HP back in the days built some number of special order customized 3458A DMM for particular customers. There is no public documentation about them , but I believe that there was special US Navy option available from HP/Agilent/Keysight to have additional aluminum shields installed over the outguard boards, to help instrument pass military EMI/RFI standards.

One of such meters recently arrived at xDevs for closer look, with serial number in 2823A0XXXX range. As always with instruments xDevs buys, it arrived pre-broken. Power up was successful, but display didn’t show anything. Meter however talked over GPIB, so there is a hope.

Exterior of the Option Special 3458A

This meter was received without rear bezel and without front lens.

Such made it obvious to see why display is not showing anything. Issue was due to unscheduled conversion of Vacuum Fluorescent Display(VFD) into Air-Filled Shards(AFS) screen. This meter also had metal mesh soldered to ground busbars around VFD screen to reduce EMI/RFI from the display, which is part of Option Special.

One can see bold crack between MATH and MRNG symbols and white barium getter that clearly indicate loss of vacuum. Time to open it up and replace the front panel/keyboard PCBA.

Interior of the Option Special 3458A

I have acquired a meter with this option and also a set of extra shields that cover all outguard boards, unlike to regular 3458A. They are rather thin aluminum sheet metal with mount points to the steel chassis frame holes on both sides. Other than shielding all the hardware and boards used in this special meter is identical to standard HP3458A units of same era.

Shields mounted with similar screws like PCBAs inside, and the screw heads left clear dents and scratches on outer covers too.

Brains in this meter are based around old-school CPU board P/N 03458-66505 Rev A with firmware revision 6 in six UV PROM chips. Dallas NVRAMs are datecoded week 30 of 1990, which is consistent with other date codes on chips and boards.

Removing shield cover from top inguard section reveals usual A1, A4, A9, A10 boards with no visible difference to regular 1990’s 3458A hardware. LTZ1000A reference does not have stickers or markings that may indicate option 002, as rear label also confirms.

Bottom side also shows usual hardware. There are some sharpie writings on A3, like XX314, A15 and U121 but otherwise it’s all normal looking.

This meter’s ADC board has older 1SJ8-0108 steering hybrid U180 chip, so only time and testing with X18 procedure would help us to determine if U180 is stable or not.

Power on and stability testing

After VFD replacement, meter was booted up and greeted with instant RAM TEST 1 LOW error on the screen. This clearly indicate faulty NVRAM chips. But since this time it was a soft error, I was able to reset the meter and finish the booting process, even without replacing NVRAMs yet. Running self-test and ACAL ALL revealed no obvious issues, everything completed normally with no errors. Time to monitor A3 stability now.

I’ve been looking this meter for some days now, and it was transported internationally from August 16, 2026 until August 21, 2026. This and perhaps replacement of the VFD and messing around the shields to take photographs above caused clearly visible CAL? 72 constant shift about 1 µV/V up. Based on this result it’s important to highlight importance of continuous operation in steady conditions during U180 stability study to have good confidence in results analysis. I hope ADC is good and we could evaluate other performance characteristics of this instrument compared to rest of 3458A’s xDevs has in lab.

Summary

For more details about typical 3458A refurbishment process, consult with this guide, that is often extended and improved.

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