[Elecraft] K3 Erratic Power Output

[Mike K8CN]

Larry, there's a good chance that the SWR bridge (FWD/REFL power measurement)
on the K3 RF Board has a problem with the level scaling feature that
switches on when you command a transmit power level above 12 watts.

Check the K3 RF Board schematic (here I'm presuming you have the K3 rather
than the K3S, but I think the SWR bridge circuits are similar), page 4 of
the schematic package, on the far right hand side for the SWR bridge
circuit.  Near the LM358 op amp voltage followers (outputs labeled VFWD and
VREFL) you'll see a pair of voltage dividers that are enabled by logic
signals /FSCL and /RSCL for higher output power levels.  

I'm going out on a limb here, but if /FSCL is not grounded when you command
an output power greater than 12 watts, the voltage follower on the FWD power
branch may saturate and effectively remove the feedback signal that the
power control loop requires to operate properly.  

The /FSCL signal is generated at the output of one flip-flop in an 8-bit
shift register - if that output won't pull down to ground R53, the lower arm
of the voltage divider in the FWD power level scaling circuit, that may
inhibit the necessary scaling of input voltage to the follower to keep it
from saturating.

The fact that the power control loop appears to work smoothly at commanded
power levels less than the 13 watts indicates that the SWR bridge rectifier
diodes are still functioning properly.  The jump to more than 100 watts
output when you command a power level just slightly greater than 13 watts
tells me the power control loop is getting different FWD power information
than what it expects to see.

Probing the SMD device leads to check that /FSCL is properly asserted when
you cross the 12 watt threshold? I'll leave that one to the pros!

73,
Mike, K8CN



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