Everett Edgecumbe Hum Metrohm This continuity and insulation tester was given to me by David Mingay who inherited it from his father's estate. I guess this dates from the mid 1950's as in 1958 the firm was introduced a transistorised version which works on the same principle. The meter faceplate is marked Patent 874672 which is a puzzle as this patent of 1961 refers to a transistorised version though the priority date (application date) is for 1957. so was this meter still in production at the same time as the transistorised version? As can be seen from the diagram in the battery compartment the instrument was intended to be powered by two 4.5 volt flat batteries connected in series. At some time in the past the connections had been modified to accept a PP7 9 Volt battery. On temporarily powering the unit with a power supply I found that the Ohms range worked well but I was unable to adjust the calibration to the red mark on the Megohm range. By placing a voltmeter across the test terminals I found that only 300 Volts was being generated and by disconnecting the tubular capacitor adjacent to the buzzer the voltage climbed to 500 Volts as it should have been. I then set about removing the rust and corrosion. After re-assembly a replacement 0.22 mFd 600 Volt working was substituted for the faulty smoothing capacitor. The unit now functions correctly the calibration is reasonably accurate and I have obtained a battery holder so that it can be powered by a set of six AA cells.
click here for the later Transistorised version