Keithley Megohmmeter, Model 500

Keithley Megohmmeter

This was a find at the Antique Science and Retro-Tech Show a couple of years ago. The box was bent and dirty, and covered with various stickers. A ‘megohmmeter’ measures huge resistances, in this case (Model 500) will test from 107 ohms to 1013 ohms, or 10 Meg ohms to 10,000,000,000,000 (ten-million Meg) ohms.
If the internals were no good, at least it could make a good project box.

Front of the Keithley Megohmmeter
Front of the Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.
Front of Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.
Keithley Megohmmeter, original and before inspection, dirty and covered with stickers.

Turns out it was good and needed only some cleaning and, because it was designed for old mercury batteries. Fortunately, there’s a fix, which I’ll include, along with the somewhat rare schematic.

The original batteries for this were 1.3V (filament) and 8V (plate). These mercury batteries are no longer available. The 8V mount was fine to accept a standard 9V Battery. I replaced the 1.3V mount so that a standard AA battery can be used.

A new battery holder for a standard AA battery and a 9-Volt connector.
A new battery holder for a standard AA battery and a 9-Volt connector.

Now, to adjust the voltages to match the odd mercury battery values:
For the AA filament battery, I soldered a 1N4017 Schottky diode, which will drop the voltage about 0.3V to 0.45V. The 5886 vacuum tube used here only takes about 0.10A (10 milliamps) of current, so the Vf will be toward the low end of the 1N4017. In addition, the 5886 tube will accept up to 1.5V at the filament.

For the 9V Plate battery, the nominal (new battery) voltage is about 9.48V. To drop this to 8.2V, I’ve soldered in two 1N4001 diodes in series. Typical silicon battery Vf should be about 0.6V and this battery is not used at higher currents. Two (2) silicon diodes should drop the voltage to an acceptable 8.28V.
Although not needed, this add protection from incorrect battery insertion.

To accommodate modern batteries, I’ve added one crystal diode (0.3 voltage drop) and two silicone diodes for the 9V.
To accommodate modern batteries, I’ve added one crystal diode (0.3 voltage drop) and two silicone diodes for the 9V.

The active element within the instrument is a type 5886 vacuum tube. This is a subminiature tube, probably made by Raytheon, but I also find data from Sylvania, GE and TungSol. This tube is readily available new-old stock. The circuit is setup to be very sensitive and uses a very low voltage (8V) to test a very high resistance. Many modern Megohmmeters use a high voltage (100 Volts, or more) to test resistances.

After setting up the substitute batteries and some cleanup, I was able to successfully test a Heathkit 336 High-Voltage Probe (used for testing High Voltage TV Tubes) which has a 1090 Megohm resistor, which cannot be tested with a conventional DVM. I was able to read the approximate 1090 Megohms of the probe.

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