[Elecraft] K1 TX output low on 2 of 4 bands

[David Ek]

My K1 (#1066, built 11 years ago) is a four-bander with ATU and internal 
battery pack. In May I built an Elecraft W1 wattmeter and used it to 
discover that my TX output (When "Out" in the menu is set to 7.0W) is 
only 1.2W on 15 meters. At that time, all other bands showed an output 
of 5W.

However, last night, after an afternoon of using my K1 while fruitlessly 
trying to tune up a new antenna, I noticed that the output on 20 meters 
was also now down, measuring 1.5W. I strongly suspect that a component 
failed due to TX'ing into high SWR while trying to adjust the antenna I 
was working on. 40 and 30 meters still seem to be okay. (My guess is 
that 15 is low for the same reason--I just didn't discover it in a 
timely fashion like I did with 20 meters).

So, anybody have a suggestion for where to start with troubleshooting? 
Should I suspect the filter board? Any components in particular?

Thanks in advance for any morsels of wisdom -

Dave NK0E