|The Heathkit story
The VTVM (Vacuum Tube Volt Meter) IM-11 came on the market in 1961 and was produced until 1968. It was the successor of the V-7A that came on the market in 1956, that in turn was the successor of the V-7. There is only minor differences between the V-7A and the IM-11. In 1968 the IM-11 was replaced by the IM-18 and was produced until 1976. 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.
I have slightly modified my IM-11. 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-18 in a similar way. On my VTVM the knobs also are replaced.
The look before it was modified
The schematic of the original unmodified IM-11
The schematic of the modified IM-11
As a new, isolated from the chassis, battery holder I use a plastic film-canister for ordinary film type 135
The battery fit in the film-canister, a plastic bracket mounted and bolt-heads used as battery contacts
The battery-canister fit inside the IM-11
The 1Mohm resistor and the test-lead terminals
The look after it was modified