Fuses

A fuse is a protectional device making sure that there cannot be an overcurrent in equipment in case is has a defect. If a fuse is "blown", you can be pretty sure that there is a defect. You can exchange the fuse by a similar one with the same value. If this blows immediately or shortly afer power on, the product has a failure for sure. Never try a fuse with a higher value or worse bypass it somehow. While doing so, there could be running a lot of overcurrent, definitely destroying the product. Note that not only the valueis important, but also the way they blow. There are basically three types: Fast (F), Medium (M) and slow (S or T for Träge in German). High peak current devices like power amplifiers mostly use the slow variants, where more constant current devices like computers, etc. use fast ones.
Regarding a Google "review" complaining about receiving a defective fuse: we, nor any other company tests every part we ship out. The manufacturers making them do, though.  The mentioned malfunctioning fuse for sure was not tested before placing, but placed in a defective unit, so it blew immediatelly as it should do. Obviously this guy believed it is as with a light bulb. If it's defective, replace it and all will work again. No, it won't, sorry.

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