I have no experience with electronics except dabbling with a
multimeter.

I wanted to silence a humming transformer, I read somewhere that
covering it in JB Weld would do the job. But I didn't even get that
far.

I struggled to open it up, in the end I managed it quite 'easily' with
a hammer.

Before even putting on any JB Weld, I put the lid back on it and
plugged it in.

Then I flicked the mains power switch and bang it blew.

I know it was the ACDC adaptor because and not a mains plug fuse
because the power cord still worked in another ACDC adaptor

Is there a way for me to open one of these things without encountering
that?
does anybody know about silencing humming / whining transformers