[Elecraft] K3 - RF board: Q2 leakage

[Matt Zilmer]

Hi Don,

A few K3 firmware revs ago, after I updated the radio, I noticed some
strange power on behavior.  I was in touch with Wayne at the time on
another subject, and he indicated these symptoms indicate a high
leakage on Q2 - RF board.

Symptoms:
Rapid-firing the Power switch only lights up the LCD backlight.  If
one cycles the external power supply between K3 power cycles, this
doesn't happen.  If I can get the K3 to sequence power up normally,
there is a _loud_ period of audio that sounds muffled, like it's on
the wrong filter setting or there is something going on with the RX
EQ.  After the strange audio disappears, things seem to go normally.

I never noticed these symptoms before, so I though there was some
change in the new firmware I loaded.  Wayne thought otherwise, and
suggested that the symptom indicated Q2 had excess leakage.

Power supply below is set at 13.8VDC.

DVM time:
K3 power off, with external power applied -
Q2-G - 13.3V
Q2-D - 0.097V

K3 power on -
Q2-G:  6.66V
Q2-D:  13.3V

So far so good.  But watching Q2-D while powering down, it takes
between 2 and 4 seconds for that net to drop from full voltage output
to about 0.5V.  Wayne had said that high leakage would be indicated by
a slow retreat of the drain voltage from full output to about 0.5V.

On the other hand, I don't know if the 0.1V that shows at Q2-D while
the K3 is powered off is right or not.  That would indicate a static
leakage value to me.  I think it should be *a lot* closer to 0V.

I have a replacement Q2 here, but don't want to "operate" on the radio
unless it's really necessary.  Do you have any recommendations?

Thanks,
Matt Zilmer, W6NIA
K3 #24
K2 #2810