FLUKE 731B, DC Reference Standard, Repair [success]

[HighVoltage]

I bought this FLUKE 731B, DC Reference Standard from the USA after finding a seller that was willing to send one to Germany. It was suppose to be in good working condition and tested by the seller to specifications.

Well, it did not work on batteries when I turned it on. (should have taking it apart first)
I opened it up and was surprised by horrible looking batteries and removed them carefully.
Then I switched the line power to 230V and applied the voltage to it, but the unit is dead.

It seems to be another repair job.
Hopefully it has only the power supply broken, but at a first test, DC volts are leaving the rectifier.

[Nuno_pt]

Nasty looking batteries.

[HighVoltage]

Yes, they are really bad.

Actually, each of these 6 visible cells are 2 NiCd cells, so a total of 12 cells x 1.2V = 14.4V
I might install some Ni-MH cells, if I find some fitting cells.
 

[ManateeMafia]

Looking forward to your repair. Don't tell TiN, he will want to throw in a LTZ reference board.

Good luck.

[lowimpedance]

Crusty indeed but otherwise it looks to be corrosion free so should clean up nicely.
So it was tested to spec. huh!.

At least you do have recent experience cleaning up 'old' NiCd driven gear.

[dr.diesel]

The 731B uses the older Motorola made LTFLU reference correct?

[ManateeMafia]

I believe they only had the SZA263 at the time which they continued using in the 732A. Fluke changed to the LTFLU sometime during the 732B production.

[saturnin]

This is how I solved the issue with old batteries in my Fluke 731B. I bought twelve of these NiMH batteries:

https://www.lidl.de/de/tronic-ready-to-use-nimh-akkus/p231445 (1.2V, 2300 mAh, AA size)

and put them into two battery holders.

According the instruction manual, the original batteries should last at least 30 hours (450 mAh), so after the upgrade it should be about 150 hours (haven't tested it yet). Photos are attached below (for reference, I added picture of the original battery pack too).

[plesa]

Why not use spot welded accu pack instead of holder? Welded battery back is better for transport as less sensitive to oxidation/contact resistance.
Is you 731 reference with hermetic fluke wirewound resistors?

[saturnin]

I chose this solution because of price - it cost me only 13EUR (I bought everything locally, i.e. no shipping). A welded battery pack would cost 2x-3x more and have to be shipped. You are right contact oxidation/resistance might be a possible problem, but rather in distant future I guess. (I will use my standard in stable conditions and transport it only occasionally.) At that time batteries will be at the end of their life anyway, so I will renew the whole set again...

Yes, my standard has hermetic WW resistors. I believe all F731Bs use them. The previous version - F731A has classic resistors and little bit different circuitry.

[ManateeMafia]

I did the same with my F730A. It was easier to build 4 packs of batteries this way and it is still good after a couple of years in service.

[HighVoltage]

Ok, this 731B has several problems ....

The power supply has no real output.

When I use a Lab PSU as a supply, I can turn the reference on and it has a working output.
But ... all values are low significantly by mV.
I can adjust the 10V output with the pot from the front, but it takes like 8 turns or so to get to 10V
But even then, all other outputs stay low by several mV and can not be adjusted by their potentiometer.

Well, I will start to fix the power supply...

[VintageNut]

1983 SZA263. Well aged.

Slow response sounds like op amp feedback loop components.

My two 731B will change immediately when the pot is adjusted.

[ManateeMafia]

Dirty contacts on the switch could be causing problems. I would be tempted to connect the 10V line straight to the + output unless one of the other outputs is also needed.

[HighVoltage]

OK, the 731B is fixed.

Power Supply had one bad resistor and one electrolytic capacitor and a couple diodes.

A new pack of Ni-Cd's is installed. (found on ebay, Euro 10 each)
This came as 6 single cells in one pack = 7.2V so I installed 2 in series.

The 1V setting was a little low and could not be adjusted.
So it required a new precision potentiometer
All working now.

Next I will measure the stability.

[Theboel]

Hi,

Looking for Your stability test result, the price of 731B seem very reasonable for a "stupid man" 732X

[HighVoltage]

Like it usually goes, if I start with one instrument, they like to multiply.
Now I have 4 of the Fluke 731B

Today I replaced all the other Ni-Cd batteries and calibrated them all as close as possible to 10V on the same meter.

Now, each of them is hooked up to a 34401A and I am data logging their 10V output.
Data logging is performed over a Prologix GPIB to USB adapter and the RF-Scientific data logging software to a PC
Will see, how it looks tomorrow.

[VintageNut]

Have you tried comparing one to another using the lowest range of a DMM? You can then see them drifting apart or closer together.

Another thing to note is if they drift the same way every day.

In my office, the 731B will drift noticeably with ambient temperature.

[HighVoltage]

OK, here is the first real test of the four 731B

Stable temperature at 24.5 +/- 0.25 degree C
No air movement
All 4 pieces 731B have the same temperature
All 4 pieces 34401A have the same temperature

Only the 731B-1 was measured with a 34401A and 3458A in parallel.
This was to be expected:  The 731B are much more stable than the 34401A

I will do this test on each of the 731B.
So, three more to go...


 

[VintageNut]

Very good information. You will see change of the 731B over temperature. 

[HighVoltage]

One day later ...

Now the second 731B is hooked up to the 3458A
All other combinations are the same
The temperature was stable as yesterday

[Dr. Frank]

Hello HighVoltage,

would you mind publishing the rms noise / StD measurements on your 731Bs?

That looks like the performance of the LTZ1000 based references.

Frank

[HighVoltage]

Hello Frank,
I will do this test, when I am done with this series. (Day after tomorrow)


Ok, here is the comparison for the third day
All the same setup with stable temperatures again and no air movement in the room

Very interesting, that all the four 34401A meters needed a couple days to stabilize.
In comparison the 3458A had almost a straight line so far.

This means that also the third 731B seems to be very stable
 

[HighVoltage]

And here is the last of 4 of the 731B measurements.
Again, the 34401A drift much more, even after a few days being ON at the same temperature, than the 731B or the 3458A
 

[HighVoltage]

Here are the comparisons between all 4 731B, measured on the 3458A

From this chart it looks like all 4 of the 731B have a 24h stability of about 1/2 PPM or better.
This of course is at a stable temperature of less than 1/2 degree C change over this time of measurements.

[HighVoltage]

And here are the comparisons of the 4 different 34401A meters
Same setup, just different days

From the previous 3458A data we know, that all four 731B standards are stable within 1/2 PPM during the measurements.
So, the changes in these graphs are based on variations in each of the four 34401A

There is a good correlation for each 34401A from day to day
What surprises me the most is the differences between the four different 34401A meters
I would have expected each one to behave about the same.

I have definitely reached the limits of the 34401A meters with this little test.
 

[ManateeMafia]

It looks like you have yourself a nice collection of references going. Do you have any plans on monitoring these with a scanner and nanovoltmeter?
I have several 10v refs that get scanned daily and it helps to see the stability without spending a lot of time measuring them by hand.

I don't remember seeing you with a nanovoltmeter so maybe that is on your list of future teardown/repairs?

[HighVoltage]

It looks like you have yourself a nice collection of references going. Do you have any plans on monitoring these with a scanner and nanovoltmeter?
I have several 10v refs that get scanned daily and it helps to see the stability without spending a lot of time measuring them by hand.

I don't remember seeing you with a nanovoltmeter so maybe that is on your list of future teardown/repairs?

ManateeMafia
What nanovoltmeter and scanner are you using?

I have been looking for a broken Agilent 34420A for a long time but no luck so far and in working condition, they are just far too expensive.
Unfortunately, Keysight will stop selling the 34420A in Europe by the end of 2016 and most likely the price for a used one will even climb up.

Hmm, .... may be I should get a new one with a good discount, if they have any left in the last week of December.
I would be very much interested to get to know your scanning setup

[ManateeMafia]

I am using a Data Proof 160A and a 34420A that I got from eBay. The Keithley 2182(A) is also a good meter as is the Keithley 182.

The software is Windows based and runs on a nano-itx headless PC which scans and saves the results to a Postgresql database. A lot more work is needed to generate plots of the results. There will be a separate program to act as a web server generating plots and reports for a remote connection . A BME280 + RPi is used as a Fluke 1620A and is queried over the network for the environmental data.

This has turned into a much bigger project than I planned.

[plesa]

I have been looking for a broken Agilent 34420A for a long time but no luck so far and in working condition, they are just far too expensive.
Unfortunately, Keysight will stop selling the 34420A in Europe by the end of 2016 and most likely the price for a used one will even climb up.

What about this one?
http://www.ebay.com/itm/HP-34420A-NanoVolt-MicroOhm-Meter-/112213078051?
BTW nice collection of references. When hooked 3458A to references I was impressed like you by it stability and noise level.

[Theboel]

Hi High Voltage and Every one,

When We measure the Fluke 731B against 34401 we can say it like compare LM399 with SZA263 off course with some uncertainty and when we compare 731B with 3458A are like to compare SZA263 with LTZ1000 again with some uncertainty.

When I see the high voltage report I believe I can say SZA263 are better than LM399 and comparable with LTZ1000, are my conclusion is wrong ?
 

[HighVoltage]

I agree so far that the SZA263 is about as stable as the LTZ1000.

But keep in mind, that the LTZ1000 is heated and the SZA263 is not.
I think when I start to take measurements over temperature, we will see much larger differences between the two.
 

[Theboel]

I agree so far that the SZA263 is about as stable as the LTZ1000.

But keep in mind, that the LTZ1000 is heated and the SZA263 is not.
I think when I start to take measurements over temperature, we will see much larger differences between the two.

My conclusion based from the same ambient temperature for 731B and 3458A but who know where is the sweet spot of SZA263 and according to story about motorola stop to made SZA263 because lack of demand I think its very hard to prove which one is really "the king" except some day a "hardcore volt nut" do face to face experiment 
for now I declare I want 731B for my birthday present     

[HighVoltage]

Well, they are not too expensive and available, so your wish should not be so hard to realize.
I was also looking for a 732B but that will probably stay a wish for a long long time, I think.

You might be right in the comparison.
I will continue my measurements for sure.