[TenTec] Corsair II Mixer Balance Adjustment

[K4MT]

I am finishing the work on my Corsair II and I decided to do the mixer balance adjustment. 
The manual says to null the tone at 21,320 but I have no tone there. I adjusted the alignment points for mixer balance thinking it might be already nulled but the null seemed to good to be true. Adjusting the null capacitor and pot produce no tone at all. I hear a tone at 21315 (yes, my dial is calibrated properly). I can null that tone but it kills the receive in that entire portion of 15 meters from around 21250 to 21400.  Surely that is not the correct one. 
Is there another way to do mixer balance adjustment or do I have a problem on that board? The radio seems to perform properly otherwise.