The VTVM (Vacuum Tube Volt Meter) IM-18 came on the market in 1968 and was produced until 1976. It was the successor of the IM-11 that came on the market in 1961, that in turn was the successor of the V-7 and V-7A. There is only minor differences between the IM-11 and the IM-18, mostly it is about new look and color. In 1977 the IM-18 was replaced by the IM-5218 and was produced until 1983. Again it was only a new look and color. The VTVM V-1 was one of the first Heathkits that came on the market in 1947, just after the oscilloscope O-1. The circuitry is very similar in all the VTVM's. The V-7 was the first with the components mounted on a printed circuit board.

My VTVM is marked as the IM-18E model. The E stands for Export and means that it is meant for 115/230V supply voltage. I have slightly modified my IM-18. I have connected the chassis to mains ground and isolated the negative terminal and the internal framework from the chassis so that floating measurements can be taken. The ¼" connector is replaced with a ground terminal and the probe resistor is placed inside the VTVM. I have modified my IM-11 in a similar way.

The schematic of the original unmodified IM-18



The schematic of the modified IM-18