so this has been driving me absolutely nuts for about three weeks now. my 7300 will just cut out mid transmission, not like a full shutdown, more like it drops to like 5 watts for a split second then comes back. sometimes it doesnt come back and i have to cycle power. happens more on 40m than anywhere else but ive seen it on 20 too.

already checked the power supply — its a Rigrunner setup going to an Astron RS-35M, voltage holds steady at 13.8 under load, i put a meter right on the radio terminals. fan is spinning, no thermal shutdown codes that i can see. tried a different coax and different antenna just in case it was a swr spike doing something weird but same deal.

the weird part is it seems worse after the radio has been on for maybe 45 minutes or so. like it runs fine for the first chunk of a session then starts acting up. makes me think thermal but i cant find anything obvious and the finals dont seem to be getting that hot. anyone dealt with anything like this before or have ideas where to start poking around?

that warmup pattern is a pretty classic sign of a failing electrolytic cap somewhere in the power chain or the PA board. they drift out of spec as they heat up and cause all kinds of weird intermittent stuff. had basically the exact same symptom on an old 746 years ago, turned out to be a cap on the driver board that looked completely fine visually — no bulging or anything — but was way out of value when i pulled it and measured it.

if you're comfortable with a scope you can sometimes catch it by probing around the supply rails while its acting up, look for ripple that shouldnt be there. otherwise it might be worth just reflowing the solder joints on the PA module connectors first since thats free and occasionally does fix intermittent power issues. icom's service manual for the 7300 is floating around and the PA section is pretty well documented. if it ends up being the finals themselves thats a different conversation but id rule out caps and connections first.

random thought but have you checked the ALC loop? i had a friend with a similar thing and it turned out his logging software was sending weird CAT commands that were interacting with the ALC in a funky way. probably not your issue but worth disconnecting the usb and running totally standalone for a session just to rule it out.

also 40m being worse — are you running more power on 40? if youre pushing closer to 100w on that band vs like 50 on 20 that could point toward a thermal thing with the finals even if they feel okay to the touch. the internal sensors might tell a different story than what you can feel.