so this has been bugging me for a couple weeks now and i finally have time to sit down and really dig into it. my 7300 has been acting weird on transmit — starts out fine, maybe 80-90 watts on SSB, but after about 3-4 minutes of moderate use it just starts tapering down. like not a sudden drop, more like it slowly loses its will to live and ends up around 20-30 watts. receive is completely unaffected the whole time.

first thing i checked was ALC and that looked normal, and the SWR on my dummy load is flat so its not an antenna issue. fans spin up when it should, doesnt feel like its overheating to the touch. i havent pulled it apart yet but im wondering if anyone has seen this pattern before — the gradual rolloff specifically. wondering if its thermal related even if it doesnt feel hot, or maybe something in the PA stage starting to go. radio is about 4 years old, not abused, mostly used for digital modes and some SSB contesting on weekends.

any thoughts before i start probing around inside would be appreciated, dont want to just start replacing things randomly

that gradual rolloff after warmup is almost always thermal — even if the outside of the radio feels okay the PA transistors themselves can be running way hotter than the case suggests. the 7300 uses RD70HVF1 finals and they do have a history of getting flaky when they start to age, especially if the thermal paste between the device and heatsink has dried out. i had basically the same symptom on a friends radio last year and reseating the finals with fresh Shin-Etsu paste knocked it back to full power.

before you do that though, check the PA board connector — theres a multi-pin connector that feeds the PA and sometimes the pins get a bit corroded or loose and cause weird intermittent power issues. way easier to check first. if you have a thermal camera even a cheap one, transmit for a few minutes and look at the PA area, you'll see pretty quickly if one transistor is running hotter than the other.

could also be the ALC feedback loop doing something weird — i know you said it looked normal but what does it look like during the rolloff, is it climbing as the power drops? if the radio thinks its getting too much RF back for some reason it'll throttle itself down. had a cap in the ALC circuit go soft on an older Kenwood and it did something similar, took forever to figure out because the symptom looked thermal but wasnt.

also worth checking if you have the RF power control set to max or if something got changed in the menu settings, ive seen people accidentally bump the output setting and then spend days debugging what was basically a user error lol. not saying thats your problem just worth ruling out quick