| Electronics > Repair |
| Fluke 87-I repair |
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| staze:
Hmm... so resistor network, or those varistors, or.... Could you check those, and see if you get infinite in both directions? Waiting for the end of the daily show before I clean up that soot. But I would think if it were conducting, I'd get lower rather than higher resistance. |
| staze:
yup, no difference. Okay, so, RV1, RV2, R29, or the resistor network. I'm guessing Pin 1 of the network is the one closest to the input jacks (all by itself). So, which pins should I be measuring between for what values? I'm not sure the service manual is very clear on this (it says pin 1, but not pin 1 to what). Though, guessing RV1 and RV2 are fine if they're in the 100+ megaohm range. I don't have a SMT replacement for R29, but I could probably colder on a replacement axial one. The part I don't get is the changing value... open resistor acting as a capacitor? Oh, and what does R2 measure in circuit for you? |
| staze:
--- Quote from: retiredcaps on February 19, 2014, 07:02:13 am --- --- Quote from: staze on February 19, 2014, 06:54:10 am ---Pictures of R29 attached, as well as the arch marking on the back where RT1 and R1 connect. --- End quote --- I would clean up those areas with some IPA and scrub away all that black residue. --- End quote --- Done, no change. There's actually a physical divot in the PCB on the backside where that arc occurred. =/ |
| staze:
okay, something is clearly wacko... measuring R2 one way, I get 850K, the other way (leads reversed), I get 1.6M. This is with the unit off. Something is clearly acting as a cap, or I'm being dumb about this... |
| staze:
So, on a whim, I set my second meter (right now, a Fluke 189) to voltage, and interestingly, I'm getting a voltage out of the 87 when it's set to DCV or DCmV proportional to the reading on the 87. In the case of the DCV, it's about -170mV when the 87 shows about -34V. And -7mV when the 87 shows -7mV. So maybe those resistors are all fine, I'm just getting voltage leaking into the circuit from somewhere and throwing off my resistance readings. Also explains the polarity issue. So... where would voltage be leaking in from? My thinking would be in section 2-4 of the manual... the voltage clamping network of Q1, Q2, Q6, CR7, CR8, and R58. Maybe one of those transistors shorted from an overvolt? (someone really blew the ass outta this thing)... but the MOVs didn't blow... RT1 did... so... what the hell? |
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