Author Topic: Fluke 8840A repair. Symptom: negative voltages incorrect on the 20V range  (Read 136 times)

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Offline martinr33

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This unit appeared to be OK everywhere except for the -20V area, where it would cap out at around -6V.
All other ranges - positive and negative - were OK.

The 20V range has its own buffer amp, with a power supply circuit that tracks the input voltage so as to avoid running out of headroom. This is why the problem only showed up on 20V.
The rails with a positive input tracked fine, but with a  negative input, the voltage reading  was capping out,

A scope test showed a very ugly -30V supply.

I tracked it down to a 50V 100uF capacitor C611 in the power supply area which, despite no visible signs whatsoever, has no capacitance. Brand is UCC, not a company I associate with this kind of problem.   Replacing the cap cured the problem.
 
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