[Elecraft] K2/100 SSB Power Control Problem

[David Honey]

At 23:07 18/12/2005, Steve Kavanagh wrote:

>Everything seems reasonable except at 11 watts, where
>the output and current drain appear to be about the
>same as at 80 watts requested.  By comparison, here
>are the numbers for 20m USB, where there is no
>problem:
>
>
>It seems to me that perhaps the power control loop may
>be running out of available attenuation on the low
>bands where there may be more overall gain available
>and perhaps is becoming unstable as a result.  Note
>that the gain at 14 MHz and above may be affected by
>the fact that this KPA100 appears to suffer marginally
>from the as-yet-uncorrected resistor problem described
>in Builder Alert 22A.
>
>Has anyone else observed behaviour like this ?  Any
>clues as to where I should look for problems ?

Steve,

I had similar issues with my K2/KPA100 and your theory is exactly
what was happening. Several others have experienced the same issue.
On 80m, the KPA100 has higher gain and the SSB module's ALC
was not aggressive enough to be able to control the level.
To see if it's the same issue, try switching the meter from RF
mode to ALC mode. In my case, and several others that have
reported here, the ALC worked much better and power was
about right. There was also an undocumented firmware
change in the main control board PIC that meant that this
enhanced ALC only happened when the meter was in ALC
mode, whereas in previous firmware revisions, there was a
control loop for it even in RF meter mode. I established that
by monitoring the serial IO bus in both modes between the
two firmware revisions.

If your K2 is suffering from this issue, the solution is to modify
the KSB2 module. Unfortunately, I can't find the details offhand.
ISTR that it involved cutting the PCB trace from P1/6 on the
KSB2 to the base of Q1, inserting a resistor there, and lowering the
value of R9. Gary Surrency at Elecraft will know the exact
values to try. ISTR using a lower value of R9 than the
initial "try this" to ensure the ALC had enough gain in
its ALC loop. Sorry I can't be more specific at the moment.

73, David M0DHO (K2/100 #4030)