[Elecraft] KNB@ non-operation

[G3VVT at aol.com]

In a message dated 30/06/05 13:35:58 GMT Daylight Time,  
rhaendel1030 at inabc.net writes:

I also  see that Q5, the transistor that shorts out the output filter tank, 
has no  bias at all. There is no DC return for the base to ground.  Is this  
correct? Maybe  the base should not be floated?  Maybe Q5 is  open? 


Reply:-------------------------------------------------------
 
Rich,
 
Q5 on the KNB2 is either on or off, there is no in-between state so no bias  
is applied to the transistor apart from when it is switched on by the one shot 
 Q3/4. Possibly looking at the base and collector of Q5 with a scope would 
help  to see the spikes of enabling voltage and suppression of the segment of 
signal  during a short duration noise burst. The circuit details with the 
schematic on  the last page of the KNB2 manual should give an insight into the 
method of  operation.
 
The problem with checking most noise blanker is generating the necessary  
test conditions to see if the operation is correct. Most radio equipment  
manufacturers are rather vague in this respect and I have yet to find a piece of  
test equipment that will provide interference pulses on demand. Possibly an  
automobile spark plug driven by an ignition coil and contact breaker pulse  source 
may be the answer. Have tried using electric drills and other brush  motors 
for generating noise pulses with only limited success. Otherwise it is  
sometimes a case of awaiting a RF noisy auto to go past as the KNB2 only  functions 
on short duration spikes of noise.
 
Regards,
Bob, G3VVT
K2 #4168 with KNB2