Archive for May, 2013

Use of light bulb dummy loads in Vintage Amateur Radios

May 29, 2013

We recently reworked a Johnson Viking Adventurer CW transmitter and an EICO 723 transmitter, both manufactured in the 1960’s. They can be seen at on the Special Sales page.
A common failure of these transmitters was the power transformers.
They can overheat from current draw in excess of specified limits. This is often caused by not tuning the transmitter quickly or by mistuning and operating out of resonance.
Back in the 1950 and 1960’s we often used an incandescent light bulb for a load. It was cheap, and the resistance was close to 50 ohms or so required for the output of the transmitter. Tube type transmitters are a lot more forgiving from misstuning than solid state types. Never use a light bulb for a dummy load on your new
solid state rig! Choose a light bulb that has a wattage higher than the expected output of your transmitter. Watching the light bulb as you quickly tune to resonance will get you properly tuned without
too much time passing. Brightness is an indicator of power out. After tuning make sure you are not exceeding the recommended current draw. If you are, decrease the antenna coupling until you
are lower than maximum recommended power. Remember the old transmitter were rated in power input, now output. Output will always be lower. Input is plate current times plate voltage. This discussion is very simplified. Stay away from high voltage and high r.f wires. It’s hoped that it will help you see how to tune your boatanchor radio. Look for more detail in our newsletters and we may put out a video on disc or YouTube.
Questions are always welcome. Glenn for Pastime Projects.