Fluke 187 intermittent error

[Mr. Pete]

Hello all,

This is my first post in this great forum.

I have on my desk a Fluke 187 with a weird fault, I want to have it fixed because it is my favorite meter, and I like to fix and learn. I bought it a few years ago second handed, so no free Fluke repair I guess. It measures within specifications on all functions, except on the mAAC setting. I'll describe it's problem measuring a constant current of 1.001mAAC (+/-0.5%) from a precise current source on the mAAC setting. The measurements are checked with other more precise and recently calibrated meters, and I can say confidently that the current source is accurate enough. This Fluke 187 is specified to have a maximum deviation of +/-0,75% and +/-20counts measuring 1mAAC on the mAAC setting, so it should return a value between 0,973 and 1,029maAC.

After power-on the returned value varies between 1.040 and 1.060mAAC, which is out of specification and it varies from time to time. While attempting to find a solution for this problem I recently discovered this. Twisting the multimeters housing with force by hand, the value can be made to drop to 1.018mAAC and it remains stable at that value for the rest of the power-on status. This value is now comfortable within its specification. Rotating the function knob back and forward a few times and back to the mAAC setting, the reading stays accurate. It can be made faulty again by twisting it again. Once re-powered, the value always starts faulty, until a forced twist of the housing is performed again. The intermittent fault has clearly a physical nature.

All the other measurement setting are stable and accurate, twisting the housing or lightly smacking the dmm does not influence any reading at all. Extra attention was given to the mADC, uAAC and uAAC measurement range: all are always stable and accurate.

I have removed the pcb from the meter. It looks good and clean, no signs of abuse, I have not seen any broken solder points etc. The rotating function knob contacts are clean as are the pcb traces underneath. My guess is a faulty coupling capacitor or resistor network somewhere on the pcb transferring the ac signal for the the mAAC setting only. If only I had acces to a circuit diagram... My reverse engineering capabilities are not very well developed for these kind of circuits.

Does someone here have an idea what components are involved in this specific measurement (mAAC) only? Or any specific ideas what to look for? Or maybe, am I overseeing something here? I measure mA's in the range of a few mA's quite a lot. It would be nice to have this functionality back on this meter.

Regards, Pete

[lowimpedance]

Wonder if there is a hairline fracture in the current shunt area etc. with the cover off and inputing a suitable current can you carefully prod (with a suitable tool ! ) the areas around the current shunts 'and them too' while looking for fluctuations in readings.