OK, I´ve got an 80s Guyatone Flip2000 Mesa Boogie copy combo.

Problem: I turn on the amp, switch from standby to on and the 500mA fuse blows immediately. The main fuse is ok, as are the power tubes.

Any hints on what and where to start looking for the problem?? I´m in an "I want to learn something" mood and therefore want to try doing this myself.

Thanks!

I'd be very careful investigating any amp problems related to high-power; such as a fuse blowing everytime the amp is turned on. To be safe, take the amp to a qualified electronics repair person.
Mucking with high-power circuits could prove quite dangerous, without proper training, etc.
Mesadude