KX1 #978, odd AC ripple and resistance readings during 1st test

[Andrea Borgia]

Hello.

Hot on the heels of my previous report, here comes the next one relating
to the first test phase.

Executive summary: it works! Unit came up with E10 as expected, display
brightness and timeout setting are ok and led shines pretty brightly.

Digging a little deeper, however, there are a few details on which I'd
like to hear your opinions:

-page 26, resistance checks: quite a few of those that had to be greater
than 10k gave an infinite reading (U1 pin 2, 6, 7, 9-12 and 15; U3 pin 1
and 8) and some of those that should have been greater than 1k were
indeed greater, only very much so! (e.g., U1 pin 4 was 140k). Is that
normal?

-page 28, ac voltage checks: I'm getting readings in excess of 20V (yes,
volts instead of millivolts!). A cursory reexhamination of the boards
confirms capacitors' polarities are correct and solder joints appear to
be fine (i.e., not always beautiful looking, but reasonably good
considering I'm

It may be worth mentioning that my friend Nicola IZ4FTB and I were using
an AMM (yes, one of those old-fashioned cutie things with the fancy
swinging needle ;-) whose battery really should be replaced, fast!

Next week, we'll get a DMM and repeat the tests before proceeding
further, but in the meanwhile I'd like to hear your thoughts on these
issues, especially considering that the little rig that could apparently
does and quite fine, thank you! 8-)

B73,
Andrea.

P.S.: pictures will soon appear on my website.

--
Homepage: http://andrea.borgia.bo.it     /    Amateur radio: IZ4FHT
A: Because it messes up the order in which people normally read text.
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A: Top-posting.
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[Ron D'Eau Claire-2]

IMPORTANT!!! Please, do not use an analog multimeter for testing resistance
unless it is specifically designed for solid state. You didn't say so, but
many meters, including those that use a simple 1.5 volt battery for the Ohms
tests, can destroy a great deal of the solid state components in your K2!!

The older ohms scales were not current-limited, which means that if you hook
up the meter to measure resistance across a transistor junction that is
forward-biased, it will often destroy the transistor. That's true of
discrete transistors or one of the gates in the many integrated circuits
used in your K2.

Voltage measurements with such meters may be off too, although it wouldn't
produce the sort of errors you saw on AC here. Before the modern DMM's
became popular, most lower-cost meters drew significant current from the
circuit under test to operate the needle.  In high impedance circuits where
only tiny currents are flowing, the readings such a meter may give you can
be grossly wrong, because the meter itself is loading the circuit. Back
then, only more expensive "vacuum-tube voltmeters" provided a sufficiently
high input impedance for such measurements. Nowadays, almost any DMM will
provide an input impedance in the 10 meghom range, which is pretty standard
now. That's high enough for decent accuracy in almost any circuits we'll
encounter in our rigs. Old Vacuum Tube Voltmeters (VTVM's) provided an input
impedance in that range too, so if you use one of those you will likely get
good readings everywhere. But, even the higher-priced VTVM's often used a
simple 1.5 volt battery in a fairly high-current ohms measuring circuit that
would destroy most transistors and integrated circuits.

The only safe way to measure resistance in the K2 is with a meter
specifically designed for measuring resistances in solid state circuits.
Even then, you may notice your resistance readings vary widely. The minimum
values are picked as value above that which any rig will show. You may find
too that the resistance will change radically when you reverse the leads,
since that will cause any solid state junctions in the circuit to switch on
or off. So you resistance measurements are fine and I suspect the voltage
measurements is indicating something wrong with the measurement setup rather
than the K2 since it seems to be working fine.

Ron AC7AC

-----Original Message-----
Hello.

Hot on the heels of my previous report, here comes the next one relating
to the first test phase.

Executive summary: it works! Unit came up with E10 as expected, display
brightness and timeout setting are ok and led shines pretty brightly.

Digging a little deeper, however, there are a few details on which I'd
like to hear your opinions:

-page 26, resistance checks: quite a few of those that had to be greater
than 10k gave an infinite reading (U1 pin 2, 6, 7, 9-12 and 15; U3 pin 1
and 8) and some of those that should have been greater than 1k were
indeed greater, only very much so! (e.g., U1 pin 4 was 140k). Is that
normal?

-page 28, ac voltage checks: I'm getting readings in excess of 20V (yes,
volts instead of millivolts!). A cursory reexhamination of the boards
confirms capacitors' polarities are correct and solder joints appear to
be fine (i.e., not always beautiful looking, but reasonably good
considering I'm

It may be worth mentioning that my friend Nicola IZ4FTB and I were using
an AMM (yes, one of those old-fashioned cutie things with the fancy
swinging needle ;-) whose battery really should be replaced, fast!

Next week, we'll get a DMM and repeat the tests before proceeding
further, but in the meanwhile I'd like to hear your thoughts on these
issues, especially considering that the little rig that could apparently
does and quite fine, thank you! 8-)

B73,
Andrea.

P.S.: pictures will soon appear on my website.


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[Andrea Borgia]

Ron D'Eau Claire wrote:


> IMPORTANT!!! Please, do not use an analog multimeter for testing resistance
> unless it is specifically designed for solid state. You didn't say so, but
> many meters, including those that use a simple 1.5 volt battery for the Ohms
> tests, can destroy a great deal of the solid state components in your K2!!

I have a KX1, not a K2, however the point is taken. Nicola told me that
meter was "low-voltage" and okay for solid state. How do I verify that
it is so? And, most importantly, how do I check whether any damage was done?


Ok, I'll quit worrying about the resistance values and get a DMM to
recheck the ripple.


B73,
Andrea.

--
Homepage: http://andrea.borgia.bo.it     /    Amateur radio: IZ4FHT
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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[Ron D'Eau Claire-2]

Andrea wrote:

I have a KX1, not a K2, however the point is taken. Nicola told me that
meter was "low-voltage" and okay for solid state. How do I verify that
it is so? And, most importantly, how do I check whether any damage was done?


Ok, I'll quit worrying about the resistance values and get a DMM to
recheck the ripple.

--------------------------------

You are probably okay then Andrea. Sorry for alarming you! There have been
multi-meters made for solid state that are not DMMs, but they are fairly
rare. As far as damage goes, most solid state either works or it doesn't. It
is possible to degrade the performance of a solid state device -
particularly insulated gate devices like some field effect transistors - but
that usually happens because of high voltages at tiny currents produced by
not following good electrostatic discharge procedures when handling them.
I've never seen a case where allowing too much current through a device -
such as an old-style ohmmeter does - 'degrades' the part. It usually kills
it. So if your rig is working, either the meter was okay for solid state or
you used it in a way that didn't damage the parts.

One way to check a suspect meter is to check the current that it allows to
flow in the ohms range using a second meter. Typically, the greatest current
flows in the lowest range. Most DMM's that I've seen allow no more than
about 1 to 1.5 mA to flow through the circuit under test at any time. My
Fluke DMM never allows more than a few hundred microamps in the ohms ranges.
By comparison, a typical pre-solid state volt-ohm-milliameter (VOM)
typically allows as much as 100 mA in the low-ohms range! Yes, I wrote 100
mA - 0.1 AMPERES - in the low-ohms range. That can wipe out all sorts of
integrated circuits and transistors!

Those older VOM's seldom allow more than a milliampere to flow on the higher
resistance ranges, so a knowledgeable user who knows a particular meter can
get away with using it for certain resistance checks. I've serviced a lot of
solid state gear "professionally" and I simply don't fool with ohms checks
with an older meter. I do have an older VOM that I prefer for "peaking" an
adjustment, but I never use it in ohms range.

Another clue is that the newer low-current meters do not need a "zero"
adjustment. If a meter has a zero adjustment, I assume it is an older VOM
that will kill solid state parts in ohms range until proven otherwise.

And one more clue is whether the meter has a separate "diode check"
function. Modern meters designed for solid state typically do not apply
enough voltage to positively turn a transistor or diode junction "on". The
resistance measured across junctions will still vary widely, but not enough
voltage is usually applied to do a valid test. So a separate "diode" check
is provided that applies enough voltage and current to determine whether the
junction is behaving properly. Still, the current is limited to about 1 mA
to avoid damaging any sensitive integrated circuit or transistor junctions.

The KX1 is a really cute rig! I have serial number 0004 which I built while
writing the assembly instructions for the manual. It's a great rig that had
been invaluable in public relations here in Oregon. I'll tell that story in
a separate post "Elecraft Rigs as Public Relations Tools" if you're
interested.

Ron AC7AC


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[Andrea Borgia]

Ron D'Eau Claire wrote:

> The KX1 is a really cute rig! I have serial number 0004 which I built while
> writing the assembly instructions for the manual. It's a great rig that had

Now I got a DMM(*) on loan and I re-ran the checks: voltage only, not
the resistance checks because I didn't want to remove the PIC: the
reading is now about 8.5mV AC ripple, well within the limit.

Apparently, the room where the assembly takes place does have stray RF
around: short-circuiting the probes gives a reading of 5mV, holding them
close together reads 300mV and spreading them 1 meter apart peaks at 530mV.

Thanks again,
Andrea.

(*) The DMM itself is a "FUKE DT890B+" (yes, "FUKE") and either it's a
"fluke" Fluke (meaning cheap copy of Fluke) or it's a priceless typo
(like some rare pink stamps 8-)

--
Homepage: http://andrea.borgia.bo.it     /    Amateur radio: IZ4FHT
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
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