Main Fuse Blowing when turning key 'ON'

[frenchy]

Gents,
I recently replaced a bad alternator & regulator/rectifier on a cb750 that i recently purchased. The previous owner had a jumper instead of a 30amp fuse at the main fuse, so i replaced that with a 30amp fuse. I noticed that the plastic behind & beside the 30amp main fuse is melted, along with small fragments of an old 30amp fuse. I just put the new fuse over top of this weave of melted plastic & bits of old fuse. When I turned the key on for the first time it was OK, but the fuse blew after trying to start the bike for about a half second. I replaced the fuse again but this time it blew right away after turning the key on.
I noticed in an old thread that loco leon had mentioned that the melted plastic could blow the main fuse. Why is this? Is this likely the problem? Any other suggestions?

Thanks guys.

 

[mixmaster]

Melted plastic is a sing of sustained high heat. Find the source and lower tghe heat. If not a short or draw perhaps the Alternator is putting out either to many amps or volts. I use a breaker in place of my fuse. Its self resets if it blows.

 

[locOleoN]

you have a ground on along the main power line some where..
Should be easy to trace back...

 

[frenchy]

Thanks mixmaster & loco leon...

Is it likely that the short is right at the 30 amp main fuse housing on the starter solenoid, where the old melted plastic & fuse bits are? Does the fuse housing need to be replaced?
Or is it more likely that the ground is somewhere along the main power line? Any words of wisdom on the best way to trace back this ground? I have never had to do this...not great with electrical.

mixmaster: where do you find a breaker to replace the fuse? Is it easy to replace?