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. Here is the multimeter I bought.
Does anyone have any ideas on how I can fix it?
"Logan W" wrote in message news: snipped-for-privacy@g49g2000cwa.googlegroups.com...
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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!!
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
I hope you used Scotch electrical tape......
-- James T. White
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?
Of course!
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!!
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.
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.
-- Of course the mains voltage couldn't _possibly_ be drifting high?
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.
-- Ah, but that's cheating! you made no mention of the second meter in your first post.
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.
-- The funny part about that is that it doesn't matter, since whatever liability they incur will _have_ to, somehow, be made up and that can only happen with rate increases, a lot of which will have to go to the lawyers, on both sides. The other side of the coin is that in order to avoid brownouts they'll have to increase capacity and that capital investment will be paid for by? You guessed it, rate increases. Do a reducto ad absurdium on the alternative, putting the utility out of business. Will that work?
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