[Elecraft] K2: V RFDET odd behaviour

[Iain Kelly]

Good afternoon,

I've been working on K2 s/n 5484 and have some off behaviour that has me
stumped.

The background is that during a KPA100 build the PC control cable was wired
up backwards and then connected to the K2. Some smoke escaped from the
control board area. The MCU has been replaced as we had been able to prove
there was damage there.

I've now got to the stage where I have 15W out again, trouble is it's all
the time when I turn the power adjust pot up while in transmit.

The pot is working ok and the K2 display shows 0.1 to 15W when adjusting
that control in receive mode. In transmit the power output shows up to 0.4W
on the K2 but 15W on an external meter. Turning the ower control to 0
reduces the transmit output.

I've measured the voltage on the cathode of D9 and that varies nicely with
output power. At the moment it is around 0.5V then jumps to 3.15V when
power out hits 15W. This happens in a jump, from what I can tell it's
because the MCU VRFDET line is still too low. When I measure VRFDET after
R66 I only get 0.8V max. Way too low.

I have removed and tested R66, R69, C77, D9 and all are correct. The path
of VRFDET is fine, the 820 Ohm resistor on the control board is fine.

R69 measures 99.9k out of circuit, but only 89k when in circuit. Taking the
output of D9 directly to VRFDET input on the MCU shows that VRFDET tops out
at about 2.3V in that case, again for 15W out, though I get a bit more
variability of output.

I have compared it with a recently acquired (and known good) K2. The only
difference I can see is that in circuit R69 shows 102k.

My plan of attack has been to get the base K2 back into a working state,
then work on any issues we find in the KPA100 module, of which there are
certainly some.

Wonder if anyone can shed any light on what I am missing here?

73, Iain M0PCB