Fluke DMM Hack Adds One Digit To Model Number

Among his many interests, [Dave Jones] likes test and measurement equipment. He recently posted a few videos on his EEVblog exploring the reasons why Fluke voltmeters are so expensive. In the process, he stumbled upon an interesting hack for the Fluke 77.

The Fluke 77 was introduced in 1983, and is an average responding meter in the AC modes. This model has become a de-facto standard for use in maintenance depots and labs for equipment which has very long lifespans — think military and industrial gear, for example. Many test procedures and training materials have been designed around the use of the the Fluke 77. The cost to change them when a new and better meter comes along is usually so prohibitive they might as well be cast in stone — or at least hammered into 20 pound fanfold paper by a WordStar-driven daisy-wheel printer. But for those unburdened by such legacy requirements, Fluke has the 17x series of True RMS reading meters from since the beginning of this century. These meters bear a strong visual resemblance to their siblings in the 7x family and are substantially interchangeable but for their AC measurement methods.

In the process of tearing down a Fluke 77 meter for another reason entirely, [Dave] finds that the resemblance it not just visual. Not only is the 77’s PCB labeled 17x, but there is an Analog Devices AD737 True RMS to DC converter IC on the board.

What gives? Why would they go to the expense to put this chip in an average responding meter?

A bit more digging into the data sheet reveals this chip can measure in three different modes: true RMS, average rectified, or absolute values. Selecting true RMS mode requires adding just a single 33 uF capacitor. Sure enough, in the Fluke schematics this is clearly shown as an optional load.

Not surprisingly, [Dave] does the modification and it works. The only downside is that after you make this change, you have to perform a complete recalibration of the meter. Annoyingly, there doesn’t appear to be any way to just calibrate the AC measurements, for example. Unless you’re a recovering Test Equipment addict like [Dave], you probably don’t have all the gear nearby to perform this calibration yourself — something to keep in mind if you try this at home. Another thing to note is that this hack was done on a modern Fluke 77 IV meter. If you have a Fluke 77 from 1985, this hack doesn’t apply.

27 thoughts on “Fluke DMM Hack Adds One Digit To Model Number

    1. I wouldn’t say this is crippleware. There’s a need for the 77, and this is the cheapest way for them to make it and have it match the specs. When used by military etc. you can’t just discontinue a model, or update it willy nilly. I think this is very smart of them to be honest.

      1. “Another thing to note is that this hack was done on a modern Fluke 77 IV meter. If you have a Fluke 77 from 1985, this hack doesn’t apply”.

        So it sounds to me like they *have* just updated it willy nilly – the internal hardware is different from the original 77. Presumably the new hardware had to meet lots of electrical specifications and go through testing, but it is still different.

        So the issue for Fluke/military etc is not changing the electronics, it is changing the user controls and just the look of the thing for documentation, operation manuals, procedures etc. I can see getting all paper documentation in the field worldwide updated reliably would be a nightmare, quite apart from the added complexity of “if you have a 77 to hand do this but if you have a 77b do that” all over the place.

        1. I think it’s less about the procedures, and more about comparability of the logs. An RMS and non-RMS meter are going to show different values for the same input. As long as the equipment has the same type of measurement and is properly calibrated they probably don’t care if a few external details are a little different.

          1. There was a time when schematics miggt specify the meter used to make measurements. The Simpson 260 VOM comes to mind. The meter could load down the circuit, so specifying a meter meant consistent readings.

        2. There are, or were, lots of competitors to the Hewlett Packard 34401 multimeter that advertised a compatibility mode under which their products had the same menu layout and SCPI remote command formatting of a HP34401 because there was such a huge installed codebase/documentation base for the 401. I can believe the same thing would happen here: decades of documentation and experience.

          HP themselves produced later-model calculators that used nice high-end ARM processors to run emulators with virtual machines to produce keystroke-identical copies of their earlier calculators.

      2. I agree with Eric Weatherby. If a manufacturer omits a 10-cent component in order to create a spurious ‘product differentiation’ that results in tens or hundreds of dollars difference in retail price, that’s crippleware.

        As for respecting Fluke, I haven’t done so for a long time. Their products are good and reliable. but their use of proprietary connectors, power bricks, and the like represents a Sony-like vendor lock-in that I have no patience with.

    2. I don’t follow the myriad models of Fluke meters. While they could be following a cripple-ware strategy in other product lines, I don’t think that is the correct characterization here. Checking briefly on Digi-Key, the model 77 is $368 and the model 177 is $366, If you agree that true RMS measurement is a better feature than average responding, Fluke is offering you a choice at essentially the same price. I’d say this is more like choosing the color of a car or right- vs left-handed scissors. I’m guessing, but I’d say they were originally maintaining two distinct designs. After awhile someone decided it would be more economical all around to merge these into one PCB. Perhaps one could fault them for not making the AC measurement style a user-selectable mode, but I don’t know how useful that would be in practice.

    3. Are they? They sell the 77 and the newer 177 at almost exactly the same price. If you just want a new meter, you purchase the 177. If you must replace a 77 with another new 77, you can do that. Fluke sells the 77 because purchasers, like the military, demand 77’s because that’s what the procedures call for and have been validated against, even though the 177 is newer and better. You get a newer 177 design internally but the same case, UI, and specs. as the older 77.

      1. Truth. Getting DLA to change specs is like pulling teeth. Getting SAE to change old adopted MILs is almost worse sometimes. It’s just easier to call it by something else.

    4. I don’t have a problem with Fluke’s approach. It is just a means to give customers what they demand. A cast-in-stone service procedure (presumably something like a US Government military service depot) requires and calls out a specific Fluke meter. The modern version of the Fluke 77 is designed to operate the same way, and most importantly mirrors the AC signal response of the original meter. Fluke is charging a premium to emulate specific 37 year old technology for a specific low volume need.

      Frankly, it is a waste of time hacking a model 77 IV to turn it into a 177 because a primary calibration standard is required to make the readings sufficiently accurate. Most casual hobbyists don’t have access to calibration equipment. Dave Jones happens to have that in his lab, but Jones also makes a lot of revenue from YouTube and can afford fancy equipment. Time is also equivalent to money and buying the meter with the higher price tag is actually less expensive.

      I have a Fluke 87 which is the true RMS big brother to the 77. I believe it cost around $350 in ~1985 Dollars. That is pretty laughably expensive in 2022. An equally performing generic meter from China is probably easily less than $100 now. Of course the Fluke 87 still works and meets its original accuracy specs after 35+ years. I doubt a generic meter would survive that long. I also have a Fluke 8060A on my bench that dates to around 1983. It went back to Fluke some time in the 1990s for a repair to the LCD driver board. At the time Fluke had a flat $70 repair rate including a traceable calibration no matter what was broken. You actually do get some benefits for spending more on Fluke meters.

      1. I also have an original Fluke 87 from about 1985 – or whenever they first came out – seems I had to wait in like 2++ months to get mine – has bailed me out of a few tough problems over the years because of its features – on time a couple of very sharp tech’s with a lot of test gear couldn’t find a problem and my trusty 87 saved the day – been more than happy with it over the years – great design – hats off to Fluke

    5. I would not call this crippleware – They have one board for multiple functions.

      When you want an Average reading meter you select a 77, When you want a True RMS meter you choose a 177.

      The last thing people want with a 77 is for it to only be available in a True RMS type. That breaks all of the standards.

    6. Analog Devices told Fluke “we’re not going to be building the old part anymore. You have to design in the 737J if you want to continue to ship something called the Fluke 77”. So that’s what Fluke did. It happens all the time.

      Nobody is cheating, it’s simply a product roll.

    7. Fluke has been going downhill for the past decade. Wait until they start selling their own brand of homegamer units at Walmart. Heck, even Snap On has garbage Chinese knives at Auto Stoners.

  1. I just checked prices for the 77 and the 177. The first thing I noticed was that the body is the same. Looks like they just change the printing and capacitor. The prices are within $20AUD of each other.

    1. I suspect it would be straightforward enough, but who would buy it? Not the institutional customers who need the Fluke 77 to remain available. Having the switch available on the meter means that, sometimes, the switch will be in the wrong position. So they would have to educate the users on how, why, and when to set the switch to each of the modes. Easier to leave in place the existing training and procedures that say, “the Fluke 77 will give the proper readings to perform this procedure”.

  2. Re: Fluke crippleware, I have an old Fluke DMM (73 maybe) that didn’t have the range hold button in the middle of the dial.
    I opened it one day and sure enough there’s interlocking PCB traces to sense a resistive carbon button, so I drilled a hole in the dial and fitted my own, saving 10-20 quid.

    Some years later though the meter stopped working entirely, so they aren’t as bulletproof as they could be. I then went off them and buy cheap crap instead, at least it’s only a few quid and you can always buy another.

  3. Can anyone tell me whether a recalibration is required every time the cap is installed or just once? For example, I only need RMS mode once in a while if I’m separated from my other true RMS meters. If I were to install the cap in my Fluke 77 in series with a slide switch, could I then occasionally flip the switch and read in RMS mode without requiring the accuracy of a recalibration, then return the meter to its normal average mode by turning off the switch? Would the old average mode calibration still be stored in the meter?

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