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 :-)