January 2021 Giveaway

Given the mention of Heathkit in the January 4 Muse, and the focus on history in both of this month's Muses, this month's giveaway is a circa 1949 VTVM from Heathkit.

Giveaway -  a 1949 Vacuum tube voltmeter

What's a VTVM? These used to be ubiquitous in labs but are unknown today. It's a vacuum tube voltmeter. Maybe you have one of those cheap Radio Shack volt-ohmmeters (VOMs). They typically have a terrible input impedance so will load circuits, giving erroneous readings. A VTVM feeds the input to the grid of a vacuum tube ("valve" to our British friends), giving the meter a very high (on the order of 10 MΩ) impedance.

The meter doesn't work. I took it apart and found the power-supply capacitor is bad; probably the others need replacing, too. The selenium rectifier works, but I'd replace it with a silicon diode. This would be a fun repair project. A manual with schematics is here.

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