![]() Martin on The End Of The Electromechanical Era.Gravis on Linux For The Paranoid Does The Work For You.nik282000 on When Does Car Hacking Become “Tampering”? The British Government Seeks Guidance.Stappers on Linux For The Paranoid Does The Work For You.Hackaday Podcast 149: Ballerina Bot Balances, Flexures Track Cat Food, PCB Goes Under The Knife, And An ATtiny Does The 555 2 Comments Posted in Repair Hacks, Teardown Tagged heathkit, vacuum tube voltmeter, voltmeter, vtvm Post navigation Some people still prefer meters that move and, we admit, for certain tasks they beat even a digital bargraph. Compare it to a digital meter today and it seems very strange, doesn’t it? If you want to read more about how VTVMs were used, there’s a copy of a 1951 Sylvania book about them online. The repair seemed pretty simple, but it was fun to see the inside of one of these. In the end, the amplifier drove a conventional analog meter, but that load was isolated from the device under test so its relatively low resistance wasn’t important. ![]() There was also a rectifier tube switched in to make AC measurements. The Heathkit meter used a dual tube as an amplifier along with some input resistor dividers to provide an 11 megaohm input. However, with a tube amplifier, a VTVM could also show a very high resistance and still make good measurements. While a good meter would have relatively high resistance, it wasn’t as high as a FET. We take that for granted today, but a typical voltmeter in the old days was just a meter with some resistors in front of it. A high resistance like that is great when you make measurements because it is very unlikely to disturb the circuit you are trying to measure and it leads to more accurate measurements. It is more likely that a very large resistor (like 10 megaohms) is setting the input impedance because the gate by itself could pick up electrostatic voltage that might destroy the device. In a properly working MOSFET, the DC resistance between the gate and the rest of the circuit is practically infinite. That means the voltage measurement probes don’t really connect to the meter at all. Today, our meters almost always have a FET front end and probably uses a MOSFET. takes us inside a Heathkit IM-13 that needed some loving, and for its day it was an impressive little instrument. At first glance, you might just think that was shorthand for “old voltmeter” but, in fact, a VTVM filled a vital role in the old days of measuring instruments. If you are under a certain age, you might not know the initialism VTVM.
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