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Old 1st July 2015, 08:27 AM   #1
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Default Unusual Measurements

How do you explain unusual measurements?

They've happened to me from time to time
with different gear, different rooms, power, etc
and have been taken with different instruments.

This time I happened to have my camera.

No more space.

I did measure 221V off the base of a 50 V transistor.
Then I unplugged the amp and moved the lead,
changed the measurement type on my TEK 2246 scope,
then changed back to the voltage measurement.

Same result 221Vp-p.
This from my HP 350D set
for 60dB of attenuation.

I had similar strange measurements taken from
my HP339A. If I recall it was measuring 60 Volts
on a 6 volt op amp output.
To confirm the measurement, I then
changed from TEK scope, to a hand held Fluke meter,
then changed to a Calibrated Fluke Scopemeter.
All had the same not possible measurements.

I'll post pics when I get some more space here
for pics.

Maybe if N.Tesla was here he could help explain it to me.
Since that isn't likely to happen, any one else have any
ideas?

10x Probe, ground to circuit ground.
Here is what I was measuring:
Click the image to open in full size.

Signal attenuated at 60 dB:

Click the image to open in full size.


Measurement from TEK 2246 Analog scope:
Click the image to open in full size.
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Last edited by SyncTronX; 1st July 2015 at 09:56 PM.
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Old 1st July 2015, 09:22 AM   #2
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I usually attribute such things to strange spherics....
Doc
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Old 1st July 2015, 09:56 AM   #3
DF96 is offline DF96  England
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Operator error is the usual cause of strange measurements.

When chasing a problem, one method of attack is to look for common factors: what is always present when the fault occurs? In this case I guess it is the same experimenter who is always present, so maybe he is the source of the problem?
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Old 1st July 2015, 10:10 AM   #4
SY is offline SY  United States
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This one seems to be whispering, "Unintentionally floating ground."
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Old 1st July 2015, 10:50 AM   #5
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+1

I would not trust the results of anything like this until verified with a known good ANALOG 'scope!
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Old 1st July 2015, 06:23 PM   #6
bwaslo is offline bwaslo  United States
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Voltages are measured relative to something, what is your [-] or ground clip connected to?
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Old 1st July 2015, 06:26 PM   #7
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Gotta be a floating ground or some unaccounted for divider probe or range setting on the instrument. I keep a 1.5V battery around for reality checks and I cross check my various measurement instruments frequently!
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Old 2nd July 2015, 02:36 AM   #8
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I'll have to get a battery, the old style.

What is also strange when it happened another time,
with the HP339a, I was using a very nicepower line
filter with dual line isolation transformers for balanced
line power.

I verified those reading by disconnecting the Tek 2246,
then connecting a Fluke 87 III meter. Still in disbelief
I then got out the Fluke 199 Scope meter and measure
the same readings.

In this amplifier case, McIntosh MC250 I quickly disconnected
the probe and moved it directly before the input attenuator
on the amp. If that wasn't enough, I then unplugged it.
The measurement maintained itself, I wanted to see what the
frequency of the wave form was.

Before you comment about it, yes I know it should have been
taken 90 degrees later, on the x-axis cross over point...I was in
a hurry to take the pic.

Click the image to open in full size.

Here is where the probes moved to, it's hard to see but I connected them
up correctly. The measurement persisted.

Click the image to open in full size.

Yes, the ground was floating so much...the unit was completely unplugged.

So, what am I missing?

NOTE: Added pics to the first post because I can.
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Last edited by SyncTronX; 2nd July 2015 at 02:38 AM. Reason: Add pics and explanation.
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Old 2nd July 2015, 02:51 AM   #9
bwaslo is offline bwaslo  United States
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where is your SCOPE's ground clip connected to? (I can't see that on the photo). That's what it measures voltage relative to. What happens if you touch the probe tip to its ground clip. Or both when connected to whatever point you're seeing voltage on? (should be zero -- if not, probe ground clip is open).
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Old 2nd July 2015, 03:27 AM   #10
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In the first post is it connected to the signal ground resistor.

In the last photo if you look you can see the alligator part
of the clip connected to chassis ground (obscured) for the input attenuators.
I was in a hurry to take the pics.

However the scope kept the same reading after moving the probe tip
and the ground to the inputs. When I disconnected the probes to move
them, the measurement stopped, until reconnected.

I think it lasted another minute then cleared itself.

These mis-measurements happen infrequently and are intermittent.
I was in the amp making sine, triangle, and square wave measurements on both channels, that is taking pics of the scope for documenting
it for other DIYers down the road.

It appears to be related to the power as it's at 61 Hz, or thereabouts.
and the sine wave a bit crooked. I don't think it is a hold over of some
triboelectricity, which I don't think is at 60-61Hz
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Last edited by SyncTronX; 2nd July 2015 at 03:32 AM.
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