19 July, 2020

Tweaking the K2 for digital modes

My Elecraft K2 which I assembled as a kit in 2001, has mainly been a CW rig, but now I wanted to use it more for digital modes like FT8 also. That required some tweaking:
  1. Wider bandwidth in SSB filter, so I took it from 2.4 to 2.6 kHz (KI6WX)
  2. More sensitive VOX (G3RXQ)
  3. RF-filtering on audio input (G3RXQ)

All of these are modifications to the tighly packed KSB2 board shown above so it took some patience to implement them. The bandwidth modification consists in replacing six ceramic capacitors that were sitting between the crystals. The VOX sensitivity modification is done by adding a pot which can be seen in the image as it sits outside the PCB on the left-hand side. I was lucky to find a mini 5k slider pot in my junk box which fitted in. Finally the RF-filtering was done by adding an inductor of 2.5 mH (right below the 5k pot) and a decoupling capacitor.

All of these modifications can be found in the KSB2 section of the guide to modifications.

The barebone K2 has a limitation with continuous transmission and the manual says:
  • Since some RTTY/data mode duty cycles approach 100%, you should reduce power to about 5W or avoid transmitting at 10W for longer than 1-2 minutes at a time. (Power levels above 10 W are not recommended.) 
  • Duty cycle 5 W, 100%; 10 W, 50% 
This should mean that it can do FT8 with its 12.6 seconds of transmission every 15 seconds at 10 W. 

The 100 W option, the KPA100, is more restricted and the manual says:
  • The maximum recommended power for PSK31 mode is 40 W, and for higher duty cycle modes such as MFSK, 25 W.
  • Key-down time: At full power (100 W), 10 sec max. recommended
The KPA100 can do FT4 at near 100 W it seems since it only transmits for 4.5 seconds, but not FT8.

Finally, changing the SSB bandwidth meant that the BFO frequencies had to be set up again. It was fun to do this again as I haven't done it for many years now. Using the nice Java program from AB3AP, K2Params, for reading the K2 Control parameters, I was also able to read out out the new values for backup. Some of them follow here. The "RTTY rev" mode is the preferred one for FT4 and FT8.

Filter settings:
  CW:       FL1 1.00C   FL2 0.70C   FL3 0.40C   FL4 0.10C   
  CW rev:   FL1 1.00C   FL2 0.70C   FL3 0.40C   FL4 0.10C   
  LSB:      FL1 OP1L    FL2 OP1L    FL3 1.80L   FL4 OFFL    
  USB:      FL1 OP1U    FL2 OP1U    FL3 1.80U   FL4 OFFU    
  RTTY:     FL1 OP1R    FL2 2.00R   FL3 1.80R   FL4 1.60R   
  RTTY rev: FL1 OP1R    FL2 2.00R   FL3 1.80R   FL4 1.60R   

BFO settings:
  CW:       04913.33C   04913.31C   04913.22C   04913.14C   
  CW rev:   04914.46C   04914.44C   04914.25C   04914.20C   
  LSB:      04913.49L   04913.71L   04913.45L   04913.54L   
  USB:      04916.66U   04916.45U   04915.62U   04916.10U   
  RTTY:     04913.55R   04913.59R   04913.59R   04913.59R   
  RTTY rev: 04916.64R   04915.97R   04915.62R   04915.44R   

This post first appeared on the LA3ZA blog.

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