[Elecraft] K2/100 power problem

[Don Wilhelm]

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
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