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General Radio 1650-A C/R/L Impedance Bridge – Professional Lab-Grade Instrument
Before there was HP, there was GR… General Radio (GenRad). From the 1920s to today, they were famous for their superior, precision test equipment. These were laboratory standard apparatus.
The 1650-A Impedance Bridge is a versatile and compact instrument capable of accurate measurements of resistance, capacitance, inductance, dissipation and quality factor over a very wide range: 1 milliohm to 11 megohm for resistance, 1 picofarad to 1100 microfarad for capacitance and 1 microhenry to 1100 Henry for inductance. Accuracy is better than 1% given for R, C and L. The 1650-A uses custom made precision components for the arms of the bridge: the CRL rheostat uses a mechanical justifying mechanism with eight linearizing cams; a silvered mica, high precision capacitor is used as standard capacitor. The switchable Orthonull device, with non-reciprocal ganging of CRL and D dials, is available for easy of balance convergence in cases of low Q or high D.
This is a transistorized and portable version of their rack-mount impedance bridge. It runs on 4 D-cell batteries. It is still a useful piece of test equipment today. There are few modern equivalents. In 1966, it cost a bit over $1000.Virtually any impedance measurement can be done with this instrument.
The condition is fully functional. It has new alkaline batteries, and two new rubber feet. The (hot) External Generator binding post had been broken off. I repaired it by boring out the old post remains; tapping new threads; adding a new brass all-thread; and re-mounting it with coaxial, epoxy paste (see picture). The unit was cleaned, and the short gasket tear was repaired with RTV Silicone gasket glue. The case shows normal wear for an item 50+ years old.
A complete 36-page operational manual is available from the IET site below: