so this has been driving me nuts for about three weeks now. my 7300 runs fine at full power for maybe 4-5 minutes then slowly starts dropping output, like it'll be doing 100w and then i watch the meter creep down to maybe 60, sometimes lower. if i let it sit for 20-30 minutes it comes back and does the same thing all over again.

at first i figured it was just thermal throttling doing its job but i put a fan blowing directly on the heatsink vents and it still does it at basically the same rate. ambient temp in the shack is maybe 72F so its not like im running it in a sauna. ALC looks normal, SWR is fine, antenna hasnt changed. i even swapped to a dummy load just to rule that out.

ive been poking around online and some people are pointing at the PA transistors starting to go, others say its a bias drift thing, one guy on another forum was convinced it was a bad solder joint on the driver board but that seemed like a guess. i havent cracked it open yet because honestly im a little nervous about voiding anything, though i think its out of warranty at this point anyway. anyone seen this exact pattern before?

yeah that pattern - works fine then slowly fades - that's pretty classic thermal runaway starting in one of the PA devices, or less dramatically, one of the bias trimmers has drifted. the fact that a external fan didn't change the behavior much is actually kinda interesting because real thermal throttling from the protection circuit usually kicks in pretty hard and fast, not a slow creep like you're describing.

if you're comfortable with a meter, crack it open and look at the PA board. on the 7300 its not too bad to access. check your quiescent current after it's been running for a few minutes versus cold - if the bias is drifting up with temperature that's your culprit and its an adjustment, not a parts swap. icom has a service manual floating around, the bias adjustment procedure is in there. i'd do that before assuming the finals are toast because new finals for that radio are not cheap and you don't wanna go there if you don't have to.

also worth checking the thermal compound between the PA transistors and the heatsink while you're in there. ive seen dried out compound cause all kinds of weird intermittent stuff like this.

had almost the exact same thing on my 7300 about a year ago. mine turned out to be a cold solder joint on the driver stage, not the finals at all. took forever to find because it only showed up when things warmed up. ended up being one of those invisible cracks you can only see under magnification with good lighting at an angle.

not saying thats definitely your problem but i wouldnt assume its the PA right away. reflowing some of the joints on the driver board cost me nothing and fixed it completely. been running fine since.