Heathkit IM 11D Valve Voltmeter

The IM 11D is a basic valve voltmeter (VTVM), using a double triode as a differential cathode follower. The IM-11 is the successor of the V-7A VTVM. It is practically identical to the V-7A: same type of case and mechanical construction, identical circuitry, but it is using slightly cheaper knobs and more modern components.

I bought an IM 11D in reasonable condition on the quarterly radio swapmeet of the NVHR. At the same occasion, I found a wreck of the same VTVM. I kept them in a box with two V7A VTVMs and only started to fix them in 2008.

It is a bit hard to date these instruments. According to the IM-11 page on the Heathkit Virtual Museum, the IM11 has been introduced in 1962 as an improvement of the V7A. The “D” in the type number probably means the kit was manufactured in Germany. The brand on the meter movement is German, and the texts on the PCBs are in German. The mains transformer looks German, too. As one can see on the photographs of the inside, the difference with the V7A is very small.

One of the IM-11's seems to have been in a fire. The black plastic handle is molten and charred. The other IM-11 was missing the front glass of the meter movement. So decided to fix the good one and sacrifice the other one to supply spare parts.

In May 2008 I started work. Trying all 4 Heathkit VTVMs, I found out all but one had a meter movement that was very much out of balance. The IM-11 that had an intact handle, also had a good meter movement. Unfortunately, it had a broken front cover (these are made out of clear plastic, probably polystyrene). I was able to find a spare meter movement, but that one was out of balance. So I just replaced the front cover.

I tried the meter and found out that the rectifier and other power supply components were OK. Both IM-11's had some charred resistors in the input voltage divider. I took the resistors I needed from the badly damaged IM-11. Some resistors were carbon compound resistors. I checked them and they were OK. Then I tried the repaired VTVM. It did seem to work! I went through the callibration procedure and became the proud owner of a working Heathkit VTVM.