[Elecraft] K2 - low power on 160m?

[ON4WIX]

Dear all

I tried to send this question to the reflector yesterday but I haven't got any responses so maybe the message did not make it onto the reflector. One more try...

I finished building K2 s/n 5361 Tuesday and managed to align it on all bands 80-10m per the specs in the manual.
First tests (RX only) showed a wonderfully quiet receiver with "good ears". TX is well within specs, up to 15W power out.
Encouraged by the FB results I started constructing the 160m extension yesterday. No worries during assembly as far as I could see.
Then, after having installed the 160m PCB into the K2 I started the alignment procedure.With 2W out all was OK both on 160 and 80m, no problems peaking output power to about 2,5W. However, when I try to crank the output power up to 10W as per the instructions on p9 of the installation manual, I am unable to get more than 6W output on 160m. 80m is OK (up to 16W output).
I have tried the alternative frequencies for peaking. 2W out is OK all the time, but I get no more than 6W when I should be reading 10W out. Cal Cur is set at 3.50A. Checked all solder joints, toroid winding numbers, what have you.

Today I did a few additional checks.
DC resistance across the two toroids L1/L2 in series (measured between pins 9 and 14 on J1) is OK, very close to 0 ohms. DC resistance between the toroid sides of C1 and C3 is also about 0 ohms.

With 2W out on 1840 kHz (where I did the peaking of L3/4) I get 1,5W on 1810 and 2,2W on 1900 kHz.

C68 is 10pF, C153 is 68pF, C75 470pF and C13/14 are 1200 pF.

I did notice that the voltage and current readings during tx on 160m are not very stable, the readings change continuously. I listed the min and max values for E and I for a number of output power levels below:
P=0,1W: E=13,9/14,0  I=0,58/0,62
P=1W: E=13,6/13/9  I=0,88/0,96
P=2W: E=13,5/13/9  I=1,08/1,14
P=5W: E=13,2/13,8  I=1,54/1,64
P=6W (Power pot at max position): E=13,1/13,8  I=1,70/1,82

I checked on other bands but this phenomenon only occurs on 160m.

Where should I look? I'm fairly confident that the problem should be located somewhere in the 160m LPF as there is no sign of any problems on any other band. I checked the forum archives for clues. I did find an article pointing to the capacitor values mentioned earlier but I've got the correct values installed.

Any ideas?

Tnx for your help.

73 de ON4WIX Glenn