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KNB2 Troubleshooting
This past week, my KNB2 failed after being
in the unit for a couple of years. Looking at the manual, troubleshooting
techniques were not included. Don, W3FPR, came to my rescue and gave me
some valuable advice on how to trouble shoot the device. I won't go into
the workings of the noise blanker, but one has to determine what has gone
wrong with the NB and there are several areas to check.
There are two main areas of the NB...the filter circuit and the noise
blanker circuit. Don gave me thetim of isolating the problem. You can
isolate each one by removing C8 to determine if the problem is in the
filter or the noise blanker circutry. In my case, it was the filter and
apparantly there was a bad solder joint that finally gave way after a
couple of years. All I had to do was to resolder all the pads in that
area of the board and the device came back to life. I have yet to determine
if the blanker part is working since I live in a very low noise area.
Gary Surrency sent me information on how to troubleshoot the noise blanker
and I want to share that to make sure the information is in the archives.
I searched the archives concerning repair of the noise blanker and got
very little information. Gary wrote:
Hi Lee,
Most of the time, the problem is due to a poor connection on the toroid
leads, a part installed wrong or the wrong part, or failure to do the
K2 RF board modifications that are necessary when the KNB2 is installed.
In some cases, I have found a bad 2N3906 transistor, U2 turned around
wrong, broken or reversed 1N34A diodes, etc. Look very closely at the
PCB and all parts to see if any of these things might be wrong. I just
repaired a KNB2 that had a short on the 6NB line, which caused Q7 and
U2 to overheat when the NB was turned on. I found the problem with an
ohmmeter. There was a tiny short on the 6NB line near Q7 that might have
been either a tiny spec of underetched copper, or a bit of solder etc.
Once I was sure no other part on the 6NB line was bad, I cleared the short
by connecting my test probes to my 13.8V power supply and then touched
them to ground and to a pad that is connected to the 6NB line. A tiny
spark was issued near Q7, and then the short was gone. So you have to
consider all possibilities when troubleshooting the circuit.
The NB takes a little investigation to insure it is working.
First, I assume the K2 can communicate with the NB, and you do not get
"Not Installed" whenever you press the NB button. And I assume
you have removed jumper wire W5 from the RF board, and also R88 is a 2.7K
resistor, R90 is removed, and R89 is replaced with a jumper wire. The
next thing to check, would be that the signal is getting thru the bandpass
filter and into the K2's crystal filter. If not, the the toroid leads
would be the first suspect thing, followed by possible poor connections
on either the NB PCB or the NB socket that is mounted to the RF PCB. You
can check to see if Noise Gate Q5 is blanking, by touching a resistor
of 1K to 2K to the base of Q5, with the other end connected to a 12V source.This
should turn ON Q5, and result in a large loss of signal, thus simulating
signal continuous blanking. Q3 and Q4 form a one-shot circuit, of differing
pulse widths based on the NB Menu selection of NB1 or NB2. With the NB
turned ON from the NB switch, pulling Q4's base to ground thru a 1k -
3.9K resistor should also turn ON Q5 hard, and cause a large reduction
in signal as when we forced Q5 ON in the previous paragraph. Pulling Q3's
base up with a 1K-2K resistor to 12V should turn ON Q5, and reduce the
signal as in the previous step. Pulling Q2's base to ground with a 1K-2K
resistor, should also reduce the signal strength greatly, as Q5 turns
ON. The only remaining checks, are that D2 is installed correctly, and
that it works. And of course, U1 (MC1350P) and its associated components
of L3, C13, R10, C14, and C17. Q1 keeps the signal level into U1 relatively
constant, by adjusting the voltage on the control control pin 5, which
is normally 2.9 to 3.0 volts. Pins 1,2, and 8 of U1 are at 6V, with pins
4 and 6 at about 1.84 to 1.85 volts, respectively. The IF signal with
noise pulses on it, are coupled into U1 with the 39pF cap at C16. U1 amplifies
both signal and noise, and D1 rectifies the noise pulse and uses it to
control the conduction of Q2-Q5 to blank the pulse duration. Longer pulse
blanking is made possible by Q6 switching in C12, and low or high blanking
threshold is made possible by changing the emitter voltage of Q2 thru
R3 and U3's pin 7 output. That pretty well covers the entire NB circuit,
so look for obvious things first and then do the tests outlined above.
--
73, Gary AB7MY
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