so this has been driving me nuts for about three weeks now. my 7300 will be running fine, maybe 20-30 minutes into a session, and then the output just drops - not to zero, but like from 100w down to maybe 15-20w. SWR looks fine on the meter, antenna hasnt changed, and if i key up on low power (like 5w setting) it seems to behave normally for a while longer before doing the same thing.

first thing i checked was the thermal stuff, figured maybe the finals were throttling back from heat. but the fan is running, the heatsink doesnt feel crazy hot to the touch, and it happens even if i only ran it for a short burst before that. so im not fully convinced its a thermal issue anymore.

ive updated the firmware already, tried a different antenna, checked the coax - all the obvious stuff. ALC looks normal on the scope on the front panel. someone in another thread mentioned the driver board on these can develop a bad solder joint around the PA driver transistors but i havent opened it up yet. just wondering if anyone's seen this and can point me toward what to actually measure before i go poking around inside.

yeah the driver board thing is real, seen it a couple times on 7300s that are a few years old. the symptom you're describing - gradual power droop after warmup, not a hard shutdown - does fit a marginal solder joint on the driver stage more than a failing final. the finals tend to fail harder when they go.

before you open it though, worth checking the DC supply voltage under load. some of the regulated supplies start sagging a bit when the caps age and it can look exactly like this from the outside. put a meter right at the radio's power connector and key up, see if you're actually holding 13.8 or if it's dipping. if the voltage is solid then yeah i'd start looking at the driver board, particularly around Q1003 area if i remember the service manual right. get the SM off miklor or wherever and have a look at the PA section before you start prodding things cold.

had something similar on mine a while back, turned out to be the RF power control pot had gotten flaky. not exactly the same symptom but worth wiggling the power knob slowly while transmitting into a dummy load and watching the meter - if you see it jump around that might be it. probably not your issue given what you described but its a quick thing to eliminate before tearing into the board.