[Elecraft] Strange behaviour in K2

[Geoffrey Mackenzie-Kennedy]

Hello Hans,

The noises which you are hearing on 40m as you tune appear to be very much 
like the noises that my K2 #3255 produces, especially above 7100 kHz when 
the BC stations are strong, or with a signal generator connected to the 
antenna jack. I had started to look into ways to get rid of the problem but 
have not had the time yet to complete the job.

I am reasonably certain that the root cause of the problem is the lack of 
shielding around the VCO and the 12.xxx MHz Reference oscillator, and / or 
the lack of RF filtering of the DC and control lines which enter the area. 
It can be demonstrated that a strong received signal will get into the VCO / 
Reference area, and at signal frequencies in the 40m band as the result of a 
process of mixing a product can attempt to take over the job of the 
reference oscillator or certainly disturb the VCO loop causing 'clicks' as 
you tune. Sometimes my K2 will lock onto a strong BC signal and only lets go 
with a 'whoop' after I have tuned away from the signal a kHz or so - how far 
depends on the strength of the BC or generator signal and the frequency.

Although I am unable at this time to suggest a simple cure for the problem, 
and a cure might not be simple, I hope that the above will help in some way. 
In Northern France my K2 does not 'click' on 40m because the level of BC 
signals received there are considerably lower than those received in this 
part of Scotland, here the individual carriers can reach -10dbm to -5dbm.

73,
Geoff
GM4ESD


Hans Ostnell  <la2moa at hotmail.com> wrote:


Hi group!

I have just finished the alignments and tests (part II) in my (second) 
K2-project. Sofar all resistance checks are ok, and the results of the 
alignment and tests have also been ok. Except for one thing: I think that 
the AGC in the radio behaves rather strangely. It's pretty nervous in the 
"Fast" position and switching to slow makes it a little bit better, but not 
perfect. I have also noticed a "chuffing" or "clicking" sound in my 
heaphones while tuning the radio aound on 40 metres. This sound (which 
indeed is very difficult to describe) corresponds with the change of digits 
on the display (regardless of tuning rate) and is very significant with the 
appearance of a strong signal. Tuning the BC-band above 7100 KHz is very 
annoying. The behaviour disappears when the antenna is disconnected, but 
occours again when connecting a signal generator or other RF-source to the 
antenna jack.

Measuring the AGC-treashold voltage on pin 5 of U2 (control board) gives a 
range from 2.65-3.74 volts when adjusting R1 from counter-clockwise to 
clockwise. The recommended voltage is 3.80 volts, which I never reach. I've 
checked the control board very carefully, looking for solder bridges, cold 
solderings, incorrect components etc, but everything seems to be fine. I've 
even re-heated all solder joints on the control board without success.

Any ideas, anyone?

Regards,

Hans, LA2MOA
K2 #6341 - unfinished