Ok guys, my turn to ask a question now (non sexual of course

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I've just trashed a Jap 50000a model Ps2 while trying to fit an Apple3. The machine did work until I refitted the screws, so I did all the obvious stuff before removing the chip & all associated wiring.
Now, all I have is a black screen. All fuses appear to be ok, but I decided to search a bit deeper & powered up the board.
With the board powered up, the PS14 fuse appears to be dead

Can anybody shed some light on why the fuse should appear ok with the power off BUT dead with the power on? Just for the record, yes, I have tried linking it with a wire.
Just in case, is anybody interested in a broken Jap pink v10