[Elecraft] [K3S] intermittent RF path interruption

Fri May 1 01:19:43 EDT 2020

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

On Thu, Apr 30, 2020 at 8:07 AM Ben Gelb <ben.gelb at gmail.com> wrote:
>
> I realized that the "Tune" wattage was set high enough to always
> switch the PA in, even when the power was dialed down to <12W.
>
> With that corrected, I now see consistent behavior.
>
> When KPA3 gets into "bad" state, I see no power out and hear no signal
> on receive. Dialing the power down to <12W results in the "clack" of
> the KPA3 bypass relays and immediately restores operation (albeit at
> QRP levels). Dialing the power up and down across the 12W mark makes
> the receive go in and out (no receive w/ KPA3 inline). Turning power
> on and off does not clear the fault - still persists after it comes
> back up. It does eventually resolve itself though and operate normally
> for a while. But have yet to determine a reliable means to "fix" it on
> demand.
>
> But it is clearly a problem in the KPA3 and not the base K3S, since
> all is cured once the KPA3 is bypassed.
>
> I have now reproduced this on multiple HF bands as well, so not just a
> six meter problem.
>
> I started digging into the schematics. T/R circuit in the KPA3 seems
> like the obvious place to start. Though I can't really see how a
> single component failure in there would cause what I'm describing (in
> particular the "memory" effect that causes the problem to manifest for
> a while and then go away).
>
> I probed the 7T and 7R voltages on connector P68A (easily accessible
> w/ top lid removed from rig). Looks fine - about 6V on either when in
> TX or RX, respectively, and ~0.3V when complemented. Also confirmed 5V
> is present on appropriate pins of P68A and 13.8V is present on P68B.
>
> I'd like to probe further on the PA board (while installed, so can see
> what is going on), but while there are many exposed vias visible with
> the K3S lid off, the silkscreen side is not visible when the module is
> installed. So its hard to figure out what to probe. Does anyone have
> access to board layout images, or otherwise a guide to what to
> probepoints are available on the heatsink side (which faces up when
> module installed) of the PCB? If not I suppose next step is to pull
> the board out and try to map it out manually when I have a bit more
> time, but hoping to save that step.
>
> For reference, schematic of interest is on pg. 46, here:
> https://ftp.elecraft.com/K3S/Manuals%20Downloads/E740323%20K3s%20Schematic%20Files.pdf
>
> Thanks,
> Ben N1VF
>
> On Thu, Apr 16, 2020 at 11:30 AM Ben Gelb <ben.gelb at gmail.com> wrote:
> >
> > Thanks all for the replies.
> >
> > - I saw the same issue on ANT 1 as ANT 2.
> > - Haven't dug out a dummy load yet... still TODO.
> >
> > New clue: Dialing the power down to 8W fixes the RX path (can go back and forth across this boundary and hear the RX go in and out, coincident w/ a relay click), but *not* the TX path when in a bad state. So I will still see 0W out when set to low power. Still have to switch to another band and transmit to "unstick" the TX path.
> >
> > So I take this to mean that:
> > - With high power out, both RX and TX path are impacted by the issue.
> > - With low power out (<8W, PA clicked out), the problem no longer seems to be in the RX path (but is still in the TX path).
> >
> > When I have a bit more time will look at the schematics and see if I can reason about what might be happening.. feels like this should narrow it down pretty far. But posting the update in case it spurs any new thoughts.
> >
> > Ben, N1VF
> >
> > On Sun, Apr 12, 2020 at 1:03 PM Nr4c <nr4c at widomaker.com> wrote:
> >>
> >> Well first I’d try with a “dummy” load.
> >>
> >> The swap antenna to Ant 1.
> >>
> >> Sent from my iPhone
> >> ...nr4c. bill
> >>
> >>
> >> > On Apr 12, 2020, at 3:23 PM, Ben Gelb <ben.gelb at gmail.com> wrote:
> >> >
> >> > A bit of a mystery w/ my K3S and I could use help figuring out what the
> >> > problem might be.
> >> >
> >> > Background: I have had the radio parked on 6m for several weeks (HF antenna
> >> > temporarily out of commission) and have left it running (receive) for
> >> > extended periods.
> >> >
> >> > Recently, I have noticed that when I do sit down and try to operate, the
> >> > radio will get into a mode where during TX the rig will indicate 0W forward
> >> > power during transmit. Following this zero power TX, the RX will also go
> >> > quiet. Then seems to stay that way for some time (not clear to me what
> >> > makes it clear up... or if it ever does if left alone).
> >> >
> >> > Some experiments I have done to try to figure out what is wrong:
> >> > - Hitting the tune button also shows 0 power (instead of usual 10-15W) when
> >> > radio gets stuck in this state.
> >> > - Dialing the power down to where I hear the 100W PA switch out of line
> >> > (relay click... around 8.0W) seems to immediate restore receive and also
> >> > causes the rig to make power again on transmit. But when I turn it back
> >> > past 8.0W, it appears to go back right back into the bad state (so suggests
> >> > whatever the issue is, is in the 100W PA module).
> >> > - Switching to another band and transmitting momentarily (think I tried
> >> > 15m) will sometimes knock the radio out of this "bad" mode. But going back
> >> > to 6 meters will eventually cause it to happen again. Usually pretty
> >> > quickly (several seconds of TX).
> >> >
> >> > Some other notes:
> >> > - Antenna always measures 1.1:1 on the K3S display (when TX working
> >> > correctly). Also checked w/ RigExpert. I don't think the antenna is the
> >> > issue.
> >> > - All my observations have only really been on ANT2 port (nothing connected
> >> > to ANT1 presently).
> >> > - Haven't really exhaustively tried to test band dependence of the behavior
> >> > because I don't have an antenna on the other bands at the moment and not
> >> > wild about trying to transmit into a big mismatch. So although it appears
> >> > 6m related based on my observations, that might not be exactly
> >> > representative of whats going on.
> >> >
> >> > Posting here in case I've given enough clues for someone to suggest what
> >> > may be wrong or how to narrow down further.
> >> >
> >> > Thanks,
> >> > Ben N1VF
> >> > ______________________________________________________________
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