

However, this unit had numerous spurs on 160m. I found the loudest one was around 1845 kHz, about S7 on the very scotch K2 S-meter. Note that the location and strength of the spurs varies with the supply load. Using snap-on ferrite cores as an experiment, I found that putting them on the power cord and the output power leads led to a reduction in the spurs.
Unit #1 - showing seven FT50-77 ferrite
toroids on power input leads.

My first modification was to de-solder the white and black power cord jumps inside the unit and slip on seven FT50-77 toroids. That reduced the spur to about S5.

Unit #1 - Output lead modifications.
I then made a modification designed by ZL2DF and published by N0SS. My unit was different from that of ZL2DF. It did not have the screw / compression power terminals. Instead, my unit had binding posts soldered directly to spade terminals on the PC board.

I pulled out the PC board and removed the spade terminals. I replace them with 16 gauge wire fed through eight type 43 ferrite beads. The wire is a bit small, but is the largest that will fit through the beads. The output terminals are bypassed across each other and to ground with .1 50v multi-layer ceramic caps.

With this change the 1845 kHz spur was just audible. Atmospheric noise covers it up at night.

Last year, I found another one of these gems at a hamfest for $20. Took it home and sure enough, a few spurs on 160m. Made similar modifications -- toroids on power leads, ferrite beads on output, caps across output terminals. Again, the spurs don't disappear, but are much less pronounced.

Unit #3 - perhaps only needs power input filtering?
This year -- you guess it -- I found another one for $5! This unit is a little different, as it has a large ferrite bead with both power leads run through it to the compression terminals. Perhaps it will only need a big of work on the power leads. We'll see when I can get it on the bench.

These are very capable little power supplies. They are easily able to power a 100 watt transceiver, and with a little work, they are quiet enough for MF and HF use.

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