so this has been driving me absolutely nuts for the past month. I picked up an IC-7300 off a local ham who said it had 'minor issues' — yeah right — and the main problem is the output power just tanks randomly. like ill be running 100w into a dummy load, everything looks fine, then it'll just drop to like 8-10w for no reason and sometimes it comes back on its own after a few minutes and sometimes it doesn't.

already pulled the finals and replaced both RD70HVF1 transistors, reseated everything, checked the bias and set it properly per the service manual. problem is still there. SWR isnt the issue, ive got it on a dummy load when testing so thats ruled out. ALC looks normal on the scope when its behaving but i havent caught it misbehaving while probing yet because of course.

im starting to wonder if its thermal — like maybe a dry joint somewhere on the PA board that opens up when it heats up. or maybe the driver stage. has anyone seen this specific failure mode on a 7300 or know what to look for next. the previous owner definitely ran this thing hard based on the fan grill discoloration.

had almost this exact same thing on a friends 7300 last year. turned out to be a cold solder joint on one of the inductors in the LPF board, not even the PA itself. the joint looked fine visually but when we hit it with a hot air station and resoldered it the problem went away completely. worth going over the whole LPF section with a magnifying glass, those boards can look okay until you flex them slightly and then you see the crack.

also worth checking the driver board connections, there's a couple of ribbon connectors in there that can get loose especially if the radio was shipped or moved around a lot. i'd reseat everything you can reach before assuming it's a component failure.

the thermal thing is a real possibility. what i'd do is get it misbehaving and then use a can of freeze spray very carefully on different areas of the PA board to see if cooling something down makes it snap back. its a bit crude but it can help narrow down the area fast. also check the thermistor that feeds into the ALC circuit, if that's drifting it could be telling the radio to back off power when it shouldnt be. dont remember the exact part number off the top of my head but its in the service manual.