TNP #49 - Keysight 34420A 7.5 Digit Nano Volt / Micro Ohm Meter Teardown, Repair & Verification

In this quick episode Shahriar repairs an HP 34420A. This 7.5 Digit meter is specialized for making low voltage & low resistance measurements. In 4W mode, it can provide excellent resistance measurements in the micro-Ohm range. This unit does not make any measurements on Channel 1 while Channel 2 reports the correct readings. An overview of the front-end schematics points to the likely culprit: the solid-state relays responsible for channel switching. After replacing the components the instruments returns to normal. Despite its age and the this repair, voltage and resistance measurements are remarkably close to their ideal values as compared with voltage & resistance calibrator instruments.

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