Teardown, Repair & Upgrade of an Agilent 3458A 8.5 Digit Digital Multimeter

In this episode Shahriar repairs and upgrades an Agilent 3458A 8.5 Digit Multimeter. The instrument has been kindly donated by AllTest Instruments Inc. You can access their website here () , as well as their eBay store (
stores.ebay.com/expresstestservices) , Bonanza store () and TestUnlimited hub (). AllTest provides the following:
- Massive inventory of test and measurement equipment with over 20,000 units to chose from.
- Full service general laboratory where repairs, refurbishment, and testing are carried out.
- An additional NIST traceable metrology laboratory with extensive T&M capabilities where OEM grade calibrations are performed.
- 5% discount on any purchase when The Signal Path Blog is mentioned for first time customers with limit one per customer.
The 3458A unit fails startup conditions with several errors. After close inspection of the boards, the initial problem points to a poorly plugged in fiber-optic cable. After this fix, the unit still fails ACAL on the AC Board. The unit’s NVRAM modules all have to be replaced. During this process the memory of the unit is also upgraded and the firmware is updated to the latest available version. After analysis of the schematics the problem is traced to a damaged JFET input operational amplifier on the attenuator flatness calibrator unit. The replacement allows the unit to pass all self-test and ACAL calibration. The instrument is then NIST calibrated by AllTest Instruments and shows excellent performance when compared with various multimeters in the lab.
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