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I'm trying to replace a blown fuse from my stereo system. The fuse has the following markings on one end: 125V 5A and the following on the other: 51MS, an H with a circle around it (I'm assuming this means Hitachi), and some other symbols including the Underwriter Lab logo, an "SA" logo and an inverted triangle with a "T" inside (drawing below) and a line just above the T. It's 5mm x 20mm

T Marking

The fuse has a wire, then a small blob of metal, then another wire (which, to my untrained eye seems a bit thicker).

I think this is an M Spot Fuse, as described by @Harry Weston in this answer: Slow Blow vs Fast Acting Fuse

As far as I can tell, this is a Hitachi FN00373. Unfortunately this is a really rare fuse. I should also mention this stereo system is about 10-15 years old.

Is it possible/advisable to use a different fuse in its place (I'm not sure I can even get the Hitachi fuse anymore). Or, is it possible/advisable to use a slightly different fuse (e.g. one that isn't "M Spot" or has a slightly different voltage/current rating)? If you can recommend one, that would be ideal!

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SimaPro
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    What size is it? Common North American fuses are 1/4" dia 1 1/4" long, while metric fuses are 5 x 20 mm (or so). – Peter Bennett Oct 13 '16 at 20:13
  • Thanks for asking. It's 5x20mm, I just updated the question as well. – SimaPro Oct 13 '16 at 20:15
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    There's one question you haven't thought to ask, which could make the whole concept of replacing the fuse null and void: *why did the fuse blow in the first place?* – Majenko Oct 13 '16 at 21:16
  • That's an excellent question, however I haven't touched this system in over half a decade. If I recall, I had been messing around using the system on different speakers, or with splicing the wires to make them longer. I think that's when it started. Anyway I just picked it up recently trying to figure out if I could fix it. – SimaPro Oct 13 '16 at 21:19

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Turns out that "51MS" is an entire family of fuses, all with the same basic characteristics but different current and voltage ratings. Assuming you can't find another "51MS-050L" from another manufacturer to replace it, you should be able to get away with any leadless, 5.2mm×20mm, medium slow-blow fuse rated for 5A and at least 125V.

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Ignacio Vazquez-Abrams
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125V 5A quick blow (51MS) Radio Shack

125V 5A 51 millisecond, available at Radio Shack, some auto stores, walmart etc. Was this soldered in, or is it just a glass fuse? Also, take a meter to it and verify that it is blown. It may be something internal.

P. Laabs
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    I'm not convinced the ms stands for millisecond, because then I would have expected "ms" not "MS". Also I think there's some subtlety here with fast vs slow and the form factor (m spot vs f type) – SimaPro Oct 13 '16 at 21:24