Fluke 77 replacement for R1 (1k 2w fusible resistor)

[robert.rozee]

hi,
    recently picked up a fluke 77 (series II) for nz$65 delivered. it arrived in a sorry state - case was filthy, crud inside too, 630mA fuse was blown and had been wrapped in metalized paper (from a cigarette packet), and R1 had been blown. i'm picking this was due to someone feeding 230v AC into it while set to the ohms range - from writing on the holster it looks like the meter had been previously owned by an electrician.

everything cleaned up nicely, and with R1 bridged with a 1/4w 1k resistor the meter seems to function correctly.

the problem is finding a suitable replacement for R1 at a sensible price. MrModemHead (http://mrmodemhead.com/blog/fluke-87-fusible-resistor/) suggests a Vishay NFR020 series, but these seem to no longer be available. another option he suggests is an Ohmite OY102KE series, but these are only available with a nz$50 minimum order from the usual suspects (element 14 in this case).


looking at the simplified schematic (see below) R1 seems to come into play under the following two conditions:

1. serious over-voltage on ohms or volts ranges (over 1800v) where RV1 and RV2 conduct. if this happens, R1 only needs to pass 45 mA or so before it blows.

2. moderate voltage input on ohms range. current flows through R1, RT1 and Q1/Q2 (acting as 7v zeners). if the voltage is relatively low then RT1 will heat up quickly enough (and hence its resistance increase) to allow R1 to survive a prolonged exposure. if RT1 can not react quick enough, then Q1/Q2 only need to be able to handle 45mA before R1's maximum power dissipation is reached and it blows. i'm guessing that R1 was upped from the original 1w used in the original 70 series because R1 was being blown too often by user 'mishaps'.


i'm considering replacing R1 with a 1/8w 1k resistor suspended on stand-offs, and sitting inside a ceramic tube. what is the opinion of this? the tiny resistor will definitely be dispatched by a serious over-voltage, while anything over about 30 volts will fry it when set to the ohms range - not a problem as i can easily replace it.

note that i have no intention of ever selling the meter, so the changed behaviour will not affect anyone else. at some point in the future, if i chance across some 1k fusible resistors, then i'll restore to original. i do have a possible 'donor' fluke meter (with blown AP75), but the value of R1 in that meter is 930 ohms, which makes me a little suspicious of the part. it too was likely killed by 230v AC on ohms range.


cheers,
rob   :-)

[retiredcaps]

i do have a possible 'donor' fluke meter (with blown AP75), but the value of R1 in that meter is 930 ohms, which makes me a little suspicious of the part.

Two suggestions.

1. Use 930 ohm resistor.  Personally, that is what I would do.  Is that measurement of 930 ohms in circuit on the pcb or out of circuit?

2. Ask on buy/sell/wanted forum for the resistor from a fellow country person.  Maybe they will just stick it in an envelope and put a stamp on it for the cheapest  possible price?

[bob91343]

I blew the one in my Fluke DVM some years ago and called Fluke.  They gave me a ridiculous price and I blew my cork, telling the person how ridiculous that is.  To shut me up she sent me a free part and all has been well since.

[bdunham7]

If you can get hold of an NTE F2W210, that's probably good enough.  I believe xDevs had an article about the origins of the custom Fluke (made by Vishay/Dale) resistors and what series to order to get as close as possible to it (there is no direct substitute AFAIK, the Fluke one is 1%, fusible and flameproof coated and allegedly capable of interrupting 1kV) but xDevs is down right now and I believe the specific series they refer to is also NLA.

The voltage ratings are an issue.  The NTE, as well as every other candidate, lists voltages in the 300-500V range, but this is 'working voltage'.  You may see 'dielectric withstand voltages from 500-900V, with some expensive models going to 1kV.  I'm not sure either is directly relevant to its function as a fuse.  I wish I could find the datasheet for the original series that the custom Fluke model is derived from, but I don't have it right now.  I actually have the OEM Fluke resistors on hand, but they are so expensive that I can't give any away or even sell them for a reasonable price. 

I think reasonable substitutes in your case are the aforementioned NTE F2W210, Vishay CMF-70 or perhaps the Vishay NFR series in 1/2 watt or anything you can find in fusible and if possible, flameproof coated.

[Hydron]

Check RS, depending on where the part is you'll get free delivery in NZ. RS UK have a minimum order now because of covid, I think it's 30 quid which is really damn annoying (i.e. it's now worse than E14's $50, especially given it used to be 0!).

Disadvantage with RS is their tendency to only have stuff in larger packs, which can push the price up a lot if you just need one. They do seem to deliver stuff more reliably in the UK than Farnell (element 14) though because they don't use UPS, who fuck me over every time I need something urgently (or drop off my scope in an apartment block mail room without a signature, or bill my customer for an unrelated order by mistake instead of me etc etc).