so this has been bugging me for a few weeks now. picked up a used 7300 off a guy at the club, he said it was working fine but honestly I think he knew something was off. anyway when I first fired it up it was doing full 100w no problem, maybe used it for a month or two and then one day just... 20w. sometimes less. the ALC meter looks normal, no fault codes showing, SWR on the dummy load is flat so its not a matching issue.

I popped the lid and had a look around, nothing obviously burnt that I can see, no smell. I did notice one of the finals (RD100HHF1 MOSFETS) looks like it might have a slightly discolored area near the drain lead but I could be imagining things. bias voltage checks looked okay from what I could read but I was kind of rushing that part.

before I go tearing into the PA board I wanted to see if anyone else has seen this kind of gradual power drop on a 7300. is it worth trying to swap just one FET or do you usually do them in matched pairs or all four at once? also not sure if icom has a specific bias adjustment procedure after replacing or if it self-calibrates somehow. I have a decent soldering setup, hot air station etc so the work itself doesnt scare me, just dont want to go replacing parts that arent the problem.

yeah the RD100HHF1s can definitely go soft like that before they fully die, you won't always get a dramatic failure. the discoloration you're seeing near the drain is worth taking seriously, that's usually heat stress. I'd pull that one first and check it out of circuit.

on the icom bias procedure -- it's not self calibrating, you do need to set it manually after swapping finals. the service manual has the steps, basically you're adjusting the idle current per device. if you don't have the SM already grab it from the usual places. and yes I'd replace them in matched pairs minimum, some guys do all four just to be safe but honestly if the other three test fine and have similar hours on them you could probably get away with just the bad one plus its pair. just make sure you source good parts, there's a lot of fakes floating around for those MOSFETs.

had something simlar on mine a while back, turned out to be a cold solder joint on one of the driver stage components not the finals at all. worth poking around that area with a magnifying glass before you commit to new parts. the 7300 runs the driver pretty hard and that board sees a lot of thermal cycling. just something to rule out, could save you some money and headache.