so this has been bugging me for a couple weeks now and im finally sitting down to actually figure it out. my IC-7300 is doing something weird where on 40m the output power just drops way off. like im setting it to 100w and the wattmeter is only seeing maybe 30-35w. all other bands seem totally normal, 20m 15m 10m 80m all read fine. SWR isnt going crazy or anything, dummy load shows the same thing so its not an antenna issue.

i ran the built in scope and the waveform looks clean, no obvious distortion that i can see. ALC seems to be behaving normally. ive already done the touch screen calibration thing just in case but that obviously wouldnt affect this. been googling around and some guys mention a low pass filter board issue but i havent cracked it open yet because i wanted to see if anyone here had seen this specific symptom before i start poking around inside.

radio is about 4 years old, bought it used so i dont know the full history. wondering if its worth sending to Icom or if this is something diagnosable at home with a decent scope and some patience.

yeah that band-specific power drop is almost always going to be the LPF board or something in the driver stage feeding that particular filter. the 7300 has a switched LPF arrangement and if one of those filter paths has a bad solder joint or a failed component it'll show up exactly like what youre describing. 40m and 15m share some of the filtering architecture if i remember right so if 15m starts going weird too that'd confirm it.

before you send it anywhere i'd pull the top cover and just do a visual inspection under good light. look for anything obviously burned or any cold solder joints near the LPF switching relays. those little relays are known to have issues on older units. a reflow on the relay pins sometimes fixes it, ive seen that work on a couple of radios. if you have a scope you can trace the RF through the stages and figure out exactly where the signal is dropping off. its not a hard job if youre comfortable with RF circuits, just take your time and dont go probing around with the power on unless you know what youre doing around live RF stages.

mine did something kinda similar but it was on 20m and it turned out to be one of the PA transistors was marginal, not totally dead but running weak. just a thought, might not be the filter at all. does it run hot on 40 compared to other bands when youre transmitting? that was my first clue something was off in the PA stage.