Yes, good job. Some pretty scary factory workmanship in there.
Wes N7WS On 4/30/2020 10:53 PM, donov...@starpower.net wrote:
Excellent detective work and documentation Ben! 73 Frank W3LPL ----- Original Message ----- From: "Ben Gelb" <ben.g...@gmail.com> To: "Benjamin Gelb" <ben.g...@gmail.com> Cc: elecraft@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
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