That switch apparenly controls the primary voltage to the power supply.
The power should not come on with the cover off, nor should you plug it
in with the power applied. (YOU CAN BE ELECTROCUTED!) See the link below
for the manual and schematics. I would unplug it , be sure to let the
High Voltage capacitors discharge, and trace the primary of the power
supply using an ohmeter to find the short circuit. Has anything changed
since you last used it? Was the primary rewired for a different AC service?

http://www.ameritron.com/man/pdf/AL-82.pdf

wrote:

Help: Ameritron AL-82 blown fuses
I bought an Ameritron AL-82 and it sat for over a year.
Worked perfect before that. Now when you turn it on
the two fuses on back of amp blow. So I replaced the two fuses
and this time I left the top cover off and it comes on. No problem.
The minute you pust the button down where the top cover goes on, it
will blow the two fuses again. The button is a safety feature.
Can someone tell me what I may need to check and replace?
Thanks and Happy New Year.............Rich



--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©

"Treason doth never prosper: what's the reason?
For if it prosper, none dare call it treason."

"Follow The Money" ;-P