VTVM (Vacuum Tube Volt Meter) IM-18
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.
is only minor
differences between the IM-11 and
the IM-18, mostly it is about
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
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.
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
terminal and the
probe resistor is placed inside the VTVM. I have modified my IM-11
in a similar way.
schematic of the original unmodified IM-18
schematic of the modified IM-18