Tube Meter Heathkit IM-18D
Since the 1950ies, Heathkit produced tube voltmeters. The Heathkit IM-18D is one of these, more from the 60ies, but I do not know the exact production year of my device. Heathkit sold these devices as kits. So you got all parts and soldered it all together. Also calibration was left to the buyer.
You can compare this tube meter with my Heathkit V-7A, which is about 10 years older but very close…
My voltmeter came in a very dirty condition, but it was all optical and after cleaning everything it looks quite well for a device which is about 60 years old.
Inside, the device looked well without any cleaning.
Small line transformer (lower left), most devices are soldered onto a PCB. 2 tubes are used.
Closer look to the two tubes. Also the power chord can be seen. Today it would not be allowed to do it in this way, I suppose :-)
For ohms measurement, usually a 1.5 volts battery was installed between the two tubes. In many cases, the batteries leaked at some point in time and this leas to oxidation. This is also the case for my device.
For ohms measurement, a 1.5 volt battery must be inserted. Because this battery leaked in many cases in the past -this is a very well known issue in all Heathkit tube meters of that kind- I do not want to insert a battery.
There are two small modifications to replace the battery with a DC voltage source taken from the filament voltage (6.3 volts). One alternative is to use a semiconductor voltage regulator, the other one just uses some resistors and caps. Maybe I will look into this later on.
Nice artwork at the voltage divider, all resistors here are 1%
A comparisation of my Heathkit IM-18D (right) with an older tube meter, the Heathkit V-7A from the 1950ies. The IM-18D was produced by Heathkit Germany. Even inside the meters are very close.