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Repair of an original Fluke 23 Multimeter.
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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. -
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 -
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.
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. -
Dave could take apart the failed resistor, is it metal film or wirewound?
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"QuoteExample- it's on Ohms function and you connect to mains whilst contemplating the meaning of life and look for your beer.
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.
I've not seen one with both the fusible resistor and a PTC. Are there any?QuoteFluke 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.
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. -
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.
Vishay's CMF70..5 seems to be almost there, with 900V operating range. The PR03 is close at 750V.
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the primary function of the 1k fusible/flameproof resistor et al is to provide survivability if someone feeds in any significant voltage while the meter is set to the OHMS range. with this in mind, i've taken a quite a different approach to what other folks have:
i found the SMALLEST 1k leaded resistors i could. the body of these resistors is 3mm long x 1.5mm diameter, with an 'official' rating of 1/6w. i suspect that 1/16w is closer to the mark. i placed one of these resistors into the ceramic tube salvaged from a discarded cartridge fuse (to help contain the fallout if/when the resistor shuffles off this mortal coil) before fitting back into the meter (a series 2 77 board, transplanted into a 23 case). following good practice, the resistor+tube sit 1/4" clear of the PCB.
whereas the official resistor should survive 240v AC being connected when set to the OHMS range (should: seems it rarely does), the puny 1/6w resistor is likely to expire at anything much above 30 volts. i'm happy with this - if i'm silly enough to connect 240v AC when the meter is set to OHMS, i want a quick disconnect, and deserve to have to open the meter up and fix it.
cheers,
rob :-)
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The surge resistor is in series with a PTC, for protecting the Ohms source output.
A long-term overload relies on the PTC to heat up and switch to high resistance, rather than causing the resistor to see steady gross overload.
Today, nobody is using fusible resistors here, because they are lacking in many areas. Their fusing point is very high for a "2W" rated part, it's 40-120W! and really a wide range. Only a few makes of fusible resistors have regulatory approvals like UL 1412.
I think the custom Fluke part is a quite a low fusing point part and no idea what interrupting voltage it can accommodate.
It should be replaced with same to maintain Fluke's original safety spec. -
Fascinating crumbs of information. The first is the resistors with the leads painted over quite a long distance. I wonder if this was a high volume version of the resistor just for vertical mounting or something ordered by Fluke. The second is the metal glaze (MG) technology for the resistors mentioned in the schematic diagram.
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Another wonderful video, Mr Dave.
@ 17:33 - where you mention reversing a screw slowly until it jumps into the thread with a tactile "click" et cetera - yeah, always done this with screws, especially ones going into inserts or other machined holes - insert the end of the screw, reverse turn it for a turn or two, and "click!" it jumps into place.
Fab mate.
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Thank you for another great presentation on multimeters.
I would love to see one discussing how multimeters measure resistance. This subject seems to be overlooked. -
I'm late to the party but I just watched the video... my first ever fluke repair was a 70 with a blown input resistor!
I've collected a pile of blown fusible resistors that show absolutely no visible signs of damage... and as a replacement I like the CMF70..5 rjsouza recommended. My friend Mark turned me on to the CMF series with some CMF60 parts he sent me for an 8060a repair (CMF601K0000FKBF64).
I've also used the SPH1001J (1k 2w) but it's not fusible... the SPF1001 is the fusible version but I wasn't able to get my hands on any.