Guess you stumped everybody -- no responses in the list! :-)
I'm not an expert in switchers, but it seems to me that the problem might be a
combination of the imperfect sine wave output from your inverter coupled with
the fact that you're putting a very low load on the switcher in receive mode.
(Assuming the switcher doesn't produce this hum when plugged into the mains.)
I think I would try an analog supply in place of the switcher and see if you
still get it. I think you wouldn't.
Second, I wouldn't necessarily trust on-air reports about your transmitted
audio where the signal strengths aren't extremely strong. You might have a hum
on there that is quite a few dB down from peak audio, and it isn't detectable
under the ambient received noise. If a local guy who is copying you S9+20 says
you have no hum, then I'd believe it (if you trust his integrity and technical
competence to make such a judgment).
Or it could be some kind of ground loop problem, but it sounds like you have
that covered. Ground current analysis in a complex home electrical/radio
system is still black magic to me. :-)
Bill W5WVO
Matt Zilmer wrote:
Hi to all,
This is not a critical issue, but it might interest some. It's also
not necessarily specific to K3's. Keywords might be "switching power
supply", "inverter", and "grounding".
The problem I describe below was never experienced with the K2, only
with the K3. I exchanged the two radios to confirm this
experimentally.
The power supply scenario is as follows.
Qty 2 - 50W Siemens solar panels on the roof above the shack supply a
12V 100A-Hr SLA battery. The battery is charged through a Micro-X
charge controller, and this arrangement has been used for a few years
with my K2. It seems to work well under all conditions. The K2 is
the low power QRP version, and generally draws less than 2.5 A from
the battery at power levels I run.
The battery is connected to a Xantrex RS-400 sine wave inverter. THD
on the 60 Hz output is supposed to be < 3%. Looking at its output via
a Tek 2465A scope shows a very clean sine wave, even under the max
400W load. The inverter is rated 400W, with surge to 800W.
The inverter feeds an Astron SL-11A switching power supply that powers
the K3/10. Its output is 13.6V, and under max load at the K3 we're
talking about 2.8A, or 39W.
All units described above are bond-grounded to each other using a 1/2"
braided strap. The strap drops 10 feet or so to an 8 foot ground rod.
The ground rod is also bonded to the house ground nearby. The house
ground is used for all main and backup loads for a much larger Solar
PV system that we use to power the house here.
When using the Battery => Inverter => SL-11A power supply => K3 power
supply path to power the radio, the switching supply hums quite a lot
in weird non-periodic ways. I can sometimes hear the same hum on the
K3 RX audio, but have had no reports of it showing up on TX audio when
I asked the other party. "Very clean audio", they would always say.
Given the long-winded narrative above, does anyone have an idea about
the rx noise I'm hearing? Seems like it's not getting in through the
DC, since it doesn't show on SSB TX. So maybe I'm hearing it over the
air?
I thought this might be common mode noise, but the TX reports I've got
with this power supply setup won't support that. Grounding can affect
that, but since low impedance paths are connecting all the inline
stuff, this doesn't seem the easy answer either.
Sorry to take up so much bandwidth, but it's possible another
Elecrafter type might face a similar issue. So it seemed worth the
bits.
BTW, connecting the K3 directly to the 12V SLA battery has none of the
above effects and works fine. Please - no flames over the easy
answer, OK? :)
73 and thanks for any thoughts,
matt zilmer, WA6EGJ
K2 s/n 2810
K3 s/n 0024
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