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Leave to 20 in there.What did you do years ago for the repair?If you have a good 'repair', and the solenoid power is going directly from the battery, through the fuse, to a relay, to the starter; then you may need to pull the starter and clean the solenoid. I think that they sometimes get clutch dust in them gumming them up.
Other than the dusting mentioned here is there anything else that could be causing the problem?
Make sure your battery leads on the battery are clean and tight, plus check the earth strap. It seems strange, but reduced current capacity going to the starter and solenoid increases the time the solenoid pulls in which in turn gives the fuse a better chance to blow.