A antenna tuner circuit which incorporates the resistance of the panel meter light into the accuracy of the meter. One must wonder why they would do this. They no longer support this unit so the bulb is hard to find. It is an axial lead bulb much like a axial lead resistor. I could replace the bulb with a resistor that is the same ohms as the bulb, I suppose. No back lit meter then but at least the meter would work again. Dumb design.
That kind of crap pisses me off daily as a tech, but there’s not much you can really do. Although the filament resistance will change with temperature and that might affect the accuracy.
That is the same question, I had as the resistsnce would change as the filiment heated up. Looking at the circuit they did not, my guess is perhaps it was not enough of a difference in output for a 1000w tuner. Especially on SSB as there is no steady carrier so your signal is just an average at best.
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u/MajorPain169 4d ago
Replacing with a resistor is not so straightforward. A light bulbs behaves more like a PTC, lower resistance when the filament is cold.