[Elecraft] K1 works great on 40m, but no output on 20m

[David L DuPuy]

I've completed building all but the ATU on my K1, and operation appears to
be normal except for no output on 20m.  I'm hearing strong signals on both
40 and 20, the receiver sounds great.  On 40m it shows an output up to 5.7
watts.  I made a brief contact on 40m (rather poor band conditions).  On
20m, it will only go up to 0.4 watts (showing "P.4").

I'm using a G5RV and an external Palstar tuner for which I know the
settings for each band.  On 40m I'm showing SWR 1.0:1 and output somewhere
between 5 and 10 watts.  No output on 20m.  I have not yet tried to trace a
signal, but presume I could use my oscilloscope in place of the RF probe
described in the manual?  Any ideas on where to look for an incorrect or
missing part for 20m only?  Sounds to me like this problem has to be on the
filter board??  Many thanks... this reflector has been most helpful.
David  AC4BN