I have built an SSB filter (k2 ssb opt. like).... it is working fine on my K1 #1423. This filter is fitted on a little board inserted into Noise blanker connector. To switch the filter, i use the XFIL/Tone voltage and a voltage comparator. When i select, on the front panel, the Filter 1, the SSB filter is switched on. If anyone is interested i send more details. 73 to all K1 gang Giuliano I0CG