[Elecraft] troubleshooting the KNB2

[Don Wilhelm]

Brian,

The K2 noise blanker works well on impulse noise - such as gasoline 
engine ignition noise, but it will not do much for powerline noise.

One of the best checks for KNB2 operation is to turn the preamp on and 
tap on the case of Q21 with a metallic tool such as a screwdriver.  
Turning the NB on should reduce the induced noise substantially.

73,
Don W3FPR

Brian Lloyd wrote:
> Sorry to have been away for so long. I've been busy.
>
> My students (5th-8th grade) and I have finished the K2, finished the 
> KNB2, almost finished the KSB2 (I hope we will have that done today), 
> and haven't started on the KPA100 yet. K2 works great. Lovely receiver!
>
> But, it appears that the KNB2 is doing nothing. Our school sits 
> practically under a 240KV power line. We have constant pulse-type 
> power line noise that kicks the noise floor up about 6dB-8dB on 
> 80M-30M. The NB on our IC-706MkII takes it right out. The KNB2 appears 
> to do nothing.
>
> For testing I have been using my heat-gun's electric motor as a noise 
> source. Noise is plainly audible. NB appears to have no effect.
>
> What I have determined:
>
> 1. 6V good (5.98V);
> 2. diode are good (conduct in proper direction) and installed correctly;
> 3. threshold line switches properly;
> 4. Pulse width line switches properly;
> 5. blanker on/off line switches properly;
> 6. 6NB switches properly;
> 7. U1 is showing good gain between input (pin 4) and output (pin 8);
> 8. No blanking pulse on GATE.
>
> Since the receiver works with the KNB2 installed, that implies that 
> the BPF is OK. Gain at U1 implies U1 is OK. All the control lines 
> switch which implies that U3 is OK.
>
> Visual inspection shows no improper component placement. Diodes, 
> transistors, and ICs are oriented correctly.
>
> I have an HP 180A analog scope. I see signal (how I determined U1 has 
> gain) but haven't had success getting it to trigger on the noise pulses.
>
> So, anyone have any suggestions for further trouble shooting?
> -- 
>
> 73 de Brian, WB6RQN
> Brian Lloyd - brian HYPHEN wb6rqn AT lloyd DOT com
>
>
>
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