This last week I was doing some loss measurements on EFHW transformers (NO antenna attached), using S11 and S21 loss figures and a calculator on Owen Duffy's website.  (I've done this many times before.)  This time, during several iterations with separate transformers, I got some credible figures.  Then I started getting odd results and noticed that with no change in inputs, the logmag reading on CH0 was varying by one or two dB from sweep to sweep.  I thought perhaps it was a computer or nanoVNA-QT problem, but taking readings directly off the nanoVNA (V2 Plus4) gave the same results.  Further checking showed other problems with CH0--for example, bad resistance readings when attaching known loads to CH0 .

I've now recalibtrated, reinstalled the firmware, calibrated again, etc.  Same problem.  Right now, with nothing on CH0, I get an SWR reading of 2.5:1 and a resistance reading of about 19.9 ohms.  With the load resistor (the same one--50 ohms--used in the calibration process) installed, it's an SWR of 2.8:1 and a resistance of 17.8 ohms.  Other measurements are just as whacky.

The only thing I can think of is that the input circuitry/hardware associated with CH0 has suffered some damage, although I'm at a loss to explain how.  (I wasn't doing anything that should have exposed the unit to static charges.)

Any ideas or solutions?

Charlie, W6JJZ

Have you checked the internal solder joints?

If they're of the common poor quality the activity may have caused a
failure.

Just a thought ...

kd4e

> On 7/18/21 4:12 PM, Charlie Lofgren wrote: This last week I was doing > some loss measurements on EFHW transformers (NO antenna attached), >
using S11 and S21 loss figures and a calculator on Owen Duffy's >
website. (I've done this many times before.) This time, during > several
iterations with separate transformers, I got some credible > figures.
Then I started getting odd results and noticed that with no > change in
inputs, the logmag reading on CH0 was varying by one or two > dB from
sweep to sweep. I thought perhaps it was a computer or > nanoVNA-QT
problem, but taking readings directly off the nanoVNA (V2 > Plus4) gave
the same results. Further checking showed other problems > with CH0--for
example, bad resistance readings when attaching known > loads to CH0 . >
> I've now recalibtrated, reinstalled the firmware, calibrated again, >
etc. Same problem. Right now, with nothing on CH0, I get an SWR >
reading of 2.5:1 and a resistance reading of about 19.9 ohms. With > the
load resistor (the same one--50 ohms--used in the calibration > process)
installed, it's an SWR of 2.8:1 and a resistance of 17.8 > ohms. Other
measurements are just as whacky. > > The only thing I can think of is
that the input circuitry/hardware > associated with CH0 has suffered
some damage, although I'm at a loss > to explain how. (I wasn't doing
anything that should have exposed > the unit to static charges.) > > Any
ideas or solutions? > > Charlie, W6JJZ

Thanks.  Yes, the same thing occurred to me.  So I opened the unit, removed battery and screen, then removed the motherboard from the metal enclosure.  I didn't see anything obvious, and didn't go any further because that would have required removing the shields surrounding CH0 and CH1 and associated circuitry.  As I assessed the shields, their removal would require some very delicate work,  Absent some very good directions on how to do it, I backed off.

Charlie, W6JJZ

Charlie,
You may find that the shields are snapped into tiny clips.  Removal/replacement may be easier than you think.
--John Gord

On Sun, Jul 18, 2021 at 03:02 PM, Charlie Lofgren wrote:

>
> Thanks.  Yes, the same thing occurred to me.  So I opened the unit,
> removed battery and screen, then removed the motherboard from the metal
> enclosure.  I didn't see anything obvious, and didn't go any further
> because that would have required removing the shields surrounding CH0 and
> CH1 and associated circuitry.  As I assessed the shields, their removal
> would require some very delicate work,  Absent some very good directions
> on how to do it, I backed off.
>
> Charlie, W6JJZ

Thanks, John.  When I return to the project later today, I'll take another look at the shields, with a magnifying glass.

Charlie, W6JJZ

Hi John,

Yes, the shields came off easily.  Thanks for the hint.

However, I found nothing that appeared loose or open.  I could have tried checking individual components (under the magnifying glass!), but I don't have the schematic and even if I did, I suspect I'd be as likely to damage something as remedy the problem.  My only experience with surface mount components has been with maybe a dozen larger ones, replacing the bridge diodes in a couple of MFJ antenna analyzers and in several kits with a few SM components.

I'll see if I get any other suggestions or more wisdom in the future.

73,

Charlie, W6JJZ

IMO, a stereo microscope is needed to inspect or work on small SMT stuff.  I use one that is has a turret objective that gives a net 10x or 30x, which is about right.   You can find decent new ones for <$200. This place has a wide selection:

https://www.amscope.com/stereo-microscopes.html?cat=335&dir=desc&order=bestsellers

Thanks, Lou.  I've saved the site address. I turn 82 in September and have a couple of rooms full of thru-hole components to use up before my expiration date, but if I take up SMT construction, I'll get the microscope.  Meanwhile, I guess I'll find another V2 Plus4 nanovna--superb instrument!

73,

Charlie, W6JJZ

Har.  I'm only 68 and have been using a stereo microscope for years for working on fine/tiny stuff, SMT has been more recent.  I used it to remove a tiny splinter from my wife's hand just yesterday.  A couple of weeks ago used it working on an HT where a battery had leaked and dissolved a trace on the board. I don't think I could have found it or repaired it without the stereo scope.  Of course it provides great views of bugs/insects/spiders/ticks/fleas, flower parts, feathers and other such things.

With magnifying optics, the temptation is to go for higher power, but it's very easy to go too high.  With a stereo microscope,10x is by far the most useful power.  Occasionally, 20x or 30x might be useful. Working on something under 10x, it's amazing how huge the tips of the tools look and how clumsy I am. 10X worse than normal, HI HI.