that sounds like what was happening to me, because initially the scope worked with a new battery, but then died, due to insufficient contact (an interruption when I moved it during re-assembly), the battery pins were bent a little as it seems when I bought them, so I bent them back and it worked. my battery was from 2013 and reads above 3v. so watch out.
on another topic re. the 7104, i have a friend who owns one too, and when he switches on the rear power switch, it does not boot, you need to press the front key in addition. mine powers on once power is applied (rear switch on) , without having to switch it on with the front button. I would like to change this, as I am powering all my equipment off with a central switch, and repowering always switches on the scope.
any ideas? the internal power jumper on the ppc board is not set, so that is not the issue. might have to do with the psu (bios and win98 are not active at that time, so no settimgs there), any ideas?