so this has been bugging me for a couple weeks now. my 7300 is suddenly only doing about 20-25w on 40m when everything else is normal. 20m, 80m, 15m all hit 100w no problem, ALC looks fine on those, but soon as i switch to 40m the power just falls off a cliff. ALC trace on the scope looks a little weird too, kind of overshooting then settling low instead of the nice flat line i get on other bands.

i already ran through the basic stuff, recalibrated power output through the menu, checked my feedline and dummy load, swapped antennas just to rule that out. issue follows the radio not the antenna so its definitely internal. been reading about PA transistor failures being band-specific but i dont know enough about the 7300 architecture to know if that even makes sense for this radio. anyone dealt with this before or have a schematic handy that shows how the PA stages are split by band?

yeah band-specific PA issues are absolutely a thing on that radio. the 7300 uses separate driver and final transistor pairs that are switched in depending on the band, so a partial failure in one of the RD70HVF1s or even just a bias drift on one of them will show up exactly like what you're describing — one band goes soft while everything else looks fine. the ALC behavior you're seeing kind of confirms it, the loop is trying to compensate but there's not enough headroom left in that stage.

before you pull it apart i'd check if the low power is consistent across the whole 40m band or if it's worse at one end. if it's worse at the low end near 7.0 it could also be a bad solder joint on one of the LPF switching relays rather than the PA itself. those relays take a beating over time and a cracked joint can cause exactly this. way easier fix if that's what it is. i'd poke around in there with some magnification before assuming you need a new transistor.

had something similar on my 7300 about a year ago, turned out to be one of the band filter relays like the other guy said. mine was 80m that went low but same symptom. icom service manual is floating around online if you dig for it, and the relay part numbers are in there. not a terrible job if you're comfortable with SMD work but those relays are tiny so if you've never done that kind of thing before maybe price out what a local repair shop would charge first.