Fluke 8840A Faulty CPU

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Here’s a little experiment: connect two meters (Fluke 8842’s) to a GPIB network and feed them with a reliable 10VDC reference. Just for comparison, feed the same reference to a ’new’ Keithley 6500 meter set-up to record DCV at NPLC = 15 continuously. Then trigger the two 8842’s using PyVISA over GPIB with the ‘group execute’ command: 

 intf.group_execute_trigger(DMM8842a, DMM8842b)

Capture 10,000 observations at roughly 1-second intervals for the 8842’s and for the same time period for the 6500 (not exactly the same periodicity) and have a look at the resultant data.

The average (mean) value for measured DCV was calculated for each meter as well as for the average of each observation for the two Fluke meters. The standard deviation of the data (sigma) was also calculated as a measure of the consistency of the measurements.

As shown in the attached table, Fluke meter ‘A’ sigma was about 50 micro-volts, meter ‘B’ 40 micro-volts and the average of each observation, about 32 micro-volts. By comparison, the Keithley 6500 measurements had a sigma of 3 micro-volts.

While this is by no way an exhaustive test of the Fluke 8842, it is strong evidence of the value in a properly maintained ‘vintage’ voltmeter in comparison to a state of the art meter with a ‘one-digit advantage’ in terms of advertised precision.

From a user’s POV, the Fluke —due to its specification of voltage ranges—is capable of ‘digit equivalency’ with the 6500 in many lower voltage measurements, particularly in the under 2VDC area. And if the reference is in a double-wall, metal case with no vents or fan, how much does the lack of an on-chip heater really matter?

[Kleinstein]

The limited resolution of the 884x may already contribute to the noise.  If random enough the quatization noise from 5.5 digits (and thus 100 µV steps) would be 29 µV RMS.  With a fixed input and little other noise, the quatization noise can be higher or lower. If the meters use digital cal factors there could be additional internal rounding errors.

One might get slightly lower noise when using the medium speed (20 SPS) mode and averaging on the PC side.

[bdunham7]

Capture 10,000 observations at roughly 1-second intervals for the 8842’s and for the same time period for the 6500 (not exactly the same periodicity) and have a look at the resultant data.

The average (mean) value for measured DCV was calculated for each meter as well as for the average of each observation for the two Fluke meters. The standard deviation of the data (sigma) was also calculated as a measure of the consistency of the measurements.

As shown in the attached table, Fluke meter ‘A’ sigma was about 50 micro-volts, meter ‘B’ 40 micro-volts and the average of each observation, about 32 micro-volts. By comparison, the Keithley 6500 measurements had a sigma of 3 micro-volts.

How many digits do the readings have over GPIB?  If there are no more than displayed, then you will have what I commonly observe with these which is that at on a very stable reference with a fully warmed up meter, you either get a flickering reading that varies by one count or a steady reading with no variation.  This could be caused by a low noise level that can result in a one-digit variance when the voltage is right at the threshold between two possible values while resulting in no variation if the voltage source is far enough from a threshold. 

It might be interesting to do the same experiment with either a 1.5V or 15V reference so that both the 8842A and the DMM6500 are displaying the same number of digits.

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The underlying purpose of the experiment--other than to just check out the 8842A meters--was to see if averaging two meters' simultaneous readings would reduce the effect of noise. It did a bit, but nothing to brag about. The published spec for the meter is 0.0030% of reading + 2 counts which is a bit over 300 micro-volts (if I did the math right). As @bdunham7 pointed out, the ticking of the 6th digit is consistent with the sigmas that I reported. Getting +/-50 microV out of a thirty-year-old meter with a 90-day spec 6X that is a testament to the quality of the design and construction and probably a bit to survivor-bias as the duds were junked years ago.

The test was done with my best meter (HP3458) and the current best reference at 10VDC (sigma ~1.7microV). The Fluke range is 2V and 20V and the 6500 is 10V and 100V, so how about 1.999V? The 2V range doesn't much above 2V and the 100V range in most modern meters is not their best spec.

[Kleinstein]

The 100 V range on modern high resolution meters is usually with a 1:100 divider and than measuring in the 1 V range. For a 5 digit or lower end 6 digit meter this is not a problem. However with the better 6 digit meters the noise of the divider alone (100 K at the lower leg) gives noise comparable to slightly higher than a good meter in the 1 V range. So the 100 V range is a bit limited and there is kind of a natural cap on how good it can be.

It can help quite a bit to have a 20 V or similar range and does not switch to the 100/200 V range so early.

The noise usually does not get much worse with age. It is only the available parts and expectations that have changed over the years. Aging may cause drift in the scale factor, especially with the high voltage and ohms ranges. The prime range (20 V for the 8840) is usually quite stable over long time.

[bdunham7]

The test was done with my best meter (HP3458) and the current best reference at 10VDC (sigma ~1.7microV). The Fluke range is 2V and 20V and the 6500 is 10V and 100V, so how about 1.999V? The 2V range doesn't much above 2V and the 100V range in most modern meters is not their best spec.

The 2V range on the 8842A is the native ADC range and thus the most accurate, so go for it.  1.9V is a calibration point, IIRC, so it should be at its best right there if you have a stable source. 

I once tested an 8842A vs my 8846A at 15.0000 volts and found them to be indistinguishable in performance at that point, although I didn't do any kind of statistical analysis.  And the 8842A spec is actually better at that point--35ppm + 2 counts  vs 38ppm + 6 counts, plus high impedance.  If my math is right, the 8842A would also have a lower spec uncertainty at 1.90000 volts-- 8 counts vs. 10. 

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The display of my used (are there any other kind?) Fluke 884X meters range from barely visible to good enough for most uses. A recent power outage in my area led me to uncover a way to improve the visibility without any effort at all.
 
Here's what happened. I was testing an eBay Fluke 8842 with a dim display to see if it was anywhere close to being usable when the power went out. The outage lasted a couple of hours and when i was putting my lab back on line, I noticed that the mains voltage was like 100V not the usual 120V. I also found that the 8842 display was blank--nothing lit at all--however, the relays were clicking as expected when changing functions. Wierd right? After a bit, the mains voltage was back to normal and the Fluke display was sorta visible again. This led to a review of the schematics, a browse of the eevblog forum and a leap of faith for the little meter. I unplugged the meter and used an isolation transformer to see what would happen if I set the ACV switch to Japan 100V. At 100V the display was dim but as I raised the voltage to 120 the display brightened considerably. No smoke, no excess heat, just brighter. Next I put the meter back on mains and observed it closely for a while. No problems, so I tried the "100V" setting on another dim 8842 with the same positive result.

I figure the 20% over-voltage in some way compensates for the lower output from ancient voltage regulator chips in the power supply. There does not appear to be any difference in calibration of volts or amps functions. Worst case, I might have to replace the voltage regulators sometime in the future, but since the display is unobtainable, I'll make do with what I've got.

[bdunham7]

You are overdriving the VFD filament which is driven directly by the transformer.  The voltage regulators are LM78xx type and will simply run warmer (not all that good) and if you go high enough, there's a crowbar protection on the 5V supply that will blow the mains fuse.  The supply voltages on these are very stable--there's a 6800uF filter cap on the 5V line.

I'm working on an LED replacement, so don't burn your meters out in the meantime!  The mains transformer is truly unobtainium and pretty tough to replace even if you had one.

https://www.eevblog.com/forum/testgear/fluke-8840aaf-8842a-vfd-display-replacement/msg3557023/#msg3557023

I intend to get back on this project in a few weeks.

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You are overdriving the VFD filament which is driven directly by the transformer.  The voltage regulators are LM78xx type and will simply run warmer (not all that good) and if you go high enough, there's a crowbar protection on the 5V supply that will blow the mains fuse.  The supply voltages on these are very stable--there's a 6800uF filter cap on the 5V line.

Correct. The filament will be seeing a higher voltage, however the resistance of filaments (generally) will increase as voltage increases, so the current flow should not get to burnout level. Plus, all my meters are on UPS transformer/battery boxes that keep the supply under 115V and capture surges and spikes. Yes, I'm skating on thin ice and I'm certainly not suggesting that anyone 'over-clock' their meter without considering the consequences. As for the heat generated by the LM78XX regulators, that would depend more on the current flow than the input voltage, so unless there is some reason to to expect the measurement side of the meter to work harder because of the input voltage to the transformer, the heat doesn't seem to be a problem. In any case, the max input for the 7800 series is like 35V and that is not likely as configured.

BTW, I've read the posts on pulsing VFD's to wake up the phosphors or revive the filament; most don't end well. My post was more of a "what's up with this" than a suggested cure.

The real solution is your LCD panel concept to replace the VFD altogether. And, while you are at it, might I suggest an LCD for the HP3478? There are many for sale on eBay--some might need a battery replacement--and an LCD (or even a 7-segment) would make the HP3478 a more useful alternative.

[Kleinstein]

A higher voltage can make quite some differene, as the voltage drop is often only some 30% of the voltage. So a 10% increase in the input voltage would make this a 30% increase in the heat.

Ideally there is a seires resistor that could be reduced to increase the filament voltage a little, if really needed, espeically if only for a temporary higher intensity.

The HP3478 has a LCD to start with, but often with poor contrast and without backlight. While it was not so good when new, there is relatively little aging with the LCDs.

[bdunham7]

Correct. The filament will be seeing a higher voltage, however the resistance of filaments (generally) will increase as voltage increases, so the current flow should not get to burnout level. Plus, all my meters are on UPS transformer/battery boxes that keep the supply under 115V and capture surges and spikes. Yes, I'm skating on thin ice and I'm certainly not suggesting that anyone 'over-clock' their meter without considering the consequences. As for the heat generated by the LM78XX regulators, that would depend more on the current flow than the input voltage, so unless there is some reason to to expect the measurement side of the meter to work harder because of the input voltage to the transformer, the heat doesn't seem to be a problem. In any case, the max input for the 7800 series is like 35V and that is not likely as configured.

BTW, I've read the posts on pulsing VFD's to wake up the phosphors or revive the filament; most don't end well. My post was more of a "what's up with this" than a suggested cure.

Trust me on these observations--I've gone over the 8840/42s pretty thoroughly when testing my LED display, and trying to revive VFDs before that.  You can turn up the voltage enough to make the VFD filaments visibly glow red, which they normally don't--so they definitely are getting hotter.  And I can assure you that the VFD filament voltage is the only one that is budging when you do this.  I can also tell you that reducing the VFD segment/grid drive voltages from 30 to 24 volts, which happened when I tried to run the LEDs in parallel with the original drivers, has almost no effect on the brightness.  I never burned any VFDs out since the you can only boost the line voltage so much before the power supply crowbar kicks in.  As for the regulators the heat produced is the product of the current and the voltage drop across them.  It's not what they're rated for that counts, it's what they're heat sinked for in their actual implementation.  The regulators should actually just shut down rather than roast, so I'd worry about the transformer more, or if the filter caps are weak they might decide to retire.  I found that while you can improve the VFD a bit with an overnight burn at a 15-18% overvoltage, the result is very uneven and doesn't last. 

The real solution is your LCD panel concept to replace the VFD altogether. And, while you are at it, might I suggest an LCD for the HP3478? There are many for sale on eBay--some might need a battery replacement--and an LCD (or even a 7-segment) would make the HP3478 a more useful alternative.

Of course you mean LED.  I have an HP 3478 but I didn't know that they had display failure issues?  Or do you just want something backlit and more visible?  I'm not sure how much demand there would be for that model.  It's basically a $100 meter and I don't think people will buy a better display for it unless theirs quits. Maybe I'll have a look once I'm done with this one.  Proof of concept is done, now it's just details.  And finding the parts that everyone keeps running out of!

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Yes, LED. My mistake, different problem; different solution.

As for the display brightness, I was just reporting what I saw (and a bit of "let's see what this does"); not as part of an attempt to find a way to brighten the display. I observed the display 'off' after the return of power; dim on the 120V setting; and brighter on the 100V setting. I did the test several times, just to verify. So I left that meter on the 100V setting and it is fully functional as far as measurement and display.

You have far more experience with these meters than I, so I trust the observations I have no plans to fool around with the voltage on the display. I don't remember exactly, but my usual practice when working on meters is to test for hot spots, particularly on voltage regulators, either with a finger on a recently unplugged unit or with an infrared thermometer on a live one if there might be an issue. There was probably a finger-check in this case. I just now checked the two 8842's running the 1.999V test @bdunham7 suggested and the outside case temperature is 37C for the meter under review and 41C for the other (the 37C is on top of the 41C, no surprise there) and no hot-spots over the transformer area. Yes, it would be a sad day if the transformer got smoked; not as sad as scrapping a good meter because of a dim display, however.

I would be happy to double-check any of the voltages for interested readers as well as verify the temperature of the heat sinks and transformer.

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The problem with the HP3478 is twofold: the calibration is maintained by a lithium battery and the display is useless in dim light. The Fluke 88xx series has neither of these problems. Both are 'closed-case' adjustable for calibration purposes. In meters 'of a certain age,' specifications matter less than actual performance. Frankly, I don't know whether the Fluke is a better meter, although the voltage reference in the 8842 seems to be held in higher regard.

How about a voltage-measure face-off? I have a GPIB network and 2 each of HP3478 and Fluke 8842. And a Python test script to trigger simultaneous readings on GPIB. What should the parameters be? Hmmm . . .

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The test was done with my best meter (HP3458) and the current best reference at 10VDC (sigma ~1.7microV). The Fluke range is 2V and 20V and the 6500 is 10V and 100V, so how about 1.999V? The 2V range doesn't much above 2V and the 100V range in most modern meters is not their best spec.

The 2V range on the 8842A is the native ADC range and thus the most accurate, so go for it.  1.9V is a calibration point, IIRC, so it should be at its best right there if you have a stable source. 

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

[bdunham7]

The 8842A is clearly superior for the vast majority of users as long as it isn't broken and has an in-cal AC Option 9 board installed.  The 3478A is limited to 300 volts (something I didn't know until it was pointed out by another member to my surprise) and the high-impedance only goes to 3 volts.  It does have some slight advantages elsewhere (AC bandwidth and 30M ohms instead of 20) but those don't make up for the shortcomings in practical use, IMO.  Measuring the filter cap voltage of a PFC SMPS may blow it up.

The calibration battery is a one-time issue and you need a recalibration if you aren't careful.  The references aren't enough of an issue to worry about for most uses, the LM199 in the HP is pretty decent, although that version is probably not as stable as the LTFLU-1 long term. I think the Fluke will beat the HP in accuracy tests, but I'm not sure most users should care.  The display does suck, but for what these sell for, it's still a good cheap meter if you can live with the limitations. 

[bdunham7]

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

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.

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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

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?

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 . . .

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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?

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

[giosif]

Thanks for your input!
I too suspect some timing type of problem, but I don't really have proof for it.

Also, you mention v4.0 for 8840A. Did you really mean v4.0?
I am asking since the highest version I've ever seen/heard of for the 8840A is v2.5.
Could you share that FW v4.0 with us, please?

Thanks!

[Kjo]

@giosif
The only specs on the UB8840M are in mostly non-English. But to my recollection when I designed the clone the UB8840M only addresses 4K on the buffered external ROM bus. But there are a few things about the Fluke 884x firmware that I have never quite understood.
In most meters there are 2 4K ROMs. The low 4K is embedded in the MCU and the hi 4K is external. The external ROM is enabled by P22 & DM from the MCU. So the low 4K in the MCU has to use port mapping signals to read the hi 4K. But there are a few late 90’s meters that have OTP or windowed Z86E21 8K EPROM MCUs where there is no device in the U222 socket. I have 3 such meters. Two of the 3 had the Z86E21 code protect bit set. Ha Ha Ha the third I could read out the firmware as an 8K block. I split this ROM in to 2 ROMs at the 4K boundary. After writing the 2-4K images into 2732A EPROMs, I put the low in a Z8613 piggyback and the hi into U222. The 8840A booted and passed all tests exactly as the original Z86E21 did.
So there is something about the operation of the Z86E21 I am unclear on. The 4.0 firmware clearly works as 2 ROMs but also as contiguous 8K memory. These Z86E21 are STM parts, and while I can read out the ROM, my programmer will not program them. So I have to send them out to get programmed.

Back to your request. Rather than dumping the ROMs here, I put all the ROM data for all the meters I have collected on my website. Anyone can grab what ROMs are there for what ever use. Not all ROM versions have code for the U202 MCU. And remember that all 3 meters have unique, non-interchangeable firmware.
HTTPS://www.Hollywoodcontrols.com/phpFluke/HC8840A_roms.php

If anyone has ROM data that I don’t have please let me know.

Kjo KO3Y

[giosif]

Thank you, Sir!

I was able to download the v4.0 firmware.
I burned the 2 x 4K files and tried them with the UB8840M adapter and the meter works fine now. 
Strange that the exact same adapter did not work with v2.3 of the firmware, but it works with v4.0.
In any case I am happy. 

On a side note, I happen to have a Z86E21, which I burned with the 1 x 8K version of the v4.0 firmware and I confirm this one works as well (I didn't install an EPROM IC at U222, as that wouldn't be needed anymore).

[Kjo]

@giosif
Glad to worked 4 you.  I have no idea what Fluke changed in the 8840A V4.0 fw.
But it works fine. What did u use to program the Z86E21 device?
Kjo - KO3Y

[giosif]

I used a less known programmer, Micromaster LV, made by a company called Ice Technology.