[Elecraft] KPA100 Hi Cur

[Don Wilhelm]

Johnathan and all,

Actually it is not negative feedback for power control, but it is a 
closed circuit control loop providing positive control of the power level.
The actual power is measured at the KPA100 output and the result is sent 
to the MCU (via the VRFDET signal line) where it is compared with the 
requested power setting.  The result of that comparison is used to 
control the level of BFO injection in order to regulate the power output.

When the KPA100 wattmeter diodes are faulty, the MCU thinks there is low 
or no power output, and ramps up the drive to maximum.  The result is 
uncontrolled maximum power output.  If operated in that condition for 
some time, the base K2 PA transistors will be stressed and will 
eventually fail.  That condition also causes distortion, especially 
noted in SSB transmissions, but also increased IMD on the transmitted 
signal because the base K2 PA transistors are no longer being operated 
in a linear fashion.

With the KPA100 in the circuit, the KAT100 power sensing is not used, 
and the KAT100 wattmeter is used only to light the SWR indicators.
The KAT100 wattmeter is used if the KPA100 is not present.  The KAT2 
wattmeter is used if neither the KPA100 or KAT100 wattmeters are not 
present.  In the basic K2 (without the KAT2 or KPA100 or KAT100), there 
is an RF Detector to measure the power output, but that detector is 
dependent on the load - a 50 ohm resistivity load is assumed by the MCU 
calculation of power so it will not be correct if the load is other than 
50 ohms non-reactive.

So that is the story of how the K2 controls the power output.  The K1 
and K3/K3S use a similar system.  As far as I know, this system of power 
control is only used by some commercial transceivers and most other 
amateur transceivers simply let the operator control the drive level or 
provide only a fixed drive level.

73,
Don W3FPR

On 1/3/2020 10:15 PM, Jonathan Taylor, K1RFD wrote:
> Don,
>
> Thanks for the tip -- that was exactly it. I checked and replaced D16
> and D17 and all is well now.  (Both were showing the same voltage drop
> in both directions.)  I gather from your description that the built-in
> wattmeter forms a negative feedback loop with the power adjustment.
>
> My diodes and I are definitely static-challenged -- I have had a
> similar problem at least twice in the KAT100 despite my best efforts
> to remember to disconnect the antenna after operating.
>
>