Hi all, first post here so please be gentle.

I have been repairing hifi since before I left school (40+years) and, over the years have successfully repaired thousands of systems, including the occasional B&O.

So last month when a friend asked if I could have a look at his BS9000 Mk II, I agreed to see why it wouldn't play CD's. It showed all the symptoms of a sick laser pick-up so I took it back to my workshop to have a proper look. I did all I could to clean the lens in situ but to no avail.

Well this is like no ordinary player and I did a fair bit of dismantling before I discovered how to get the pickup assembly out - most of it unnecessary I now know!

Long story short, I ordered a replacement pickup which arrived a couple of weeks later; fitted it and.... nothing! The 'sledge' moved across to position 6, stopped and the lights went out. Now dead. A few checks revealed a o/c 100mA fuse on the transformer primary. A tentative try with an ammeter showed 250mA being drawn - hence the dead fuse! But why? Had I dropped a screw somewhere? Unlikely, as I am generally very careful and I had all screws accounted for. A quick search on here returned this thread  and indeed, all 4 mosfet's are s/c. Panic over for now, at least I know why it's dead.

SO my question is.. what causes these to go? I am nervous about just popping in new ones until I know what is likely to have caused their demise.

Suggestions gratefully received, TIA.