My Argosy 525 analog has a birdy at 21.320 also. I have replaced the main 2.4kHz filter with a 500hz cw filter and have a 250 hz cw filter in the second position. This is now a CW only rig 500 hz primary and 250 hz secondary plus the audio filters. The filters do attenuate the incoming signals. 73, Don, nn8b