Inside a Selective Voltmeter

[Martin Lorton] has a vintage Harmon 4200B selective voltmeter that needed repair. He picked it up on eBay, and he knew it wasn’t working, but it was in good condition, …read more

May 9, 2025 - 21:19

[Martin Lorton] has a vintage Harmon 4200B selective voltmeter that needed repair. He picked it up on eBay, and he knew it wasn’t working, but it was in good condition, especially for the price. He’s posted four videos about what’s inside and how he’s fixing it. You can see the first installment below.

The 4200B is an RMS voltmeter and is selective because it has a tuned circuit to adjust to a particular frequency. The unit uses discrete components and has an analog meter along with an LCD counter.

The initial tests didn’t work out well because the analog meter was stuck, so it wouldn’t go beyond about 33% of full scale.

Since there are four videos (so far), there is a good bit of information and detail about the meter. However, it is an interesting piece of gear and part 3 is interesting if you want to see inside an analog meter movement.

By the fourth video, things seem to be working well. You might want to browse the manual for the similar 4200A manual to get oriented.

Forgot why we measure RMS? You weren’t the only one. RMS conversion in meters is a big topic and there are many ways to do it.