[Elecraft] [K2] KNB2 vs. electric fence.

[NU7B]

Joe:

I just ordered a KNB2 and looked at DL7MAJ's modification. In looking at the
schematic, it seems like the shortest time constant for the one-shot is
above 10 µs. The DL7 mod noted that Q2 was being loaded by the base of Q3 -
possibly there is a load transient associated with charging C11,12. The
2222A has a fairly low beta, and Q3 has no RE, so its base current could be
fairly high. Raising the beta seems like it would reduce the loading on Q2.

I'll measure the beta on what comes with the KNB2. I've got 2n3904 (B=100)
and 2n5089 (B=400), so I may substitute. The 5089 has a lower fT of 50 MHz,
but for a 10µs pulsewidth, it should be fine.

Steve NU7B


Joe Planisky wrote:
> 
> Thanks for the responses, Dale and Vic.
> 
> I tried increasing C12 to 0.1uF, no change.
> 
> I tried turning on the preamp, again no change.
> 
> I did some research on the web and it seems the effectiveness of the  
> KNB2 has been the subject of some discussion in the past.  A handful  
> of folks even shared my suspicion that their KNB2 was not working at  
> all.   I was able to verify that my KNB2 was indeed working by  
> connecting a short piece of wire to the antenna jack and scraping it  
> against various metallic objects in the shack with the NB on and off.   
> (I tried the light dimmer trick, but I must have the worlds quietest  
> light dimmer.  Never thought that would be a problem!)
> 
> What did seem to help was the mod suggested by DL7MAJ:
> 
> http://www.dl7maj.de/KNB2-Mod.pdf
> 
> which adds a 0.1uF capacitor between the emitter of Q2 and ground.   
> This increases the low-threshold sensitivity.  Before, I never noticed  
> any difference between the LEVEL=LO and LEVEL=HI settings.  Now there  
> is most definitely a difference and the LO setting takes out at least  
> some of the pops.
> 
> Thanks again for your responses.
> 
> 73
> --
> Joe KB8AP
> 
> 
> On Aug 3, 2008, at 2:16 PM, Dale Boresz wrote:
>>
>> Turn on your preamp, and then try the NB.
> 
> On Aug 3, 2008, at 2:07 PM, Vic K2VCO wrote:
>>
>> Some electric fences produce wide pulses, so you should try using  
>> the NB2 setting. If that doesn't work, you might consider increasing  
>> one of the capacitors that determines the pulse width in the KNB2;
> 
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