[Elecraft] K3 - RF board: Q2 leakage

[Don Wilhelm]

Matt,

Wayne have more intimate knowledge of the K3 hardware and MCU firmware 
than anyone else.  If Wayne believes that replacing Q2 will fix the 
problem, then I would replace Q2.

73,
Don W3FPR

Matt Zilmer wrote:
> 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
>   
>