[Elecraft] KNB2-Noise Blanker...the answer

[Gregg Lengling]

First off I want to thank a couple people who sent me e-mails that
helped me troubleshoot my Noise Blanker.  Especially Sverre Holm LA3ZA
who gave me a test to see if it really was working.

First off the test to see if your noise blanker works, open the radio
(as per instructions and set something on the right side of the radio to
fold the lid/amp onto (I use a phone book).  
Power up the radio and make sure NB is off.
Right near the NB board is Q21 the RF Preamplifier, this is a metal can
transistor.  Use a screwdriver and scratch on the can and you will cause
static in the receive.  
Now turn the NB on and do the same, if the Noise Blanker is working
properly, the static will be removed or at least reduced in level.

Well it made no difference on mine, so I knew that something was wrong.
I know I asked the list for voltages to check stages, however no one
ever provided them, and to tell you the truth as long as you have the 6
volts and IC U3 the controller changes properly on pin 5 to turn on the
NB, it's pretty useless.

To properly troubleshoot the Noise Blanker you will actually need an
oscilloscope  to see the noise pulses and the blanker pulses.  The
blanker actually has a delay built in to allow the blanking pulses to be
added back in the IF stream at the same time the noise is sent back to
the receive stages, thereby effectively canceling them.  With the scope
you can follow the noise pulses into the RF amp (U1) and then into the
pulse amp (Q2) which will then trigger the one shot circuit (Q3 and Q4)
which is then fed to the noise gate (all of this has gain control for
lo/hi threshold using a FET (Q6).  Q5 the noise gate then adds the
canceling pulse back into the delayed IF stream.  And thus blanking is
accomplished.

Now what was wrong with mine.  Even an experienced
builder/engineer/technician can screw up and make a bad solder
connection and that's what I had.  After using the scope to trace the
pulses, I found I had the pulse on Q5 side of C8 but not on the RF out
side....as Homer Simpson would say...DOH.  Resoldered C8 and the blanker
started working.  Took the rig out to the car and got on 17 meters, all
quiet, turned the engine on and bango 20+ noise level.  Turned the
blanker on and the noise went away the S meter went dark...no noise just
signals.  Tuned in a weak station on 18.157 and listened to them, then
turned the NB off and gone they were.

We'll I'm a believer in the circuit now, works great.  At one point I
thought I was going to have to modify it or something, but once I fixed
my goof-up, that scenario went away, because it works great.

Great design out of the guys in Aptos...thanks again for the help and
hope this helps someone else out. (I know a few of you e-mailed you that
you found the NB useless...but I think you'll find you made a boo-boo in
construction...it really does work.)

73,


Gregg R. Lengling, W9DHI, Retired
Administrator http://www.milwaukeehdtv.org
K2/100 S#3075
Politics is the art of appearing candid and completely open, while
concealing as much as possible.   -States: The Bene Gesserit View