[Elecraft] [K3S] intermittent RF path interruption

[Wes]

Yes, good job.  Some pretty scary factory workmanship in there.

Wes  N7WS


On 4/30/2020 10:53 PM, donovanf at starpower.net wrote:
> Excellent detective work and documentation Ben!
>
>
> 73
> Frank
> W3LPL
>
> ----- Original Message -----
>
> From: "Ben Gelb" <ben.gelb at gmail.com>
> To: "Benjamin Gelb" <ben.gelb at gmail.com>
> Cc: elecraft at mailman.qth.net
> Sent: Friday, May 1, 2020 5:19:43 AM
> Subject: Re: [Elecraft] [K3S] intermittent RF path interruption
>
> And now the exciting conclusion....
>
> I decided the most likely spot for the problem to exist was in the T/R
> switching circuit in the KPA3A. Schematic on pg. 46 of
> https://ftp.elecraft.com/K3S/Manuals%20Downloads/E740323%20K3s%20Schematic%20Files.pdf.
>
> I removed the KPA3A to take some pictures and figure out how to probe
> some points of interest, then re-installed it, and took following
> measurements:
>
> Drain of Q4 - 0.25V during RX, 122V during TX
> Drain of Q6 - 13.7V during RX, 0.24V during TX
>
> ... so far seems reasonable ...
>
> Measured both sides of L3 and saw 7.42V during RX. Also seems reasonable.
> Then measured both sides of L1 and saw that it appeared to be floating... hmmmm.
>
> L1 and L3 should basically see identical voltages during RX. L1 should
> not be floating. This means that there's no DC voltage being applied
> to D2 or D5 anode. That would mean no connection from antenna input to
> either PA output or RX path. Fits the problem description.
>
> I took the KPA3A back out again and did some continuity checking.
> Shortly I was able to discover that one of the ends of R5 (facing L1)
> had a failed solder joint to the PCB. When I touched it with the
> probe, the bad end actually came completely free and lifted up off of
> the PCB. After starting at it for another minute, I realized that
> there is a mismatch between the component package size and the
> footprint on the PCB. The pad spacing on the PCB is too narrow, and
> even when perfectly centered, there is almost no overlap of the
> package pins on the resistor and the solder pads that it is supposed
> to attach to. This is also true of R6 and R12 (though they seem to be
> connected, at least for now - though notably R12 has a green wire
> attached to one end).
>
> This seems like an assembly defect. Doesn't seem very surprising that
> with a poor mechanical connection that the solder joint would not hang
> on. Maybe it can (or already has been in a later run?) be corrected by
> selecting a resistor that has a package that matches the PCB
> footprint? Seems likely to me that more KPA3As out there would fail
> similarly.
>
> I noted by contrast that R18 uses the same package as the R5, R6 and
> R12 components, but appears to have a PCB footprint that correctly
> aligns with the component.
>
> Anyhow, I removed R5 and soldered it back on (turning it at a slight
> angle, and borrowing the nearby pad of C17 to ensure it had a good
> solder connection). Now all seems to be well again. Hear lots of
> signals on RX w/ amp inline an make power out on TX. Hooray!
>
> Here are a few pix of the adventure. Hopefully they might be helpful
> to someone else.
> https://drive.google.com/drive/folders/1kP9cR6HbBqP1Xy35QOOd945Bm2KgYW7T
>
> 73,
> Ben N1VF
>