Redistribution and use of this article or any images or files referenced in it, in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
- Redistribution of article must retain the above copyright notice, this list of conditions, link to this page (https://xdevs.com/fix/hp34420a/) and the following disclaimer.
- Redistribution of files in binary form must reproduce the above copyright notice, this list of conditions, link to this page (https://xdevs.com/fix/hp34420a/), and the following disclaimer in the documentation and/or other materials provided with the distribution, for example Readme file.
All information posted here is hosted just for education purposes and provided AS IS. In no event shall the author, xDevs.com site, or any other 3rd party be liable for any special, direct, indirect, or consequential damages or any damages whatsoever resulting from loss of use, data or profits, whether in an action of contract, negligence or other tortuous action, arising out of or in connection with the use or performance of information published here.
If you willing to contribute or have interesting documentation to share regarding pressure measurements or metrology and electronics in general, you can do so by following these simple instructions.
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.
With Original LM399
With new LM399A
Calibration and verification
|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 FP plastic||Keysight||1||$20|
|34401A Rubber boot set||Keysight||1||$27|
Modified: Aug. 16, 2021, 5:26 a.m.