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General Radio 1685
Digital Impedance Meter
General Radio 1685 Digital Impedance Meter

Available from 1978 to 1978+

Catalog History
 Document Year Page
Catalog78 1978 28
(All manuals in PDF format unless noted otherwise)

The General Radio 1685 Digital Impedance Meter was available in Catalog 1978 only.

The Type 1685 is a combination digital and analog impedance measuring device. The CRL values are displayed by a five-digit LED display, while D and Q use an analog system.

Capacitance and inductance are measured at two selectable frequencies, 120 Hz and 1 kHz, while resistance is measured using DC voltages. The 1685 uses an eight-position range switch for capacitance and inductance measurements and a ninth position for high resistance measurements.

Above the range switch are illuminated arrows pointing to the range of greatest resolution. The analog DQ dial uses similar illuminated arrows guiding the user towards the null.

The 1685 has a comparator for Go/No Go testing of components. Limits are set with five decade switches for low and high values and indicator lamps for the results. Also provided is an indicator for high D or low Q if these values exceed the DQ dial setting. The four connections to the unknown are provided a special connector at the rear of the instrument, a test fixture or special cable are required. The Type 1685-P1 is the recommended test fixture.


  • Capacitance Range: 0.01 pF to 20,000 μF
  • Inductance Range: 0.01 μH to 2000 H
  • Resistance: 0.1 mΩ to 20 MΩ
  • Dissipation Factor: 0 to 10
  • Q Factor: .1 to infinity
  • Basic Accuracy: 0.1% basic accuracy for dc and 1 kHz; 0.5% for 120 Hz measurements