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Fluke 8840A/8842A and AF Differences?

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Original:
Is it possible to convert the rear inputs of an 8840A/AF to normal?

J-R:

--- Quote from: Original on June 25, 2023, 08:42:00 pm ---Is it possible to convert the rear inputs of an 8840A/AF to normal?

--- End quote ---
The 8840A/AF has other hardware changes and a different firmware, so I don't see this being a realistically possible mod.  Also, you'd end up with a hacked up instrument with multiple new limitations.

bdunham7:

--- Quote from: J-R on June 25, 2023, 12:06:34 am ---The 8840A/AF has:
- "expanded environmental envelope"
- meets some MIL spec
- true RMS is standard
- 1000V RMS AC input (vs. 700V)
- enhanced EMI/RFI protection
- DC voltage ratio

I don't see that the GPIB board is a standard option on the 8840A/AF, both from the catalog and from image searching.

--- End quote ---

It appears you are right about the GPIB as the 8840A/AF has a spot on the back to indicate the presence or absence of the option.  I don't remember where I got that they all had this, but perhaps the government contract included them and only the few retail versions actually appeared without.  Here's one in real life:  https://www.ebay.com/itm/175756230145

The 1000VAC statement is interesting, but the one spec sheet I have says otherwise.  A 1000VAC range would imply that there is a third version of the AC board.  I don't have an 8840A/AF in hand to look at or test, but if anyone does and is willing to give it 800VAC to see whether the display flashes I'd be curious.

https://nscainc.com/wp-content/uploads/pdf/F_8840A-AF.pdf


--- Quote ---Tinkering a bit back and forth between the 8840A and 8842A, it's a tough sell to convince me the 8842A is really that much better.

--- End quote ---

It is an incremental improvement to be sure, but needing offset to zero out a range with 100nV resolution isn't at all unexpected.  That is the equivalent of the last digit on a typical 6.5-digit DMM.  And I've found that out of about a dozen examples, the 8842A long-term calibration stability is noticeably better.  But I'll agree that any of these in good condition is an excellent choice for a budget bench meter.

J-R:
I believe the 8840A/AF 1000V AC input is correct.
First, the labels on the 8840A and 8842A list both 1000V DC and 700V AC, but on the 8840A/AF it only states 1000V MAX and does not split out AC/DC ratings.
Second, if you look at the 1990 Fluke catalog, under the 8840A specifications section it states: "Maximum Input: 700V rms, 1000V peak or..." but under the 8840A/AF it states: "Maximum Input: 1000V rms or...".

Given the age of these units, maybe they have stabilized enough that there isn't as much of a difference between the 8840A and 8842A that there may have been in the past.  But really to clarify my point on which to select, I'm just saying I'd rather have a known-good unit with a fresh calibration and data rather than blindly choose one model over another.  Also, the speed comment is because one of the main advantages of the VFD and this model is the fast update rates, but the low range is achingly slow in S mode.

bdunham7:

--- Quote from: J-R on June 26, 2023, 03:02:55 am ---I believe the 8840A/AF 1000V AC input is correct.
First, the labels on the 8840A and 8842A list both 1000V DC and 700V AC, but on the 8840A/AF it only states 1000V MAX and does not split out AC/DC ratings.
Second, if you look at the 1990 Fluke catalog, under the 8840A specifications section it states: "Maximum Input: 700V rms, 1000V peak or..." but under the 8840A/AF it states: "Maximum Input: 1000V rms or...".

--- End quote ---

I agree it looks like there is some difference there, but the specifications also show the highest AC range to be 700VAC.  So I'd be curious to know if it will measure all the way to 1000VAC without flashing or if it is simply rated to withstand that much voltage without blowing up.  AFAIK there isn't an applicable schematic available to resolve the issue, so perhaps some day I'll get the opportunity to dissect one and see for myself what is in there.


--- Quote ---I'm just saying I'd rather have a known-good unit with a fresh calibration and data rather than blindly choose one model over another. 

--- End quote ---

Hmmm, I sort of think that the whole point of getting these meters is that you can rely on them to be accurate without recent calibration, provided that they aren't broken.  Calibration sounds nice, but it costs money and cheap calibrations don't prove much.  I've yet to see one that is out of spec without having an obvious major malfunction, and in every case where I've repaired them they go back to being very accurate.  The only way I know of to get one to be slightly inaccurate is to swap in a different AC board.   

Of course, my opinions date back to when you could scrounge up a fairly decent 8842A with AC and GPIB for under $100.  With a handle.

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