K2 w/100W amp has temperature related xmit shutdown

[wa4ywm]

I have a K2 serial no. 4729 with the 100W amp which I have been using for almost 10 years.

It recently developed a problem with the following 2 symptoms:

1. After operating for some time (perhaps 1-2 hours) the Tune function will begin to display 1 W with a 1.0-1 SWR on all bands. It normally displays 22-23W with the correct SWR shown.

2. At the same time, if I attempt to transmit, the K2 will apparently begin to draw too much current and it shuts down and re-starts. The Tune function also shuts the K2 down at this point.

I observed the same behavior on a dummy load - to eliminate an antenna or feedline issue.

I also left the power supply on while letting the K2 cool down to see if it was the source of the problem. Apparently not since the K2 would operate for a significant time after turning it back on.

I have not tried any other troubleshooting. I thought I would see if anyone could point me in the right direction before I took the shotgun approach.

TNX for any help & 73,
Jim WA4YWM

[Don Wilhelm-4]

Jim,

That sounds like the classic problem that is produced when the KPA100
diodes D16 and D17 succumb to static coming from the antenna.
The K2 has a loss of power control and will try to transmit at the
maximum power possible.
You would see that if you connect an external in-line wattmeter and set
the power knob to somewhere between 20 and 40 watts - I suspect the
actual power output would be in excess of 120 watts.  You will also
likely see HiCur messages - and that is causing your shutdowns.

You can 'take my word on it', or you can use an external wattmeter to
verify that you have no power control and the power output is always at
the maximum the K2/100 can deliver.

Replacement of D16 and D17 in the KPA100 is in order as the first step.  
There are other failure points in the KPA100 power output reporting to
the base K2 MCU that can cause similar responses (failure of KPA100 U6),
but the most common problem is D16 an D17 which can be damaged by static
charges on the antenna feedline.

If the problem does resolve to the failure of D16 and/or D17, you might
want to consider measures to prevent future failures.  Put a DC path
across each antenna feedline to bleed off static.  A non-reactive 5k to
50k resistor will do fine.  Then disconnect antennas from the K2 when
not in use - a switch to connect the K2 to a dummy load is ideal.

73,
Don W3FPR

On 7/5/2015 7:28 PM, wa4ywm wrote:

> I have a K2 serial no. 4729 with the 100W amp which I have been using for
> almost 10 years.
>
> It recently developed a problem with the following 2 symptoms:
>
> 1. After operating for some time (perhaps 1-2 hours) the Tune function will
> begin to display 1 W with a 1.0-1 SWR on all bands. It normally displays
> 22-23W with the correct SWR shown.
>
> 2. At the same time, if I attempt to transmit, the K2 will apparently begin
> to draw too much current and it shuts down and re-starts. The Tune function
> also shuts the K2 down at this point.
>
>

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