[Elecraft] Strange loss of output...

[Ron W]

Hi all,
         I wonder if anyone can suggest what might be wrong here.....here is
the scenario:
For a while now the power out on 20m has been low. Today I can max about 35
watts, so I took the long suffering K2 apart and went through the transmit
alignment again, put it all together. No change.
I then removed the KPA100 completely, powered the set from the old "QRP"
jack and looked again.
I can set the L8 and L9 for max power output on both 20 and 30m as
described. No problem there. I then peak C21 and C 23 for max power . Once
again no problem.
I can up the power to 5 watts and all bands show the same output on the watt
meter.
I set the power level to 10 watts and all bands show 10 watts (near enough)
*EXCEPT* for 20m which still shows 5 watts.  I just can't get 20m to give
any output above 5 watts.
I guess a problem like this reeks of dud soldering on a toroid (?) , but if
30m ,( which shares the LP and BP filters) is fine , then where?
Having posted this I will go over the connections of the BP and LP filters,
....and that means taking off that gosh-awful heatsink panel

Any ideas appreciated in the meantime.
Cheers......Ron ZL1TW