Fluke 87 Fusible Resistor

Fluke 87 Fusible Resistor (R1)Properly-designed multimeters that are suitable for professional and trade use have a number of components, usually near their input jacks, that are for the sole purpose of protecting both the user and (secondarily) the multimeter itself from over-voltage and over-current situations.  Everybody knows about fuses and can probably recognize them right away.  But the fuses are only used to protect against having too much current passing through the meter and test leads when using the Amp and/or milliAmp jacks.

[Recommended viewing: EEVblog #373: Multimeter Input Protection]

There are a number of other protective components you may find near a multimeter’s input jacks.  Some are connected in parallel with the input source, and are meant to trigger when the voltage across the input reaches a dangerous level, becoming a low-impedance path to essentially short-circuit the energy away from the rest of the meter’s sensitive circuitry.  These components are usually spark gaps and/or varistors (MOVs).

Others are connected in series, and will become an open circuit or high impedance when something goes awry and too much energy is being absorbed.  These components are usually PTC thermistors and/or fusible resistors.  The latter component masquerades as a regular high-wattage power resistor, and is sometimes also described as “flame-proof”.  Thermistors should return to their normal impedance after the overload is removed and they have cooled down, but once a fusible resistor has “blown”, it no longer functions.

Many (but not all) models of Fluke multimeters include a fusible resistor in series with the Volt/Ohm input jack.  The meter shown in this repair is a first-generation Fluke 87.  The metal-film fusible resistor is designated R1 (part #832550) in the model 87 parts list.

F87FR_S01_R1specs

In the service manual it is described generically as a “1kΩ 2W fusible resistor.”  This resistor is a common failure point for a unit that has been subjected to an over-voltage situation.  Tried to measure the output of an electric fence, maybe?  (Don’t do that.)

Disclaimer: The following describes an amateur repair on a component that is critical to the safety specifications of the device. In cases where safety ratings are mandated, it is advised that repairs be performed by authorized service centers with manufacturer-approved parts.

A meter with an open fusible resistor may power up and look normal, but upon trying to make measurements, it can act rather goofy.  Let’s look first at the symptoms of an open fusible resistor, and then find and replace it.

[Click on any of the following pictures in this article for a larger view.  All graphics used in this article are also available as an image gallery.]

Symptom #1

DC voltage is selected on the function switch, but when a DC voltage is applied the meter always reads zero.



Symptom #2

Resistance is selected and the leads are left open. A functional meter would display a steady OL, but this one displays positive and negative random numbers and sometimes flashes OL.  If the beeper is turned on, it will beep randomly.



Symptom #3

Shorting the leads would normally cause the meter to read a value near zero, say about 0.2 ohms.  But this one flashes OL rapidly.  (The dim digits in the photo is a result of the flashing.)



Symptom #4

On the diode check range, with no leads connected, the meter would normally read OL.  Instead this one seems to be stuck at  0.000V.



Symptom #5

Attempting to measure 120VAC mains results in a very small unsteady voltage being displayed.  The broken resistor is capacitively coupling a small voltage into the front end.



Begin repairs by disassembling the meter.  For this particular repair, you may get by without removing the shields as shown.  But I like to set aside the plastic parts while waving a soldering iron around.  If you take off the shields, pay particular attention so as not to break the fragile bits, marked on the photo at left.  Breaking one’s fragile bits leads to a bad day.



Locate the fusible resistor.  It is usually light brown in color with two color bands.  (Don’t ask me what the colors mean, I’ve never figured it out.)



Do not confuse the fusible resistor with the Ohms sense resistor, which will almost always be very close by, since it also connects directly to the red input jack.  This one is light blue and is 909K ohms.  The ohms sense resistor in most Fluke models is usually at or near 1 Meg.



Using another multimeter, we can confirm that this fusible resistor has failed open.  A good resistor would read about 1K ohms, this one shows open.



The replacement I’ve selected is a 1kΩ 2W Vishay metal-film fusible resistor.  It is 5% tolerance instead of the original 1%, but the value of this resistor does not affect the calibration.  (It is in series with the voltage divider’s 10 Meg input impedance, which far outweighs any variations in this resistor.)



De-solder the old resistor and clear the holes of solder.  A solder-sucker and/or solder wick is handy for this.



Solder the new resistor in place.  Be sure to clean any flux residue left on the board with IPA.  Residues and contamination on the PCB are not a good thing in the front end of a digital multimeter.



Reassemble and test the repair.  Here, an 806-ohm precision resistor mounted on a dual banana plug gives me a quick means to check resistance accuracy.  Looks promising.



Checking VDC calibration with a 5.000VDC reference.  One count off.  Not a problem.



Measuring 120VAC mains now gives a good reading.


Goofy meter is now acting sane again!

Update: After I prepared this article I discovered that DigiKey (and Mouser, Newark, etc.) no longer stocks the replacement resistor I used. The following are possible sources of replacement parts.  Note that non-Fluke replacements are not guaranteed to return the device to its original safety specifications.

Part No. Description  Voltage Possible Source
832550 Original Fluke part Fluke Service
Ohmite OY102KE Ceramic Composition Resistor 2watt 1K 10% Surge 400V Mouser, Digi-Key
 NTE F2W210 2W Fusible Power Oxide Resistor 300V Moyer Electronics

 

About modemhead

Fixologist and multimeter junkie.

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29 Responses to Fluke 87 Fusible Resistor

  1. supporter says:

    Dear Mr. ModemHead first of all respect of your skills to repait dmm model and your blogger site. I found your blogger site after i looked for adding a summer to my low cost dmm (Digital Multimeter DM 830 E) when I testing a wire to hear a sound and not looking on my dmm all the time.
    Please can you help me to add a summer for DM 830 E and explain how it works?

    Thank you
    supporter

    • modemhead says:

      I assume you refer to a ‘beeper’ or ‘buzzer’ for continuity testing. You will need to add a piezo element, a comparator, an oscillator, and a gate of some sort to allow the comparator to turn the piezo element on and off. When a low resistance is placed across the inputs, the voltage across it drops close to zero. The comparator detects when this voltage goes below a set threshold, and turns the beeper on. Here is a schematic for a simple 830-style DMM with continuity feature. I added notes to point out the how the continuity works.

  2. supporter says:

    Hi yes your ´re right! thank you for the fast answer!!!!!
    ok I will try !!!

    Keep doing your blogger site !!! To repair dmm is good to ask peaople like you, you are very special.

    best regards
    supporter

  3. Harvey says:

    Back again to sample your wisdom and humor 🙂

    I too have two of the cheap 830 digitals that “supporter” is in the process of modifying, and yes they are cheap but for a simple icl7106 clone blob chip they work quite well really.
    The dc voltage and resistance is accurate enough for ordinary go/no go testing.

    I must admit ive never noticed the fusable resistors before, but im not in the habit of applying voltages where they should not be, yet!! ha ha 🙂

    Hmm fluke 87, a wheelbarrow resident? 😛

    I must get a fluke portable multimeter before you kidnapp all the available ones floating around :-), the trouble is the plethora of models confuses my small and often confused brain.

    I have also noticed your 806 ohm resistor that appears to be marked 805.8 ohms, your standards are slipping by calling it 806 ha ha 🙂 🙂 leg pulling 🙂

    As allways a delight to read your writeups 🙂

    • Harvey says:

      Dear harvey, you are a plonker, you have given the impression that the two 830 multimeters you have are being modified by “supporter”.

      You are english and your spelling&punctuation is rubbish.

      With no respect

      Harvey the confised 😉

    • modemhead says:

      Glad you enjoy my write-ups. Come back anytime. I have a few more in the queue at the moment. Being confised doesn’t hurt, does it?

      The ICL7106 is a wonderful invention. Makes it simple as pie to make a very accurate DC voltage measurement. And it’s what, a 30+ year old design? That’s why the 830 cheapies are even useful at all. Safety and reliability is of course, something else entirely, and not to be found in those ubiquitous little boxes.

      You’re right, Fluke model numbers can be confusing. That’s what happens when the marketing department gets involved I guess.

      As for the resistor, well look at this and then you decide what to call it. 🙂

  4. Harvey says:

    Ha ha sorry ive been hitting the wrong keys again 🙂

    Confised… ha ha thats my kind of confusion+pissed, ha ha 🙂

    Yep your right (no supprise :-> ), 806 ohms at +/- 2.015 ohms, and i bet its even better than that, manufacturers tend to show the worst values on some things.

    Im sorry im so crap at spooling and p.un’c”tu;at!!.ion mate, ive allways had trouble with reading/writing/maths oops math and general learning, butt im shure u stand under wat im slaying lol 🙂

  5. N Clark says:

    Hi Mr Modem I have several Fluke 25 meters.I can not find a source for the 1k 2w fusible resistor. is there another way around this problem,I do not want to fit a standard resistor because of the safety issue.Thanks for your help in the past.N Clark

  6. N Clark says:

    Hi thanks for reply.I tried moyer they will not accept orders outside there country.I have located the F2W210 resistors from Wholesale electronics.inc.thanks again for your help N Clark

  7. Keith Ostertag says:

    Thanks for this writeup I would not have known what that part was! I have a Fluke 27/FM that needs a similar fusible resistor in series with its input. Moyer does have the NTE part you mention, but they want $10 to UPS this $1 part.

    How about putting four 4.7K 1/2 watt fusible resistors in parallel for this purpose? They’re 54cents and are carried at my usual supplier (DigiKey). I think I can fit them in there.

    http://www.digikey.com/product-detail/en/NFR25H0004701JR500/PPC4.7KBCT-ND/614262

    Thanks,
    Keith

  8. Paulo Souto says:

    Hi, Mr Modemhead,
    Thanks for all these helpful informations! I have a Fluke 27/FM that needs a the fusible resistor and the one beside it (220k) which is also open. I did not know that component was a fusible resistor until I read your article indicated by one guy (Marvin) from Estonia (http://www.eevblog.com/forum/testgear/fluke-27-27fm-side-by-side-tear-down/msg386093/#msg386093).
    Thanks a lot!
    Paulo

  9. Paulo Souto says:

    Hi again!
    Please, the rt1 (.022k 1000) in series with the fusible resistor is also open. Can I replace it with a fixed 20 Ohms resistor?
    Tkx

    • modemhead says:

      RT1 is a 1K-ohm @25C PTC (positive temperature coefficient) thermistor rated 1000V, 40% tolerance. It acts like a self-resetting fuse. Replacing it with a 1K fixed resistor will allow the meter to function, but will greatly compromise its input protection scheme.

      • Paulo Souto says:

        Thank you so much for your prompt response and nice explanation.I wrongly thought the .022k stamped on the component meant its ohmic value.Before doing the “adaptations” I’ll try to get the right parts thru a friend that works with electronic instruments. He may have some used spares.
        Best regards.

  10. Paulo Souto says:

    Sorry to bother you with the same matter but I found out that the component I was talking about is a capacitor (C3) and not the RT1. The problem is that I can’t find the service manual for Fluke 27FM. Instead, I was looking at the Figure 4-2 ( A1 Main PCA ) of Fluke 27 service manual which has RT1 at the same place of C3 on the 27FM main board. After analysing the schematic diagram of Fluke 27 I noticed it does not have the fusible resistor R1 (it’s a 3.5K 5W) and R2 has a value of 1M (220k for Fluke 27FM). Anyway, I made a stupid mistake. If you take a quick look at link below you’ll see what I’m talking about:
    http://www.eevblog.com/forum/testgear/fluke-27-27fm-side-by-side-tear-down/msg385939/#msg385939
    In case you know how to get the Fluke 27FM Service Manual over the internet, please be so kind to informe me.
    Thanks and best regards.

    • modemhead says:

      I think the “/FM” model was only for the military, so there may not be a service manual circulating for it. My PDF copy looks like it’s for the more recent yellow 27 models. I do have a hardcopy military-style “Operator’s Manual” for the 27/FM, but it has no service information. I’m reasonably sure the 8025B will be very close to the 27/FM, except for the RMS converter section. The manual for that is available.

      I now understand why “RT1” and the “.022K” didn’t make sense. C3 is a 0.022uF capacitor, the K is the tolerance (10%), and it is rated for 1000V. It is the coupling (DC-blocking) capacitor for the AC functions. The thermistor on this older model is the gray rectangular component just north of the 5 green blobs which are varistors.

  11. Paulo Souto says:

    You are correct, the “/FM” model was for the military. According to the seller, this one came from the military when they sold a bundle, some in bad, others in pretty good condition. I will call a friend of mine that works at a Air Force PMEL first thing on monday to see if he can get me some scraps and a copy of the service manual that can send to you in case you’re interested in. By the way, I took a PMEL course in 1984 at Lowry AFB and worked with calibration til 1998. I worked most of the time repairing Tek Scopes, specialy 465 M, signal generators and Spectrum Analyzers before changing to another area. So, I’m having a lot of good memories reading your posts.
    Thanks again for all your valuable informations.

  12. qwertzui says:

    Hi,

    I have a Fluke 75-2 with a blown R20 Fusistor. Fluke part 740662.

    This Fusistor (0.36Ohm, 2W, +-10%) seems to be a safety circuit for the >300mA input jack, together with a glass fuse (630mA, 250V).
    See Service manual: http://assets.fluke.com/manuals/77______smeng0100.pdf

    Since the original part is no longer available, nor being manufactured I am looking for a replacement. I am not an expert regarding circuits etc….

    If I am thinking right, the glass fuse is protecting the multimeter’s <300mA input of amperages over 630mA. The fusistor that is directly behind the input jack in the circuitry should be protecting the multimeter of higher voltages than 0.85V… correct!?

    So any replacement fusistor, protecting the multimeter's <300mA input of higher voltages than 0.85V should work fine in this safety circuit, right? E.g. a fusistor with specifications like 0.33Ohm, 1,5W, +-10% would only tolerate a voltage of 0.7V, right?

    Thanks for helping me out and given short advise on that matter. Would be a shame to dispose a fine working multimeter only because of a broken fusistor….

    Regards,

    • modemhead says:

      My opinion is that the function of R20 is to interrupt the circuit in a massive over-current situation. The small 630mA fuse has a low “interrupting rating”, meaning that a sufficient amount of overload current could simply arc over the fuse element even after it breaks. R20 should then provide a secondary method of interrupting a severe overload. It will dissipate 2 watts at 2.36A, so at some point beyond that it should open up. The voltage drop across R20 is not particularly important, except that it will contribute negatively to the “burden voltage” of the DMM for the 300mA range, so the lower the better.

      Your suggested replacement seems reasonable to me.

  13. Taylor Vandergrifft says:

    Be aware that all non-Fluke 832550 resistors carry a much lower voltage rating than the voltmeter – some as low as 200V ! The original (Fluke 70-series era 1K fusibles, PN 650085) were available commercially, but no longer. Fluke went to the 832550 to ensure the CAT rating on the meters is maintained – very important in this litigious society. One shock/incident with a modified meter is all it takes. Spring for the Fluke resistor (~$9 from Fluke these days) and you’ll never go wrong.

  14. Pingback: Roached my Fluke 83 | Buttons, Switches, Knobs & Lights

  15. Uggesen says:

    Hey mrmodemhead 🙂

    Will you be so nice to link me to a site where I can find the fusible resistor for Fluke 77 series 2? 🙂

    Thanks so much
    Kind regards uggesen

  16. neville hardy says:

    hi i have a Fluke 27/fm and i over voltaged it so i found the 1k resistor as described and
    it was o/c so i replaced it with a 1/4 watt 1k ohm r and all worked fine
    but a few days later i didd it again over voltage !!! 1 k ohm went o/c so i replaced it
    meter works ok on Ohms and on diode setings but on AC and DC range its all odd goofy readings , can you offer any help
    thank you kind, regards Nev

  17. Gene Giddings says:

    Thank You for this site. I was very upset when my 87 failed. It had been a great meter for many years. I thought I was going to have to replace it and did not like the idea of that. Moyer Electronics sent me the parts in 2 days. The repair was very easy and it is back working perfectly again.
    So again, I thank you.
    Gene

  18. Richard Walker says:

    Hello all I’m new to the fluke bug and got bit. I received a fluke 87 very first ones. Any way I have a question or two that I hope you can answer. I hope the pics can be uploaded. There is 2 components one looks like a box with two legs on one side and one leg one the other side and it has the letters JCP. The other is a box with the numbers 511 on top.

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