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EEVblog 1424 - Fluke 23 Multimeter Repair

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EEVblog:
Repair of an original Fluke 23 Multimeter.

Excavatoree:
I don't make videos, but I've been kicking around the idea of making a very short repair video of this repair.  That's what I get for procrastinating.  It's a very common repair, especially when one buys the cheaper meters from the well known auction site.  I've changed a few of these.

One of our meter repair experts on the forum, forgive me for forgetting who, found a source for fuseable resistors, but the ones I've beeen buying from Digi Key were discontinued.  (but I haven't checked lately)  I've got a couple old Fluke parts, and haven't needed any lately.

Per Hansson:
Here is a blog article on this issue, with some recommended replacement resistors:
http://mrmodemhead.com/blog/fluke-87-fusible-resistor/

However I'm curious, a surge resistor like used in later models Fluke's input protection, I think they use WS5M
How would that compare to the already used fusible resistor?
Does a surge resistor basically act like a MOV when you overload it and a fusible resistor like a fuse in laymens terms?
https://www.bourns.com/products/resistors/pulse-power-resistors/product/WS%20Series%20Surge%20Withstand

floobydust:
Dave could take apart the failed resistor, is it metal film or wirewound?

It's an important part with a safety function, being fusible and flameproof. Some data says it's wirewound, other says metal film. Hard to know what the fusing power point and fusing voltage limit is for the part.
All the fault current through it also flows through the diode-connected transistor clamp and PTC, until it heats up and current backs off, if there is time for that.
Example- it's on Ohms function and you connect to mains whilst contemplating the meaning of life and look for your beer.  :popcorn:
1k resistor plus 1k cold PTC and around 7V diode clamp, at 240VAC mains is initially 117mA almost 28W, or 14W for the 1W rated resistor. I'd guess the resistor fuses before the PTC heats up. So the part is kind of a nuisance in that it fails first.
Later, manufacturers got rid of the fusible requirement. Today it's not the Bourns WS5M, Fluke 87V uses KOA PCF-1C102K 1k 1W 16.5mm wide, alternate is Ohmite OX102E, as the ceramic green-body surge resistors. Unknown if they are MF or WW construction. But not fusible, I'd guess the PTC is expected to look after that and has the agency approvals.

The surge resistor doesn't act as a MOV. It is to safely withstand and drop most of the surge/overload voltage, until the PTC heats up, in the event of a long term overload.
Fast transients, the spark gap or MOV might clamp to 2kV and joe has studied this.

Fluke 474080, IRC/TT SPH series "General Purpose Failsafe Molded Wirewound Resistor"
Fluke 832550, "1k fusible 1% MF 100ppm fuse/flmprf" I think it's a Fluke custom Dale/Vishay part, high voltage.

On the Web people are using NTE Inc. F2W210 "fusible power oxide" but no specs on the part really, for fusing current or voltage  :-- Who knows if it is a safe replacement.

Mr. Modemhead used Vishay/BC Components NFR0200001001JR500 (PPC1.0KDCT-ND) but it's obsolete. That's a 500VRMS rated and fuses at >16X power (32W) in 60 seconds or less. Meh.

bdunham7:

--- Quote from: floobydust on October 05, 2021, 08:33:18 pm ---Dave could take apart the failed resistor, is it metal film or wirewound?

--- End quote ---

That would be the first one I've seen failed that isn't burnt to a crisp.  Fluke designates it as "RES,MF,1K,+-1%,100PPM,FLMPRF,FUSIBLE 474080"


--- Quote ---Example- it's on Ohms function and you connect to mains whilst contemplating the meaning of life and look for your beer.  :popcorn:
1k resistor plus 1k cold PTC and around 7V diode clamp, at 240VAC mains is initially 117mA almost 28W, or 14W for the 1W rated resistor. I'd guess the resistor fuses before the PTC heats up. So the part is kind of a nuisance in that it fails first.

--- End quote ---

I've not seen one with both the fusible resistor and a PTC.  Are there any?


--- Quote ---Fluke 474080, IRC/TT SPH series "General Purpose Failsafe Molded Wirewound Resistor"
Fluke 832550, "1k fusible 1% MF 100ppm fuse/flmprf" I think it's a Fluke custom Dale/Vishay part, high voltage.

On the Web people are using NTE Inc. F2W210 "fusible power oxide" but no specs on the part really, for fusing current or voltage  :-- Who knows if it is a safe replacement.

Mr. Modemhead used Vishay/BC Components NFR0200001001JR500 (PPC1.0KDCT-ND) but it's obsolete. That's a 500VRMS rated and fuses at >16X power (32W) in 60 seconds or less. Meh.

--- End quote ---

A few years ago I acquired a surplus lot of 10 of the 474080 resistors.  There are no substitutes that I'm aware of and people are using inferior parts to make these repairs.  IIRC the NTE one is 300V and fusible but not flameproot, the other one I don't know the details of but obviously 500V is a deal killer.  Fluke also specifies 1% and 100ppm TC.  I've fixed meters that have obviously been hit with some pretty high energy and the correct Fluke resistor stops the damage.  Repairs without it are just gambling that it the same thing won't happen again.

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