Semi-Open Fuse?

[wn1fju]

I was fixing a generic Chinese function generator (Instek GFG-8020H) and after replacing the shorted transistor in the output amplifier, I hooked it up to the scope with an unterminated BNC cable and everything looked reasonable.  When I properly terminated the cable with 50 ohms at the scope, the output vanished.  I turned the unit off and put an ohmmeter across the output jack.  Expecting to see 50 ohms, I was a bit surprised to measure ~400K.  The culprit was the protective fuse in the output circuit which measured 400K after I pulled it.  It is a standard glass 200 ma fuse (and looks intact).  Didn't know that fuses can pop partially open.  Guess I learned something today.

[SeanB]

The fuse can fail as nearly open, as the wire breaks or oxidises at the crimp or at a break at the end, and while it will break down at mains voltage and work again at low voltage it is an insulator. Also you can get poor contact at the connections to the holder, which can be hard to trace as the slightest touch ( like picking the equipment up) makes it work again.