Noise Blanker Malfunction

Good Morning Everyone....
 
My KNB2 has quit working...or...might be working all to well.  On FD, the signals on my K2 disappeared and the RX died.  I took the rig home after FD (I did not get to make one Q with the K2) and started looking at it with the email help of W3FPR.  Well, come to find out last night when I removed and jumpered the KNB2, the RX was good to go...better than ever...hotter than a pistol...smokin...well you get the picture.  With the KNB2 in the signals are reduced significantly...I can hear some loud signals....but they are very weak and there is not S-Meter or AGC action at all.
 
I cleaned the header pins and tried it again.  The RX was deaf.  I pulled it again, and put inthe jumper and...she was jumping off the table....hot RX again.  With the KNB2 in there was not S-Meter action....and now I have a lot of action.  The KNB2 is recognized by the K2 as installed when it is in...so that part of the circuit is working.
 
Reading the manual...and...reading the archives...I found very little information on troubleshooting the innards of the KNB2.  I understand the circuit...did some continuity checks with the DMM...I do not have a scope...and found nothing really apparent.  I plan to go through the KNB2 tonight and touch up each solder joint to see if that is what is wrong.
 
Does anyone have any info on how to troubleshoot the KNB2?  Or, should I buy a new one for $39 and not worry about it.  I hate to do that...but....
 
Lee Buller
K0WA
 


Common sense is in short supply - get some and use it.
If you can't find any common sense, ask for help from
somebody that has some common sense. - Lee Buller
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[Don Wilhelm-3]

Lee,

Pull out the KNB2 scheematic and follow along to identify the components
that may be at fault on you KNB2 board - there are only a few.

The signal enters on pin 1 and goes to the junction of R8 and R9 (check
these for soldering and value) - then it hits the parallel tuned circuit
consisting of L1 and C3 (make certain L1 has the right number of turns and
is well soldered and that C3 is 820 pf [821] and is properly soldered).  The
signal goes through c4 (100 pf [101]) and then on the the parallel tuned
circuit of L2 and C6 {1500 pf [152]) and C7 (1800 pf [182]), and then
connects to the output at pin 6.

The output at pin 6 also has capacitor C8 (0.01 uf [103]) in series with the
collector of Q5 shunted across it.  When Q5 conducts, the output signal is
shorted to ground, so make certain the base of Q5 is quite low under normal
conditions which should hold Q5 in the cutoff region.  If the base voltage
of Q5 is more than about 0.2 volts, the circuits driving the base are at
fault somewhere, but if teh base voltage is low, Q5 itself may be shorted.
To check for a shorted Q5, temporarily remove C8.  With C8 removed, the
bandpass filter should still be in the circuit and the K2 should receive
when the KNB2 is re-installed.  If the signal is still attenuated greatly
with C8 removed, the bandpass filter is at fault.

73,
Don W3FPR

> -----Original Message-----
>
> My KNB2 has quit working...or...might be working all to well.  On
> FD, the signals on my K2 disappeared and the RX died.  I took the
> rig home after FD (I did not get to make one Q with the K2) and
> started looking at it with the email help of W3FPR.  Well, come
> to find out last night when I removed and jumpered the KNB2, the
> RX was good to go...better than ever...hotter than a
> pistol...smokin...well you get the picture.  With the KNB2 in the
> signals are reduced significantly...I can hear some loud
> signals....but they are very weak and there is not S-Meter or AGC
> action at all.
>
> I cleaned the header pins and tried it again.  The RX was deaf.
> I pulled it again, and put inthe jumper and...she was jumping off
> the table....hot RX again.  With the KNB2 in there was not
> S-Meter action....and now I have a lot of action.  The KNB2 is
> recognized by the K2 as installed when it is in...so that part of
> the circuit is working.
>
> Reading the manual...and...reading the archives...I found very
> little information on troubleshooting the innards of the KNB2.  I
> understand the circuit...did some continuity checks with the
> DMM...I do not have a scope...and found nothing really apparent.
> I plan to go through the KNB2 tonight and touch up each solder
> joint to see if that is what is wrong.
>
> Does anyone have any info on how to troubleshoot the KNB2?  Or,
> should I buy a new one for $39 and not worry about it.  I hate to
> do that...but....
>
> Lee Buller
> K0WA
>

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