so this has been bugging me for a couple weeks now and i cant figure it out. my 7300 will start a transmission at full power (running about 90w into a dummy load to keep things simple) and then somewhere between 10 and 30 seconds in it just drops to like 20-30w and stays there until i unkey. keying up again it goes right back to full power, then drops again after a bit.

already went into the menus and watched the ALC meter and it doesnt look like its hitting the ceiling, and the finals temp according to the radio looks fine, not even close to where it should be throttling back. tried two different cables and a different mic just to rule out something stupid. happens on every band ive tried so its not like a filter issue or anything band specific.

one thing i noticed is it seems to happen faster when i run higher duty cycle stuff like JS8 vs just voice, which kinda makes me think thermal somewhere but the radio itself isnt reading hot. wondering if maybe a driver stage component is drifting when it warms up, or possibly something in the PA bias circuit. anybody seen this before or have a idea where to even start poking around on this thing?

yeah that pattern sounds a lot like a thermal drift issue in the driver stage, not the finals themselves. the 7300 has that intermediate driver board and there have been a few cases where the bias resistors on there start to drift as they warm up which causes the driver to back off and the PA never gets the full drive it needs. the radio's thermal protection is monitoring the heatsink on the final transistors so if the problem is upstream of that it wont see anything wrong.

if you have a scope you can probe the driver output and watch it over time to see if the signal level is actually dropping there or if its further downstream. if you dont have a scope, honestly your best bet might be to pull the top cover and carefully feel around the driver board area after you've reproduced the fault -- if somethings running surprisingly hot thats a hint. icom's service manual for the 7300 is floating around online and the driver board schematic is actually pretty readable. wouldnt start replacing parts blind though, at least try to narrow it down first.

had something similar on an older 746 pro once, turned out to be a cold solder joint on one of the driver transistors that only showed up under thermal stress. took forever to find because visually it looked fine under normal light, only saw it when i used a magnifying glass and moved the board a bit. not saying thats definitely your problem but worth keeping in mind that it doesnt have to be a failed component, sometimes its just a marginal connection that opens up slightly when things expand a little from heat.

also -- did you check if its doing it in the internal tuner on or off? probably doesnt matter since youre using a dummy load but just covering bases here