When I try to run SSB on 80, 40 or 30m (the latter for
purposes of completeness only !) at power levels above
10 W but below 100 W I find the power control is not
working properly. There is no problem on CW, or above
14 MHz, or below 10 watts. S/N is 4509.
Here are some typical measured numbers by way of
explanation, resulting from saying helloooooooo rather
loudly into the mic (for long enough that the power
control loop should have settled). In the tables
below I used the following abbreviations:
Pset = requested output power
Isupply = average current from power supply at about
RF = reading on K2 LED bargraph in RF mode (watts)
ALC = reading on K2 LED bargraph in ALC mode (no. of
Po = average power output (measured on SWR meter with
relative power scale, calibrated against K2 on CW set
for 25 W)
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
On 80/40/30 I have also observed that the power supply
(Astron SS-25M) buzzes when I transmit at the 11 W
(requested) level, as if the current drawn is being
pulsed at perhaps 10 Hz or so. The transmitted audio
(as heard on a second receiver) is probably slightly
distorted as well. The problem can occur on both LSB
and USB but is noticeably more severe/probable on LSB.
The setting of SSBA has little effect other than I
have to talk a bit louder if it is set to 1.
Once in a while it goes away (I haven't figured out
what made it do this yet...sometimes transmitting for
a while at 100 watts seems to help), which has me
thinking perhaps a bad solder joint is involved.
However, cycling through modes or turning the K2 off
and on will always reset it so the problem is present,
so it is not a simple temperature dependence. The
requested power level above which everything seems
fine appears to vary in a manner which I can not yet
A careful solder-quality inspection was made of the
KSB2 board (and a few dubious joints fixed !) but this
made no difference. I haven't tried anywhere else yet
since the other boards are much less accessible. Nor
have I looked for missing/wrong/backwards components
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 ?
>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
>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 ?
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.