HP/Agilent/Keysight 34420A Nanovoltmeter repair


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Overview and exterior design

Manuals and datasheets

Teardown and repair


For high gain stage used in 1 mV and 10 mV ranges HP designers used ultra-low noise JFET matched pair in hermetic TO-78 package from InterFET, IF3602. This N-type JFETs are based around monster large 2×2 mm geometry die N3600L and can sport voltage noise level of just 0.5 nV/√Hz. Next best device of such class is Linear System’s LSK389 and LSK489 with 1.8 nV/√Hz voltage noise specification. This low noise levels come at a price of huge capacitance.

Enough to say, this device is also not cheap, at $64 USD retail price per piece and lead time at least 28 weeks. There are also single JFET versions of with this low-noise device, such as IF3601.

This instrument had faulty channel 1, did not measure any input signals. Tracing signal from the input connector to front-end quickly revealed a culprit, which was bad solid-state switch. After ordering replacement LH1510AAB SSR relay from Digikey (P/N LH1510AAB-ND) and replacing both Channel 1 started to work normally.

Second issue was faulty communication interfaces. After replacing GPIB drivers SN75ALS160DW and SN75ALS162DW and resoldering TMS9914A controller these issues were also resolved. GPIB implementation is pretty much follows reference in 34420A and similar instruments like HP 33120A, 34401A, etc.

Performance tests

With Original LM399


With new LM399A


Calibration and verification


Repair BOM

LH1510AAB SSR RELAY SPST-NO 200MA 0-200V LH1510AAB-ND QTY 2 $2.92ea
SN75ALS160DW IC TRANSCEIVER HALF 8/8 20SOIC 296-9932-5-ND QTY 1 $3.13ea
SN75ALS162DW IC TRANSCEIVER HALF 8/8 24SOIC 296-9934-5-ND QTY 1 $6.80ea
34420A Lens Keysight 1 $10.34
34420A FP plastic Keysight 1 $20
34401A Rubber boot set Keysight 1 $27
Author: Ilya Tsemenko
Created: Aug. 3, 2020, 3:41 p.m.
Modified: April 10, 2022, 12:40 a.m.