K2/100 power problem

[william stewart-3]

I am trying to repair a K2/100 that I believe was damaged in shipment to
me. The power output was erratic - sometimes almost no power out,
sometimes about half power. I investigated the SWR circuitry in the
KPA100 and replaced R29, which was discolored but still had the correct
resistance. After carefully reassembling the unit, the power output is
now greater than maximum rated power regardless of the power setting
(just keyed momentarily and measured with a peak-reading wattmeter).  
The resistance from either side of R28 to ground is about 92 ohms, the
same for the cathodes of D16 and D17. Is this correct? Is U5 blown?

Thanks,

Bill K5EMI
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[Don Wilhelm-4]

Bill,

The resistance measured to ground on the cathodes of D16 and D17 should
be in the vicinity of 100k ohms when measured in circuit (you should be
actually measuring the resistance of pots R26 and R27) .
The behavior you are describing is typical if those diodes are not
functioning as diodes.  The K2 thinks there is no power output and
increases the drive to maximum in an attempt to obtain the requested
power output.  Typically under those conditions, the KPA100 will develop
150 to 180 watts - not good for the longevity of the PA transistors.

Replace the diodes, then null the wattmeter (C1 adjustment - see the
manual) and then re-calibrate the setting of R26 to make the K2 display
match your external wattmeter.  Set R27 to match unless you have a load
that will produce a known SWR.  The KPA100 manual will tell you how to
do the calibration (see page 48 for the details, and I recommend the
Optional SWR fine adjustment), then go to page 52 and do the optional
calibration at higher power.

73,
Don W3FPR

On 3/24/2013 7:08 PM, Bill and Gail Stewart wrote:

> I am trying to repair a K2/100 that I believe was damaged in shipment
> to me. The power output was erratic - sometimes almost no power out,
> sometimes about half power. I investigated the SWR circuitry in the
> KPA100 and replaced R29, which was discolored but still had the
> correct resistance. After carefully reassembling the unit, the power
> output is now greater than maximum rated power regardless of the power
> setting (just keyed momentarily and measured with a peak-reading
> wattmeter).  The resistance from either side of R28 to ground is about
> 92 ohms, the same for the cathodes of D16 and D17. Is this correct? Is
> U5 blown?
>
> Thanks,
>
> Bill K5EMI
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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[Don Wilhelm-4]

Bill,

That kind of damage is not typical of shipping damage.  I would rather
suspect that it received a static surge (wind, rain, snow static or
nearby lightning) before you got it in your hands.

Put a DC path across your antenna feedline - 27k to 100k resistor or an
RF choke (100 uHy is good) to prevent future occurrences, but put it at
your antenna switch or some other place, but not inside the KPA100.  If
you have no other choice, put the resistor or choke inside a PL-259 body
and use a UHF Tee adapter - the resistor goes on one side of the Tee,
the antenna goes to the other side..

73,
Don W3FPR

On 3/24/2013 7:08 PM, Bill and Gail Stewart wrote:

> I am trying to repair a K2/100 that I believe was damaged in shipment
> to me. The power output was erratic - sometimes almost no power out,
> sometimes about half power. I investigated the SWR circuitry in the
> KPA100 and replaced R29, which was discolored but still had the
> correct resistance. After carefully reassembling the unit, the power
> output is now greater than maximum rated power regardless of the power
> setting (just keyed momentarily and measured with a peak-reading
> wattmeter).  The resistance from either side of R28 to ground is about
> 92 ohms, the same for the cathodes of D16 and D17. Is this correct? Is
> U5 blown?
>

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