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