Electronics > Repair
Fluke 8840A Faulty CPU
giosif:
Assuming you are referring to the FW on the adapter board (i.e. U202), yes, I am certain I burned it correctly.
That's assuming the U202.bin file from the Ko4bb web site is good in the first place (just to check, the MD5 hash of the file I have is 79cebebdf156a9b1219a6c9f26da09e0).
Just to be on the safe side, I burned the FW once again, this time on a different 2732 IC.
I also bought an AT28C64B, burned it and tried with that.
In fact, I also burned the U222 FW again (on another 2732 IC).
I tried with these new ICs in their corresponding places, but the result is exactly the same.
Another thing I did was to order another batch of UB8840M, this time from a different seller.
I put in one of these newly arrived ICs but, as expected, the behaviour is the same.
In terms of shorts, I very much doubt it as I did both a visual inspection and continuity checks.
Also, I now have 3 boards that behave the same way, so it's unlikely I shorted them the same way.
To summarize:
1. The rest of the 8840A meter is working fine (confirmed with using an original Fluke CPU IC & associated U222 IC - true, those are FW 2.5, but I don't think that matters).
2. U222 IC (i.e. 2732 burned with corresponding U222 FW 2.3) is fine.
3. On the CPU adapter board:
3.1. UB8840M IC is good.
3.2. 2732 IC with U202 firmware is good.
3.3. I checked the adapter board connections are according to the schematics and it all checked out.
The only remaining parts which are left are really the 3 capacitors on the adapter board (I am using 1 x 10 uF electrolytic, 1 x 100 nF film and 1 x ceramic cap whose value I don't remember now, this last one on the 2732 IC power rail), but I doubt it they have any influence in this.
Any other suggestions, please?
cheapskate:
I briefly checked the circuit I uploaded against the boards I got manufactured and I don't see any differences in wiring. The only change between them is the cutout on the left side for access to test points. Here are the UB8840 datenblätter I used when designing the board.
https://hjs.lima-city.de/DDR/
http://www.blunk-electronic.de/datasheet/GDR/datenblaetter_ddr.pdf
View[+]Finder:
--- Quote from: essele on September 02, 2020, 01:44:05 pm ---Hello ... I thought I'd summed it up nicely...
"I'm going to get my three 8840A's up and running and then I never want to see another one ;-)"
I got them up and running and then sold them.
... and I've resisted the temptation a few times when some 8840A's and 8842A's came up on eBay .. I'm working on some other things at the moment and don't plan on doing anything further with this, ever, even if my life depended on it. ;-)
Lee.
--- End quote ---
A disadvantage of becoming a 'recognized expert' in any domain is that one can never retire . . .
So, this question is for anyone other than Lee: is there a possibility of extracting hidden digits from the ADC? The quality of the ADC and voltage reference in the Fluke 8842a show stability nearly equal to 6.5 digit meters like the Keithley 6500 and Keysight 34465a in tests I have done. (Depending on the choice of DCV input of course.)
bdunham7:
--- Quote from: View[+]Finder on May 18, 2021, 07:07:52 pm ---So, this question is for anyone other than Lee: is there a possibility of extracting hidden digits from the ADC? The quality of the ADC and voltage reference in the Fluke 8842a show stability nearly equal to 6.5 digit meters like the Keithley 6500 and Keysight 34465a in tests I have done. (Depending on the choice of DCV input of course.)
--- End quote ---
Probably not, at least not directly. The Recirculating Remainder method is different than dual or multislope integrators which often do generate extra digits internally since they measure time. If I understand it correctly, the ADC on the 8840A/8842A models generates the equivalent of either 19 or 20 bits, which is about 500K or 1M counts, and then displays 200K counts--so there's not enough for another digit. An indirect method would be to use an averaging algorithm on the readings and synthesize another digit--the older 850x monster meters do exactly that to generate either a 'CAL' digit (used only for adjustment) or an extra digit in the 10 volt range (8505B/8506B).
The 8842A does have remarkable stability, especially long term because it relies on aged and characterized references and resistors rather than an oven. However I think short term stability, tempco and noise might not be quite good enough to merit another digit. Of course there are plenty of actual 6.5 digit DMMs that probably don't merit their last digit either.
Kleinstein:
The 8840/8842 still use the slightly odd reciculating reminder ADC. In theory with 5 passed and 4 bits each + maybe 2 bit extra from the last pass, there my be just 6 digit resolution. The slower conversions use the average over many such conversions - so in theory they may be a 6th digit, maybe even more. However it is not clear if the linearity and noise is OK all the way to the end - DNL may be limited by the adjustment of the ADC.
I would not consider the ADC especially good - it has some merits at high speed or with sampled readings, but not that great with slow measurements and mains hum suppresion can also be tricky.
With the computer one may use medium speed and get one more digit by averaging at the computer (e.g some readings).
The reference is very good - noise wise better than the usual LM399 found in 6 digit meter, though without a oven stabilization and not as good with the TC.
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