K1 Builder Alert #4: RX Noise from KNB1
13 June 2001 

Impact: All KNB1s shipped before August 2001.


The KNB1 noise blanker can be a source of audio noise during keying under certain conditions. This is most often noticed on the 20m or 17m bands, around 30 to 50 kHz up from the lower edge of the bands. This document describes a simple modification to insure this does not occur.

Technical details:  

The 6R/AUX line goes low and high when the K1 is keyed. The 12C509 KNB1 PIC normally sleeps except when it is sent an auxbus command. It interprets the keying transitions on the 6R/AUX line as a possible command. In doing so, the microcontroller exits sleep mode and briefly turns ON its on-chip RC oscillator. The added .01uF capacitor suppresses oscillator harmonics on U1's Vdd pin that can enter the RX signal path.


  1. Remove the KNB1 from the K1. 

  2. On the bottom side of the KNB1 PCB, install a small .01uF disk or monolithic cap across pins 1 & 8 of the 12C509 NB PIC, U1. Use as short of leads as possible, and press the capacitor flush against the bottom of the PCB with the cap centered between the two rows of pins for U1. 

  3. Reinstall the KNB1.

  4. Test the KNB1 to be sure it works as before, and check to see if any noise is now heard across the tuning range.