160 M Harmonic Problem with K2/100 Driving TenTec Titan

[Jim Brown-10]

I've recently been using a K2/100 to drive a Ten Tec Titan as a CW contest rig. I only
need about 25 watts drive to get 800 watts or more out of the Titan on most bands.

On 160 (and maybe on 80), the K2/100 goes into some sort of unstable condition that
causes it to overdrive the power amp and create a pretty strong second harmonic (a
ham from Wisconsin emailed me to alert me to it, and said I was about S6).  Some
testing has isolated it to the K2, thanks to a hint from Bill, NY9H. The Titan is clean as a
whistle driven to full power with my TS850 into the same antenna.  

I'm also seeing some hint of this when operating on 80, and need to investigate it
further.  I really want to fix this -- the K2 is my favorite CW contesting rig, and I really
need its DSP to fight my local noise on 160.

Some troubleshooting thoughts.

1) On 160, my only antenna is an 80/40 trap that I must load as a long wire against a
radial system (I'm on a city lot, and that's the best I can do). This puts lots of RF in the
shack.  I suspect this may be related, but maybe not. I also often load the antenna this
way on 80.

2) As I increase the drive from the K2/100, I hear instability as the control system shifts
from the barefoot K2 to kick in the K2/100 amp.  I can also listen to the harmonic on the
850 and hear it increase from barely audible (with no antenna) to quite strong when the
rig goes unstable.

3) The K2/100 runs about as warm at 25 watts as it does at 100 watts.

4) There are times that the instability in the K2/100 will cause the overdrive light in the
Titan to come on, indicating more than 120 mA of grid current, even though the stable
tuning was more like 10 mA or less. This is clearly coming out of the K2/100. This
instability is happening with roughly 20-25 watts of drive from the K2/100 and 500 watts
out of the Titan. The Titan is working into a big Ten Tec tuner, and a VSWR that is 1:1. I
recently replaced all the fixed caps in the tuner with 6 kV parts.  

5) I have observed some pin 1 problems in the K2, and have fixed one of them (at the
mic connector). Maybe I need to fix the one at the headphone connector too.

Any thoughts or experiences with any of this from the group?


Jim Brown  K9YC


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[n6wg]

Jim
Can you test your system into a dummy load instead of an antenna?
This would help determine whether it is truly rf feedback from the antenna
into the rig.
73, Bob N6WG
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For what its worth,
 
You might try to choke all cables connected to the K2.  Sounds like RF is getting is somewhere.  You might have such a high RF field in your shack because of your antenna system that it might be impossible to cure on the short term  Choking all the cables might be a start.  I use RG174 for all keying lines and such.  You might want to choke your headphones as well.  I have had trouble in this area too until I got a more resonate antenna installed on my city lot.  It is a true inverted-L...around 130 foot long.  You can use an UNUN to get to the right feed point impedance or use a tuning cap/inductor arraingement.  With your Titan (goooooood amp) the components would have to be pretty healthy.  As for the second harmonic, I was in a 160 meter contest several years ago where a guy came on and said we were 20 of S9 on 80 meters.  We were into a 1/4 wave verticle on 160 with a very good match.  I think that sometimes RXs are pretty good that --46 to -55 db down is not enough.  He
 ck, what
 do I know though?  These are just random thoughts. I've had a neighbor compain that when she was curling her hair with a curling iron (coil of hot wire) that she could hear me in her hearing aid!  
 
Lee - K0WA
 
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[Jim Brown-10]

On Thu, 2 Dec 2004 09:17:06 -0800, Bob Tellefsen wrote:

>Jim
>Can you test your system into a dummy load instead of an antenna?
>This would help determine whether it is truly rf feedback from the antenna
>into the rig.

Good question, and yes, I did that test yesterday to try to eliminate the antenna
system and the tuner from the list of possible causes. The K2/100 and Titan
seem quite stable into the dummy load.

Jim


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[Jim Brown-10]

On Thu, 2 Dec 2004 12:07:06 -0800 (PST), Lee Buller wrote:

>I think you have way to much RF in the shack if there is no trouble with the
>system when you have a dummy load hooked up.

Yes, I do have a lot of RF in the shack -- note the description of the antenna. It's
essentially an end-fed wire that works against "ground," so it MUST put a lot of RF in
the shack if it is going to work. In my case, "ground" consists of five driven rods, a
dozen radials of various lengths, and a big wrought iron fence (roughly 135 linear feet)
that runs around the front of my city lot. All of this is bonded together on a big bus bar a
6" below the operating desk, and bonded to each radio, the amp, and the antenna tuner
with 1" wide tinned copper braid.  

But radio gear should reject that RF if it is built properly.   When radio gear has a
problem with "RF in the shack," a common cause is what in the pro audio world we call
a "pin 1 problem," which is a mis-termination of the shields of input and output cables.  
A shield SHOULD go straight to the chassis of a radio (like a BNC or SO239 mounted
to the chassis). Unfortunately, several decades ago it became standard practice to cut
corners by mounting connectors to the printed circuit board and connecting the circuit
board to the chassis with mounting screws. This is "the pin 1 problem."  

Why is this construction a problem?  Simple. RF current flows on that cable (you call it a
mic cable, but mother nature knows that it's an antenna). That RF current flows along
circuit board traces to get to the chassis and to "ground" or whatever completes its
return path. Those circuit board traces have inductance, so there will be an IZ drop
across it due to the current flow. And that RF will get impressed on various parts of the
circuit, depending on where the circuit board layout puts it.  

The K2 has a pin 1 problem at the mic connector and the headphone connector --
neither of those connector shells touch the chassis if built according to instructions.  I
suspect that there is also a pin 1 problem at the RS232 connector, but haven't looked at
that yet. I've fixed the pin 1 problem at the mic connector.

BTW -- lots of ham gear has pin 1 problems, including all the TenTec gear I've looked
at.

Thanks to those who have responded re: instability in the K2/100 power amp stage. All
ideas are appreciated.


Jim Brown  K9YC


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[Stewart Baker]

> The K2 has a pin 1 problem at the mic connector and the headphone connector --
> neither of those connector shells touch the chassis if built according to
> instructions.  I
> suspect that there is also a pin 1 problem at the RS232 connector, but
> haven't looked at
> that yet. I've fixed the pin 1 problem at the mic connector.

>> Hi Jim,
>> I am interested in how you fixed the "pin1" problem on the K2 mic connector.
>> The only mod that  I am aware of to addresses this is connecting the socket
>> shell to the Front panel board earth plane. As you comment this can cause
>> an interference voltage to be developed across the board circuitry.

>> 73
>> Stewart G3RXQ

> BTW -- lots of ham gear has pin 1 problems, including all the TenTec gear
> I've looked
> at.
> Thanks to those who have responded re: instability in the K2/100 power amp
> stage. All
> ideas are appreciated.
>
> Jim Brown  K9YC
>
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[Jim Brown-10]

On Fri, 3 Dec 2004 10:27:46 GMT, Stewart Baker wrote:

>>> Hi Jim,
>>> I am interested in how you fixed the "pin1" problem on the K2 mic connector.
>>> The only mod that  I am aware of to addresses this is connecting the socket
>>> shell to the Front panel board earth plane. As you comment this can cause
>>> an interference voltage to be developed across the board circuitry.

I sent Stewart a photo of what I did. I can send it to others, but I don't think the list accepts
attachments.


Jim Brown  K9YC


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[Dan Barker]

I can host photos for list members. Just email me directly the photo, and
it'll soon be available at http://www.visioncomm.net/images/<yourPhotoName>.

Dan / WG4S / K2 #2456

PS: We also host the ECN website.
<snip>
I sent Stewart a photo of what I did. I can send it to others, but I don't
think the list accepts
attachments.

Jim Brown  K9YC
</snip>

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