Multimeter giving incorrect readings

[Logan W]

I have a DMM which I got a while back for $20 or so, from Jameco.
Lately, it has been giving me incorrect readings - when the test leads
are not connected to anything, it will say there are 800v, and this
does not go away when the test leads are detached. If I open up the
case, then it works normally, and if I do not have the back fully
snapped in, it works, but as soon as I do that, it starts saying 800v.
<a
href="http://jameco.com/webapp/wcs/stores...toreId=10001&catalogId=10001&productId=119212">Here
is the multimeter I bought.</a>

Does anyone have any ideas on how I can fix it?

 

[DaveM]

I have a DMM which I got a while back for $20 or so, from Jameco.
Lately, it has been giving me incorrect readings - when the test leads
are not connected to anything, it will say there are 800v, and this
does not go away when the test leads are detached. If I open up the
case, then it works normally, and if I do not have the back fully
snapped in, it works, but as soon as I do that, it starts saying 800v.
<a
href="http://jameco.com/webapp/wcs/stores...toreId=10001&catalogId=10001&productId=119212">Here
is the multimeter I bought.</a>

Does anyone have any ideas on how I can fix it?

DMMs frequently have a foil shield that helps to protect the instrument from
outside interference. These foil shields can sometimes make contact with
the circuitry and cause strange operations, such as you are experiencing.
When you open the case, make sure there are no holes in the shield that
aren't supposed to be there, caused by parts of the circuit board piercing
the foil.
Another thing to look for is a broken circuit board. Look at it under a
magnifier and see if there are any cracks. If you find any, that's likely
the cause of your problem. If your DMM has a multilayer board, you might be
able to fix it, but it's not likely to be easy. A soldering iron with a
very fine tip is mandatory here. Bridge the breaks in the copper traces
with strands of very fine wire (30 Ga wire-wrap wire is a good choice here).
Also,make sure that there aren't any foreign substances
(coffee/coke/beer/etc) that has found its way inside the case. If you find
any gunk of the sort inside, a thorough cleaning is necessary, the last step
being a flush with distilled water. Allow to dry completely ( an afternoon
of exposure to the warm sun should work).
As a final suggestion, you might flex the display (assuming that it's an LCD
display) and see if it causes your erroneous display symptom. If so, you
might have an intermittent contact by the Zebra connector, which is a
polymer strip of alternate layers of conductors and insulators. This strip
is placed between the display module and the PCB, and makes electrical
contact between the two. If that's your problem, you might try cleaning the
contacts on the PCB, the Zebra strip and the LCD module with pure (99%)
isopropyl alcohol. Be very careful with the LCD contact area.. no
scrubbing on it. just a few swipes with a clean soft cloth saturated with
alcohol.

Good luck with your DMM.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[Logan W]

DMMs frequently have a foil shield that helps to protect the instrument from
outside interference. These foil shields can sometimes make contact with
the circuitry and cause strange operations, such as you are experiencing.
When you open the case, make sure there are no holes in the shield that
aren't supposed to be there, caused by parts of the circuit board piercing
the foil.
<...>

Good luck with your DMM.

<...>

Thank you, this was the problem! I fixed it by putting a couple layers
of scotch tape over the affected portion of the shield. Will this
affect the multimeter in any way, or is there a better solution?

Thank you,
Logan Williams

 

[James T. White]

Thank you, this was the problem! I fixed it by putting a couple layers
of scotch tape over the affected portion of the shield. Will this
affect the multimeter in any way, or is there a better solution?

I hope you used Scotch electrical tape......

 

[CWatters]

I have a DMM which I got a while back for $20 or so, from Jameco.
Lately, it has been giving me incorrect readings - when the test leads
are not connected to anything, it will say there are 800v

Many meters give strange readings with the probes open circuit or in the
presence of RF fields. Got a wireless lan?

Put it on the voltage range and connect the two probes together. It should
read zero volts obviously. What does yours read? Hook it up to a 9V battery,
what does it read?

 

[Logan W]

I hope you used Scotch electrical tape......

Of course!

 

[DaveM]

Thank you, this was the problem! I fixed it by putting a couple layers
of scotch tape over the affected portion of the shield. Will this
affect the multimeter in any way, or is there a better solution?

Thank you,
Logan Williams

No, that shouldn't affect the DMM unless you taped over an area that is
supposed to make contact with a prong on the PCB. Enjoy your DMM.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[[email protected]]

For $20, throw that piece of crap into your nearest trash can.

Now run out and purchase a quality name brand DMM from a mainstream
reputable supplier. Wavetek is good, Fluke and Tektronix are better.

A DMM that produces wildly inaccurate measurement is about the last
thing that you need in your life!

I have an inexpensive Wavetek Model 5XL (around $35) that travels with
me and produces measurements with errors of less than 1% overall. I
would prefer to own a Fluke or Tektronix, but simply can't justify
their cost.

Harry C.

 

[[email protected]]

To that I would add, hook it up to the power line and see what it
initially reads, and while leaving it connected see what voltage it
reads at 5 minite intervals. If you see the voltage reading gradually
increase, immediately throw it in the trashcan because its readings are
meaningless and you can't repair such a fundamental design defect.

Harry C.

 

[[email protected]]

Not likely John, since I have a conventional analog meter that
constantly monitors line voltage to compare the DMM reading against.

Also, it is not typical of the power lines here in the US to start at
115V and half and hour later have drifted up to 140V and still
climbing, while the analog meter still reads 115V.

Then too, cheap DMMs are well known to be labeled for 1 or 2% accuracy,
when the reality is that if they can measure voltage to 10% accuracy
they are doing quite well. On the other hand, a Fluke is usually a safe
bet!


Actually, my inexpensive Wavetek Model 5XL (which I purchased as an
expendible throw-away for field work) does quite well, IF you remember
to keep the batteries fresh. IIRC, it sells for between $30 and $40.
Its readings are generally within 2% of those measured with the Fluke,
but I don't know if this is normal for the model, or if I just got
lucky.

Kindest regards, Harry C.

 

[[email protected]]

John posted:

"With a single meter I would expect to the see the mains rise from
dusk (as air conditioners switch on less and less), and we do have a
+/- 10 spec on the mains, which means _most_ of the time you can
expect to see them somewhere between 108 and 132V."

Actually John, the residential service requirement here in the
Northeast (and most other parts of the US) is 108-126V, with a nominal
of 117V. The electric utiliity becomes liable for damage and/or loss of
service for voltages outside these limits provided that they are
notified that either specified limit is being violated and take no
action to correct the problem.

Typically they can resolve the issue by changing taps on the
distribution transformer. More extreme solutions (constant voltage
transformers) are sometimes temporarily applied to end of line load
variation problem until the gauge of the feeder lines can be increased
to reduce IR drop in the line. (Usually this temporary fix is done only
in rural areas still governed by federal REA requirement.)

I note that the electric utilities are still lobbying for liability
immunity during brown-out situations, but fortunately these efforts
appear to be falling on deaf ears, since the entire responsibility for
brown-outs falls squarely on the shoulders of the electric power
providers themselves.

Kindest regards, Harry C.

p.s., John, the reason I have a dedicated line voltage meter is a
carry-over from the days when I dabbled in color photography, where
color enlargers needed to be powered at a constant voltage of 125V,
otherwise the color temperature of the prints will vary. The Variac is
now gone, but I kept the line voltage monitor as well as the Triplett
630-NA (mirror scale) used as both a sanity check as well as a trusted
secondary voltage and current standard.