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Overview and exterior design

Manuals and datasheets

HP/Agilent/Keysight 34420A datasheet with specifications

HP 34420A User’s manual, Edition 1

Agilent 34420A User’s manual, Edition 2

Keysight 34420A User’s manual, Edition 3

Keysight 34420A Service manual, Edition 3, with poor quality schematics

HP/Agilent/Keysight 34420A scanned schematic pages

HPAK 34420A Safety/Regulatory Addendum

34420A SPRT Addon

Making Low Resistance Measurements using the B2961A and 34420A

Precise Low Resistance Measurements Using the B2961A and 34420A

Fluke/HP 34420/SPRT and 34420/PRT NanoVolt/Micro-Ohm Meter Instruction sheet

Teardown and repair

p=.

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

https://xdevs.com/ks34420a_test_aug2020/
https://xdevs.com/ks34420a_test_10mV_aug2020/

Calibration and verification

Summary

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