TAVO Ohmmeter (Metrawatt)
ser no. 1389761

instructions on the rear for extending the range of the meter


scaleplate removed from defective meter

 interior of the switch section


meter movement

Ex WW2 German ohmmeter 5 ranges from 1 ohm to 10 Mohm which had been taken apart and the meter section had been converted into a 0-120  (0-120mV) meter. The resistance selection had lost two knobs but otherwise was complete. The original scale had been removed and replaced with a hand calibrated one. I had married the two parts together and after some experimentation discovered how it may have been intended to work. I made a new scale for the four lower ranges and fitted two replacement knobs. The unit takes two 1.5 volt cells which no longer seem to be available, they are shorter and larger diameter than AA cells. I don't know for sure but I imagine it came from the same souce as the E52b communications receiver also on this site.

In 2015 I purchased an identical meter  which had an open circuit meter movement so I was able to use the scaleplate selector switch knob and resistor coil to put the original instrument back as manufactured.

For the low ranges the unknown resistance is connected to the black terminals and the left hand knob is turned clockwise until the pointer reaches the calibration point, the right hand button is then depressed and the reading multiplied by the appropriate constant as shown in the table gives the resistance. For the higher ranges (in red) with the right hand button locked in the depressed position, the meter is used in the conventional manner, shorting the terminals and adjusting for full scale deflection before connecting to the unknown resistance.

This is what I think is the circuit.

This is one of a range of 'Tavo' instruments made by the German firm Metrawatt in the 1940s and 1950s having similar meter movements and housed in two parts  see below.