Repair of Agilent 3458A Option HFL DMM


Hard to believe but it’s been six years since xDevs’s first rusty 3458A repair project . That “Golden grade” meter still going strong in lab, helping with daily measurements and calirations. Most use is done at main 10V DC range, capturing biggest benefit of 3458A DMM with excellent ADC performance and very low thermal coefficient errors.

This article will show a worklog for yet another 3458A, this time from Agilent manufacturing days. It’s not standard meter but the elusive Option HFL that was sold and maintained by Fluke back in early 2000s when they just gobbled up poor Wavetek instrument division but didn’t finished yet the transforming 1281 into ugly-looking Fluke 8508A. And to follow our best tradition, unit was received broken and non-functional. There is no fun in buying working equipment. No room for learning how to fix stuff! :)

As usual, all photos are clickable for high-resolution version.

Recently many threads pop up, with question “Is it worth to buy bad 3458A for $1000?. Well, simple answer is – No, it’s a big risk investment. If you asking yourself about this, then you are not ready for it. Buying broken or used 3458A for “good price” and fixing it might not be an easy way of getting good 8½ DMM, when multiple hidden costs considered. If unit stated as “untested/unknown/don’t know how to test” and not to have shown self-test result, it could have hardware failures. Paid cost for such a unit WITH proper repair can easily exceed cost of buying a fully tested unit. If unit’s photo shows it as powered up but you can see “ERR” on display, it also means there is fault detected. Keysight have standard 3458A repair + recalibration cost at $2660 USD. Calibration alone of fully functional unit is likely to be over $1000 USD. Also old units manufactured earlier than 2005, would have Dallas NVRAM batteries already near end of life, with risk of losing calibration data any day. So even if you buy meter for $1000, you can be easily be held back by another $2000-3000 to fix it and get properly calibrated.

There are also special versions of 3458A with options, such as:

Option Description
3458A-001 Extended memory option
3458A-002 Improved DC reference stability 4 ppm/year
3458A-H01 Special 1000 Vrms ac maximum input voltage
3458A-H52 US Air Force NSN
3458A-HFL Fluke version 3458A/HFL with 2ppm/year stability and better resistance
3458A-E02 Special version
3458A-OGC Precision calibration, intended for metrology use only

Table 2: 3458A options and versions


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If you willing to contribute or add your experience regarding HP/Agilent/Keysight instruments repairs or provide extra information, you can do so following these simple instructions

Manual references

Service notes

There are service notes/engineering changes were published during years of 3458A’s lifecycle. If your unit is old, worth to check if any of them required to do.

Initial photos and conditions

Like a wizard with all-see crystal sphere, here are list (likely not complete) of issues present with the box.

  • Dim VFD TBD
  • Not working resistance function Fixed
  • Not working correctly voltage function Fixed
  • Busted mounting for front binding post terminals
  • Large offsets and AC failures on all AC functions. Fixed

Power checks and initial diagnostics

Instrument was throwing “REFERENCE LOOP FAILURE: 201” codes and sometimes “Error 114: balanced rundown convergence” errors upon first power ups. Resistance function and AC voltage functions were not functional and DC voltage was errornous.

A1 DC/resistance board assembly repairs

A2 AC assembly troubleshooting and repairs

Most of the troubleshooting process was recorded during livestream. Beware, it is not edited or skipped footage, so it has nearly 7 hours of pondering around 3458A guts.

After spending 15 or so hours all AC voltage issues were narrowed down to one broken jelly-bean 3904 NPN transistor :)

Meter verification and calibration

SN18 ADC stability test

Comparisons to other lab gear

Thermal infrared images

Thermal analysis and understanding of hot spots and thermal gradients present in metrology-level instrument like HP/Agilent/Keysight 3458A is vital to achieve stable and repeatable ppm-level performance. Some of thermal images are captured using Fluke TiX560 320×480 imager.

Image 68: Infrared photo captures

Image 69: More infrared photos


Any second-hand broken 3458A buying decision should weight in probable cost of A3 A/D PCBA replacement. There are reasons why many of 3458As that end up on eBay have faulty boards or part mules. But in this Option HFL unit we happen to avoid the trouble with A3 swap and it was just relatively simple cheap component level repair.

Project cost:

Item Cost Shipping Supplier
Dead Agilent 3458A $1800 $100 eBay
Dallas/MAXIM NVRAMs 1 x DS1220, 2 x DS1230 $80 N/A Maxim
3 x ADG441 mux ICs $15 N/A Digikey
1 x OP07 operational amplifier $5 N/A Digikey
4 x CD4094 shift-register IC $2 N/A Digikey
2N3904 transistor $0.1 N/A Digikey

Table 9: Project cost

Total restoration cost for this meter: $2003 USD. Not cheap by any measure, yet still in range of secondary-market value for 3458A.

Date Activity Time spent
05/05/2021 Received unit, initial teardown and checks, repair for ohms functionality 6 hour
06/05/2021 Repair for DCV functionality, finished A1 repairs 5 hour
12/05/2021 Troubleshooting A2 issues 3 hour
13/05/2021 Test other boards with good donor A2, repair bad A2 15 hour
19/05/2021 A2 repaired, started calibrations and performance tests 12 hour
20/05/2021 Testing SN18 start 8 hour
26/05/2021 Started datalog or 10VDC base range to verify mid-term A3 stability 150 hour
Documentation, photos, calibrations, article write-up 12 hours

Table 10: Timelog summary

I would like to express my gratitude and appreciation to supporters of this project: Todd Micaleff, Mr. T, Nikonoid and of course all our readers. Stay tuned, many more interesting ideas are going online at in coming months. Discussion about this article and related stuff is welcome in comment section or at our own IRC chat server: (standard port 6667, channel: Web-interface for access mirrored on this page.

Author: Ilya Tsemenko
Created: July 2, 2021, 6:27 p.m.
Modified: Jan. 27, 2023, 8:40 a.m.