I called Fluke, asking for advice. They said this unt is more than 15 years old. It better to buy a new than sending in for repair. Nobody said anything about warranty.
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I have not disassembled the inputblock. It seems to be a compact-piece, and has 8 soldering point.
However, when I take another ohm-meter and put one test probe in the inputblock, and the other probe on the soldering-point on the rear, or a component along the input-stage, I get very low ohm reading, so I concluded there's no problem along that signal path...(?!)