Howling/Ringing in OMNI VI 500Hz Filter

[hlserra@pwa.acusd.edu (H. L. Serra)]

Thought I'd toss this one out to the OMNI VI users: I have one of the
early OMNI VIs, and it has always displayed filter "ringing" ONLY in the
500Hz filter position in CW mode. None of the other filters do this. Also, in
500Hz, when I use the PBT control, a continous "howling" occurs which
changes slightly in pitch as the PBT is tuned. Any similar experiences or
fixes? 
73, Larry N6AZE