Electronics > Repair

Fluke 8840A Faulty CPU

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bdunham7:

--- Quote from: View[+]Finder on June 03, 2021, 01:58:29 am ---Attached are results of a test with both Fluke 8842's at 1.9999V from an HP3245 precision source. The Keithley 6500 meter was pretty much spot on for the 10,000 observations: 1.99989765V

--- End quote ---

So which meter, A or B, is the one set to 100V mains?  They both appear to be in spec, I'd just be curious if anything changed at all if you changed the mains voltage.

Also, can you do a run at 15 or 19 volts?  I know it isn't the Keithley's best range, but since I claim that the 20V range on the Fluke is a plus, I'd just like to see how they actually compare.  I assume your precision source is compared to the 3458?

Kleinstein:
From the input secion of the Fluke 8842/8840 the best range is the 2 V range. The 20 V range uses an additional (though likely good quality) divider behind a buffer.
The 3478 is a bit odd with the 3 V range as it's best range. As a postive side it has a 30 mV range and can thus resolve down to 0.1 µV, which can be a plus.
With only 5.5 digits the reference (LM199 vs. unheated LTFLU) is not so much the limiting factor. The Fluke is better with noise, the Lm199 is lower TC - but neihter is visible with these meters. Both have a pretty good reference but limited resolution ADCs.

View[+]Finder:

--- Quote from: bdunham7 on June 03, 2021, 02:47:43 am ---
--- Quote from: View[+]Finder on June 03, 2021, 01:58:29 am ---Attached are results of a test with both Fluke 8842's at 1.9999V from an HP3245 precision source. The Keithley 6500 meter was pretty much spot on for the 10,000 observations: 1.99989765V

--- End quote ---

So which meter, A or B, is the one set to 100V mains?  They both appear to be in spec, I'd just be curious if anything changed at all if you changed the mains voltage.

Also, can you do a run at 15 or 19 volts?  I know it isn't the Keithley's best range, but since I claim that the 20V range on the Fluke is a plus, I'd just like to see how they actually compare.  I assume your precision source is compared to the 3458?

--- End quote ---

The "A" meter has the input voltage set to 100V. Nothing appeared to change when the input mains setting was changed and the meter restarted. I only tested volts and low current amps.

The precision source is an HP3245 that is calibrated (volts and amps) by a Python script with my HP3458. That makes 10V (for example) on the 3245 source show as 10V on the 3458, and all across the full range of the 3245. The 3458, in turn, is auto-calibrated on DCV daily or as needed if there has been a change in ambient conditions (I've taken steps to eliminate that effect). Finally, the 3458 is DCV calibrated monthly (with adjustment) against a 10VDC reference that runs 24/7 in a "Thermal Stability Chamber" with temperature maintained at 25.25C by a Thor2000 TEC controller. Also an ohms calibration (with adjustment) against a 10kOhm reference. It is the best I can do with what I have to work with and I believe good enough to verify 6.5 digit used gear.

The 3245 that I have does not have the HV option, so the max DCV is about 10.1V. What I have used for higher voltage reference is a combination of a LiPo battery and the best voltage source in my lab, the Keithley 230 to maintain the LiPo. The test of the Fluke 8842 would be close to 19VDC and would be monitored by the HP3458 as well. Maybe later today for overnight?

UPDATE: Results of short run (1000 obs) with longer 10,000 observations run tonight perhaps to include the 3458 . . .

View[+]Finder:

--- Quote from: bdunham7 on June 03, 2021, 02:47:43 am ---Also, can you do a run at 15 or 19 volts?  I know it isn't the Keithley's best range, but since I claim that the 20V range on the Fluke is a plus, I'd just like to see how they actually compare.  I assume your precision source is compared to the 3458?

--- End quote ---
Voltage source was a Keithley 230 set at 19.99VDC with an 8400 micro-farad capacitor in parallel. DUT's were two Fluke 8842 meters (A & B) independently connected to the source. The source was also monitored by an HP3458 and a Keithley 6500. Ambient temperature was 25C +/-0.2C. The test comprised 10,000 observations at 2-second intervals using GPIB group command. Mean (average) and sigma (variation) were calculated from the raw data for each instrument as shown in the attached table.

Some reflection on the results: The 100V range is not where the HP3458a earned its reputation, no surprise finding lower precision than on its 10V range. The Fluke 8842 performs well on its 20V range, probably as good as it does on the 2V range. As a general-purpose precision bench meter, the Keithley 6500 excels at data collection and the ability for the user to display, analyze and export data easily over a browser interface. Far better than the Keysight competitor in that area.

UPDATE: Another run with a source of 12VDC. I was trying to stay in the 10V range of the HP3458.

Kjo:
I am always amazed how islands of projects can co-exist with little crossover. I just found this thread after several years of working on 884X projects through the Groups.io forum for Fluke hardware. I have more than 20 of these meters and some with Z86 issues, 700013 issues and bad displays. Several years ago I too found the  UB8840 clone John.ccac built for a disk drive. That had me motivated to design such a clone for the 884X.

The posts in this thread are not absolutely clear on the ROM versions used. But it should be clear that 8840A, 8840AF & 8842A all use different ROM versions that are not cross compatible. My UB8840 clone worked with all 8842A ROMs, but not all 8840A ROMs. In fact, only the 8840A V4.0 ROMs worked flawlessly in the 8840A meters. I have a suspicion, but not confirmation, that there is a timing difference between the Z86 and UB8840 such that data transfer to the display controller is incorrect. There is likely timing differences in older ROMs that are improved in the V4.0 version.


As mentioned elsewhere, if you can find a Z86E21AF microcontroller with a glass window, it can be programmed with the complete 8K ROM, though I have only verified this with the 8840A V4.0 ROMs.

Because the 700013 quad analog switch is the most likely failure point in these Fluke meters, I have, over the last 2 years, build 5 different clones in an attempt to duplicate this unobtainium part. I began with a Microchip PIC and DG212B. This processor has 4 programmable logic blocks independent of the program counter. It worked, sometimes, in some locations. Unfortunately, some obscure timing or noise issue kept it from working universally. But it would have been a easy 2-chip solution.


I made 2 other versions using 74HC & 4000 series with single chip inverters and a DG212B. These also worked, sometimes. And were a bear to assemble.
The 5th version used a PLD and a DG212B. It is also a 2-chip version, much easier to assemble. This version was condensed onto a PCB about 30% bigger than the original IC. It works flawlessly in U301, U302 & U303. Should also work in U40X positions.

I have made this available as a kit or fully assembled in limited quantity. (I need to recover some of the expenses incurred)
https://www.hollywoodcontrols.com/phpFluke/HC700013P.php

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