KPA100 Behaving Badly...

[John Huffman-3]

I'm trying to fix a friend's K2/100.  I realigned it, CAL PLL, CAL Filters.

It seems to work fine on QRP.  But, when I strange things happen when I
go to about 30 watts and above.

Symptoms - Worked a station at 14.022 at 10 watts.  All is well.  Tune
up or down the band.  Raise power over 30w.  After a few letters on CW,
the K2 mutes, the display flashes and I'm back on 14.022.

Attempted to tune the external manual tuner on 40m.  SWR on the K2
display and external meter is 1:1 when tuned at low power.  Reflected
power is off the scale of both meters when the KPA100 is activated.

There is no menu PA E___ error code.

On 20m the SWR is 1:1 at both high and low power.

At this point I'd like a little guidance before trying to collect more data.

Any help is greatly appreciated!

73 de K1ESE
John
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[Don Wilhelm-4]

John,

The resettable fuse in the base K2 is tripping - it trips at 5 amps, so
your base K2 is drawing too much current for some reason or another.
The reason for high current along with low output power is typically a
problem with the Low Pass filter or T4.  But there are two LPFs involved
here - one in the base K2 and another in the KPA100.  So physically
remove the KPA100 and check the base K2 first.  Check  the base K2 at
maximum power - with CAL CUR set at 3.50 Amps, you should not see HI CUR
messages - well maybe you will see them on 40 meters, 80 meters and 160
meters if the actual power output is in excess of 16 watts, but make
certain you can do a TUNE at 12 or 13 watts without the HI CUR (check on
all bands).  If it passes that test, then look for the problem in the
KPA100.

Now one other item that you should understand while doing this
particular troubleshooting is how the K2 controls the power output - if
the power output sensed by the K2 is not up to the level requested, then
the microprocessor will increase drive in an attempt to make more power
- and that can cause the base K2 to draw excessive current.  That means
if the wattmeter in the KPA100 is not properly balanced and calibrated,
it can cause an indication of low power output when in fact the power is
quite high - use an external wattmeter to check for that condition - if
the actual power is high, then look at the KPA100 wattmeter for any
faults and be sure it is balanced and calibrated.

OTOH, if the wattmeter is functioning properly, AND the base K2 passes
the prior tests, you can possibly have a problem with the KPA100 low
pass filter, or you can have a problem with the T/R switch.  In a rare
instance, it is possible that one of the PA transistors has 'lost its
smoke'.
A LPF problem will usually show up on only 1 or 2 bands (the 2 bands
using the same LPF).  A T/R switch problem can produce a parasitic
oscillation - that usually will not show up when operating into a dummy
load, but will show up when working into a resonant circuit (like an
antenna or an antenna and tuner).

It is also possible that KPA100 Q6 and Q7 are not working properly which
can turn the KPA100 into a big oscillator at about 860 kHz.

If you do find evidence of a parasitic, and if the KPA100 has the old
T/R switch design (with red toroids at L15 and L16, the usual cure is to
add the KPA100MODKT.  If the T/R switch is the newer design with blue
toroids at L16 and RFC1, then check the value of C31 - if it is .033 uF
(33n), it should be replaced with a .22 uF capacitor.

That is most of the 'normal' list of things that could be wrong and
cause your symptoms.  I hope that helps.

73,
Don W3FPR

John Huffman wrote:

> I'm trying to fix a friend's K2/100.  I realigned it, CAL PLL, CAL Filters.
>
> It seems to work fine on QRP.  But, when I strange things happen when I
> go to about 30 watts and above.
>
> Symptoms - Worked a station at 14.022 at 10 watts.  All is well.  Tune
> up or down the band.  Raise power over 30w.  After a few letters on CW,
> the K2 mutes, the display flashes and I'm back on 14.022.
>
> Attempted to tune the external manual tuner on 40m.  SWR on the K2
> display and external meter is 1:1 when tuned at low power.  Reflected
> power is off the scale of both meters when the KPA100 is activated.
>
> There is no menu PA E___ error code.
>
> On 20m the SWR is 1:1 at both high and low power.
>
> At this point I'd like a little guidance before trying to collect more data.
>
> Any help is greatly appreciated!
>
> 73 de K1ESE
> John
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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[Bill Coleman-2]

On Nov 17, 2009, at 11:56 AM, John Huffman wrote:

> Symptoms - Worked a station at 14.022 at 10 watts.  All is well.  Tune
> up or down the band.  Raise power over 30w.  After a few letters on CW,
> the K2 mutes, the display flashes and I'm back on 14.022.
>
> Attempted to tune the external manual tuner on 40m.  SWR on the K2
> display and external meter is 1:1 when tuned at low power.  Reflected
> power is off the scale of both meters when the KPA100 is activated.

I see this problem on certain parts of certain bands when using certain antennas. I believe it is caused by RF getting into the K2. In the first scenario, it is causing a reset of the rig, much like it just powered on. In the second scenario, it is interfering with the computer dialog between the microcontrollers between the K2 and the KAT100. Perhaps just resetting the KAT100.

If I switch to other antennas / frequencies the problem will go away. It will also go away if I reduce power somewhat.


Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
Web: http://boringhamradiopart.blogspot.com
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901

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