[Thomas] picked up a Marconi TF1245 with dents and dings. We have to admit that we had not heard of a “circuit magnification meter,” but apparently, this was a thing in the late 1960s and early 1970s. Turns out, we have heard of this kind of meter before, but it was called a Q meter. The device works using a very low-impedance resonant circuit and a very high-impedance voltmeter. It measures the ratio of the voltage across the known circuit and the unknown circuit. This particular meter needs an external signal source with very special characteristics. You can see the well-built device in the video below.
The unit didn’t seem to work, but we suspect that it didn’t like his normal signal source. According to a comment in the manual, the matching signal generator delivered 0.5V into a 0.5 ohm load. You could also use a matching transformer to get to the required match.
Like anything made by Marconi, the build quality is amazing. There are four tubes and four diodes inside, along with beautiful meters, transformers, wafer switches, and some amazing mechanical linkages. As far as we can tell, if [Thomas] had a matching transformer, the device would work.
Of course, there were other Q meters. You could measure Q with a grid dip oscillator, too.
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- Source: https://hackaday.com/2023/12/09/marconi-circuit-magnification-meter-gives-up-secrets/
