Project:  BUILDING THE ST. LOUIS SWITCHER - Bill, WJ5O

From the May 2002 edition of QST Magazine (Matt Kastigar NØXEU)- see the pictures below

Since I had a couple of old computer power supplies available, I decided this would be a good weekend project.

It took only a few minutes into the project to discover that the author described the procedures using an AT power supply and MY units were both XT power supplies.

Checking voltages I was able to determine that the green wire from the connector to a main board was the one that turned the unit “ON & OFF.”

OK, now the procedures were the same ...  Although, I found that a 20 Watt, 3 Ohm resistor got very hot in the 5Volt regulator control ...  so I paralleled a couple of 6 Ohm 20 Watt “sand” resistors and bolted them to the upper “grill” of the power supply.  With the unit assembled the cooling air passes over both resistors and they only get warm.

I found a pot on the supply board that would permit an adjustment of the 12 volt output. Full clockwise however would only adjust the 12 volt output to 13.03 volts.  When testing the power supply with a Radio Shack HTX-10 25 watt rig, the voltage would drop to 12.87 on FM transmit . The voltage drop was only a “flicker” when transmitting SSB.  The same drop to 12.87  was indicated using KDK 15 Watt FM rig.

The only “minus” I’ve found was attempting to use the Yaesu PA-6 DC/DC supply/charger and the Yaesu FT- 411 Handie.  The PA-6 has an internal regulator to control vehicle voltage down to 7.2 volts. I connected a cigarette lighter receptacle to the “switcher” power supply and plugged in the PA-6 unit. The battery charging side worked but as soon as I keyed the rig, the “switcher” power supply “shut down.”  I’m assuming that there is a high frequency component on the DC output and it is sensed by the PA-6 which causes a high current draw to ground within it’s regulator.  Doesn’t seem to damage anything ... after a minute or so and an “ON/OFF” the “switcher” again puts out 13.03 volts.

All in all, it was a very satisfying weekend project.  I’m planning on using the “switcher” on my ten meter beacon.