Fluke 732 series DC Voltage Standards service, modifications, testing



In this article legacy, but still very good Fluke 732A DC Voltage standard will be serviced and tested. We have multiple units so inter-comparisons will be performed to obtain relative drift rate.


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Manuals and service information

Comparison for known specifications of production resistance standards:

Specification Fluke 732A Fluke 732B Fluke 732C Fluke 734A or 734C Datron 4910 Fluke 5720A calibrator
Output voltage, DC 1V, 1.018V, 10V 1.018V, 10V 0.1V, 1V, 10V 4 × 732B or 732C 4 × 1.018V, 10V up to 1100V
Stability (1y, 10V) ±6 ppm ±2 ppm <±2 ppm ±1 ppm ±2 ppm
Stability (30 day, 10V) ±0.5 ppm ±0.3 ppm <±0.3 ppm ±1 ppm ±2 ppm
Temperature coefficient, ppm ±0.05 ppm/°C ±0.04 ppm ±0.04 ppm <±0.1 ppm/°C ±3 ppm max (18-28°C)
Adjustment range 50µV, <0.05ppm None Programmable 7½-digits
Reference design solution Motorola SZA263 LTFLU-1 up to 4 x Linear LTZ1000ACH 2 x LTFLU-1
Max load current 12 mA 200 mA 10 mA 418 mA
Output noise <1 µVRMS, 0.1-10Hz <0.6 µVRMS, 0.1-10Hz 3.5 µVRMS, 0.1-10Hz
Construction type Rugged half-rack module Rugged ¼-rack module Full-width 19” rack frame Rugged half-rack module Full-size rackmount
Active thermal compensation Yes, +45° C oven assembly LTZ1000 internal oven Yes, multiple ovens
Temperature sensor for monitoring Yes No
Operation range, ambient +0…+40 °C 0…+50 °C
Power requirements 100,120,220,240VAC or 24-40VDC 100,120,220,240VAC or 12VDC 200W mains powered
Backup/offline power supply Internal ±12V Battery bank, 12 hours life Internal ±12V Battery bank, 72 hours life None
Dimensions, weight 191 × 221 × 603 mm, 12.3kg 254 × 206 × 311 mm, 4.8kg 86 × 105 × 127 mm, 730g
MSRP ~$2k USD [EOL] $9k USD [EOL] $15k USD $40k+ USD $10k+ USD [EOL] $70k+ USD

Fluke also provided special order service to replace 1.018 VDC output to 1.000 VDC on their 732B units.

Fluke 732A Design and construction

Model 732A is rather bulky beast that demands deep bench or rackmount installation.


Reference oven PCB repairs and teardown

Fluke 732A using famous Motorola SZA263 Ref-amp hermetic zener-based reference core, while newer 732B and 732C use redesigned and refined version of ref-amp, designed and manufactured to this day specifically for Fluke instruments by Linear Technologies under part number LTFLU-1.

What is the Ref-Amp?

Ref-Amp consists of an NPN transistor in series with a zener diode. When biased properly, the combination has a extremely low temperature coefficient. The reference voltage that the can vary from +6.5 VDC to +7 VDC, depends on actual production bath. Also since both zener and transistor are located on same substrate and enclosed in hermetic package, they are tightly thermally coupled and protected from ambient humidity. This allows to improve stability over long time spans.

There are two well-known examples of this device family, Ref-Amps used by Fluke in high-end instrumentation equipment. These are Motorola SZA263 and Linear LTFLU-1H/LTFLU-1AH.

The Motorola SZA263 is a “Ref-Amp” that was made by Motorola, and later discontinued after getting out of high-volume semiconductor business. This forced Fluke to find a partner to design a replacement for the obsolete SZA263, as Motorola would not sell the design files and masks of original design. Linear Technology was happy to help big customer like Fluke with the design, and that’s how LTFLU-1AH Ref-Amp emerged. It’s pin-2-pin and function compatible with the original SZA263, but the LTFLU-1AH use bit aluminum alloy for the interconnects, which is not the same one Motorola used. This rendered in different long-term stability, visible by positive drift over time on SZA263, but negative in LTFLU-1AH (same as LTZ1000/LTZ1000A and the LMx99 ICs).

If you have a bank of 4 × 732A’s and a bank 4 x recent 732B’s that you get calibrated once a year, you will clearly see these drift patterns. Care need to be taken though, as early batches of Fluke 732B were still using left-over SZA263’s, but later all production 732B were updated to LTFLU-1AH.

Main differences summary between the SZA263/LTFLU-1AH IC and the market-available Linear LTZ1000 are:

  • Different package. LTFLU-1AH is 4-pin hermetic can, the LTZ1000 in an 8-pin TO-99. They are far from drop-in compatible.
  • SZA263/LTFLU-1AH has the transistor for temperature compensation in series with the zener, not parallel as LTZ1000 design.
  • Due to different manufacturing process, Motorola SZA263 have positive long-term drift, while Linear LTFLU-1AH has negative long-term drift.
  • Opposite to LTZ1000A, there is no on-die heater in SZA263/LTFLU-1AH
  • SZA263/LTFLU-1AH require much more time (vs LTZ1000 design) and care for support resistor matching and tempco testing

Last item is due to different tempco compensation transistor arrangement in SZA263/LTFLU-1AH circuit, which needs more attention for temperature and current compensation than LTZ1000 circuits. LTZ1000 design is fine even with standard datasheet reference schematics, without any analog black magic or voodoo.

All this above of course less a problem than zero availability of LTFLU-1AH, as Fluke have exclusive rights for this design and chip, and Linear is not allowed to sell Ref-Amp on open market. So Motorola SZA263/Linear LTFLU-1AH are less friendly solution to implement a stable DC reference, as even if you get LTFLU-1AH chip, lot of time and money for resistor matching and temperature testing is required.

LTFLU-1CH die photo. Courtesy branadic (Dipl.-Ing. A. B├╝lau) from the EEVBlog

On the die photo we can see much more complex design than just diode and transistor.

Instruments list using Ref-Amp

Known instruments to implement SZA263/LTFLU-1AH as primary DC reference:

There is also great thread here on EEVBlog created by lymex, revealing guts of various DC voltage standards.

Repairs and refurbishment for the 732A

One of TIP transistors on power supply regulator card was cooking over +130 °C due to fault and wrong value resistors. It was obvious even without fancy thermal imaging from strong burn smell and heat radiated by massive heatsink.

This issue was resolved by replacing offending transistor and all cabron composite resistors and electrolytic capacitors on both boards.

And collection of all replaced and bad parts from both 732As:

Replacement for front terminals on 732A

Poor standards had some rough handling by previous owners, as they were aquired from second-hand market.

Replacing posts in theory is quite simple, but in practical implementation is a bit tedious because there are number of rigid copper solid core wires coming out of oven assembly and the length of this interface is just long enough to tilt front panel out. Each wire is carefully marked with identification label, but it is still little messy because there are number of sense wires, especially to 10V output terminals.

To replace post one would need to desolder the wire from copper post, replace the terminal, resolder stuff back in, hopefully not melting binding post plastic insulator in process. Using soldering iron with wide tip and plenty of power (80+ watts) is best in this case, to save on heating time applied to the terminal.

After reassembly and testing that all electronics in 732A work both units were intercompared and verified against other DC voltage standards.

In photo above one such intercomparison is shown with 7 different zener 10V standards (Fluke 7004T, 732A, 732B and xDevs.com’s 792X) using Data Proof 160A nanovolt scanner and Keithley 2182A nanovoltmeter as differential detector with sub-ppm sensitivity.

Battery replacement using 4 × 6V 4.5Ah VRLA for 732A

Simplest way to replace old dead batteries in Fluke 732A is to source four suitable 4.5Ah 6VDC lead-acid batteries. One of such examples is these from DigiKey for $15.76 USD/pcs. They are rated for floated voltage at +6.90V, which is lower than old original 732A batteries.

Charge floating voltage level in 732A can be adjusted by R20 potentiometer on A4 PCBA (one with transformer and pre-regulator circuitry). Without this adjustment 732A will cook new batteries pretty fast, destroying new pack in a year or so.

Installation takes about 10 minutes. Disassemble frame, remove old batteries, and install new ones on plate. Bend battery terminals to about 60° angle for easier cable connections. Then carefully drop top metal frame with holes for terminals, without shorting any terminals to metal plate.

Connect all red wires to positive terminals on battery pack and black wires to negative. Keep order like original. Output voltage at positive and negative pad of interconnecting PCB should be around +26 VDC for new batteries with good charge.

Now pack is ready to install in the unit.

Should be good for 3-5 years of lifetime in normal metrology lab conditions.

Battery replacement using 2 × 12V 4.5Ah VRLA for 732A

Another alternative for battery pack rebuild is to install two larger 12V rated lead-acid batteries. This require some additional cutouts for the top metal work to allow safe terminal exposure and wiring.

Fluke 732B Design and construction

xDevs and friendly labs also have number of more modern and much smaller Fluke 732B DC Voltage standards. They are smaller and more refined version of large and deep 732A but provide essentially same functionality and performance.

Compared to 732A thermistor output and earth chassis 5-way binding post terminals are no longer available on front panel. Instead thermistor is routed to DB9 port on the back of 732B and earth chassis connections provided by screw near mains receptacle input.

Overall reference schematics is rather simple:

732B units repairs and teardown

Let’s dig deeper, towards the zener reference oven core.

Fluke 732C Design and construction

Fluke launched refresh DC standards line and released new 732C on June 26, 2018

Model 732C is latest reincarnation of Fluke 732 standard series. It replaces 1.018 VDC output with 1 VDC output and adds 0.1 V output in addition. Both of these outputs are provided from resistance passive voltage divider and support only high impedance loading.

Four-standard enclosure mainframe is also refreshed to new Model 734C but it has no physical design changes, other than now cheaper yellow Fluke label on front.

There are few more internal changes in 732C design compared to predecessor 732B but all specifications and operation procedures are otherwise identical between these two units. Core of the 732C is still based on good old Linear Technologies LTFLU-1ACH refamp zener core.

Here are some additional photos of Fluke 732C unit, showcasing key elements in design.

Charger has some small changes compared to 732B design.

There is additional internal switch SW503 located near REF01 reference IC, implemented to disable voltage regulator for the reference core. This feature added to save some battery power when this PCB is installed in charger power pack 732C-7001 module, which does not have zener reference core.

Interesting to note that PCB design have date code 2016. So it took Fluke about two years from design win to actual product release, which is pretty quick for a demanding product like 1ppm/year capable DC voltage reference.

Zener core is connected to front panel copper low-thermal posts same way as with older 732B:

Oven block has own controller PCB with heater regulator, few opamps, some passives and interconnects for hybrids with ceramic substrate.

Heaters on the side are same as older model reference. They are epoxied to massive aluminum box to get slower thermal response and more uniform oven operation.

Power transistor is mounted on the short side of the aluminum frame.

And star of the show, actual zener reference with all precision hermetically sealed thin-film NiCr resistance networks:

Precision wirewounds in round hermetically sealed brass enclosures are now gone and replaced by cheaper thin-film networks.

LTFLU-1ACH Ref-Amp in this particular 732C was manufactured around week 18, year 2018. There is OP97F nearby, thermistor bead and same thin-film network as on 732B.

Comparisons and long-term drift data


Summary and conclusion

Many younger engineers and graduates today may neglect and disregard old era instruments, such as 40-year old Fluke 732A as antique boat anchor boxes. But thinking that modern shiny replacement is better in world of metrology often a deception. Well cared Fluke 732A is still as capable as newest and fanciest Model 732C and actually capable offering something than NO new standard from factory can do. This capability is proven history of operation with decades of predictable and stable drift. All variants of 732 standards are still in wide use at calibration laboratories, used as DC voltage transport standards for transfers of the Volt between states and industry customers. Quantum standards are still quite expensive and specialzed capital investment to appeal wider customer base. Hopefully in next decade or so this would change, but until then many of us can still rely on glorious Fluke 732A/B/C boxes.

Special thanks goes to long-term xDevs.com supporters Todd M., Igor O. and many more for participation in interlab comparisons for resistance and voltage metrology. What are your results with 732 type standards? How do you keep track of DC voltage stability in lab? Do you have history and data to share about your 732? With these and any other related questions – feel free to reach out to us at our own IRC chat server: irc.xdevs.com (standard port 6667, channel: #xDevs.com) or via email

Author: Todd M.
Created: May 31, 2017, 4:01 p.m.
Modified: April 20, 2023, 6:38 a.m.