Electronics > Repair
Fluke 73 III repair
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bborisov567:
I got the unit with leaked battery. Fortunately there was little damage to the board. I cleaned the board very well, even removed the analog chip and cleaned under it. Now the unit is in somewhat better condition but still there is a problem with the ohm meter function. In open circuit it reads about 8-10 megaohms and when shorted it read about 40 ohms and the value is gradually going to zero, it's like there is a charging capacitor. All other modes are working accuratelly. When measuring voltage it needs about 3-4 seconds to settle. I tried removing the MOVs at the input but there is no change. Also when i short the "OHMS" pin to ground it reads around -40 ohms, note the minus. The 33pF cap between OHMS pins and ground is changed. The refference voltage and the supply voltages are also stable. I don't have the exact schematic but i've been using the schematic for the 73 ii, which is quite similar. I have checked Q1,2,3 with a transistor tester and they look okay.  Can somebody suggest what might be wrong? |O
Kim Christensen:
What did you clean it with? Usually for alkaline battery leaks, cleaning with some vinegar and a flush with clean water works well. For the remaining greasy residue I use a 10% ammonia/water mix with a few drops of dish soap. Scrub with a short haired paint brush, rinse well with water, and blow dry with air.
Then dry in a very low temp oven / drier (100C / 212F) for an hour before powering back on. (Take LCD off first)
There could also be some contaminants in the yellow sparkgap next to the fuse. You could run a piece of paper soaked in 99% alcohol through the gap.
bborisov567:
I cleaned it with vinegar after that with alcohol and in the end i rinsed the board with soap water and let it dry. I tried cleaning the spark gap with paper soaked in alcohol. I am wondering if the acid could be in the pcb material. But still that would explain the 10 meg reading but not the 40 ohm.
Kim Christensen:
Remember that an ohm meter is just a volt meter and a current source. So if there's any voltage leakage into the ADC, then that'll look like a resistance. Does the meter read zero on the 300mV DC scale?
Here's a link to the Fluke 77/73 manuals.
Diagram is from the 77 manual, but the 73 will be of similar in design.
Paceguy:
I could be wrong,but other than some residue from cleaning it could be an open fusible resistor. I circled the two resistors that could be suspect. Also bad contacts on the rotary switch can cause this kind of problem.
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