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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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mechanical zero on a small meter
I have an old MFJ SWR meter with a built in counter. It has an LCD
display to read frequency and it uses a small 2 inch korean made plastic analog meter to read SWR. I keep this unit in my service truck wrapped up, however it may have gotten jarred because the unit still works fine but all of a sudden mechanical zero is up about ten percent of full scale. There is no zero adjustment accessable through the front of the instrument. This is a nice little unit however to adjust mechanical zero you have remove the analog meter and open the case. This requires complete disassembly of the entire instrument and it is a real pain. I did this and opened the small meter and adjusted mechanical zero. This is a simple procedure however the problem is static electricity. Depending on how I hold this meter, touch it, rub my finger or even a piece of cardboard near it, mechanical zero may shift. At times I think that I have it zeroed and I close the case up only to find that zero has shifted. Then I touch it wirth my finger and zero will sometimes shift again. So the problem is that I don't know if after I touch this thing if It is totally discharged and it is really at zero or if static electricity has influenced the position of the needle. This simple job is really driving me nuts. It is very dry in the house and I'm sure that this may have something to do with it, but I just don't know how to compensate for this. Obviously it is important that my adjustment is correct as there is no access to it once the unit is re assembled. Can anyone offer any advice? Thanks, Lenny. |
#2
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mechanical zero on a small meter
On 2/18/2010 3:00 PM klem kedidelhopper spake thus:
So the problem is that I don't know if after I touch this thing if It is totally discharged and it is really at zero or if static electricity has influenced the position of the needle. This simple job is really driving me nuts. It is very dry in the house and I'm sure that this may have something to do with it, but I just don't know how to compensate for this. Obviously it is important that my adjustment is correct as there is no access to it once the unit is re assembled. Can anyone offer any advice? I think you alluded to the answer yourself when you pointed out that the house is very dry: take the meter to a more humid environment, which should cut down on that annoying static electricity. -- You were wrong, and I'm man enough to admit it. - a Usenet "apology" |
#3
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mechanical zero on a small meter
David Nebenzahl writes:
On 2/18/2010 3:00 PM klem kedidelhopper spake thus: So the problem is that I don't know if after I touch this thing if It is totally discharged and it is really at zero or if static electricity has influenced the position of the needle. This simple job is really driving me nuts. It is very dry in the house and I'm sure that this may have something to do with it, but I just don't know how to compensate for this. Obviously it is important that my adjustment is correct as there is no access to it once the unit is re assembled. Can anyone offer any advice? I think you alluded to the answer yourself when you pointed out that the house is very dry: take the meter to a more humid environment, which should cut down on that annoying static electricity. Wipe the face with an anti-cling dryer sheet. -- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#4
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mechanical zero on a small meter
On Thu, 18 Feb 2010 15:00:15 -0800 (PST), klem kedidelhopper
wrote: I have an old MFJ SWR meter with a built in counter. Which one of these? http://www.mfjenterprises.com/Catergories.php?sec=13 The only one that has both an LCD and meter is the MFJ-259B: http://www.mfjenterprises.com/Product.php?productid=MFJ-259B Any particular reason why you find it necessary to hide the model number? You generally get much better answers if you disclose such information. It has an LCD display to read frequency and it uses a small 2 inch korean made plastic analog meter to read SWR. I keep this unit in my service truck wrapped up, however it may have gotten jarred because the unit still works fine but all of a sudden mechanical zero is up about ten percent of full scale. See if you can find a similar meter. Note that the jeweled bearings at the ends of the meter needle shaft are thin brass cantilevered arrangements, which are easily bent by a high G shock load. It's not unusual for the meter pivot point to literally fall out of the jeweled bearing. More commonly, the arm gets bent, causing the meter to move erratically. It's also possible to "tangle" the two springs that zero the meter. That results in a zero drift, similar to what you're seeing. Swing the meter back and forth sideways and see if the needle moves smoothly. There is no zero adjustment accessable through the front of the instrument. This is a nice little unit however to adjust mechanical zero you have remove the analog meter and open the case. This requires complete disassembly of the entire instrument and it is a real pain. Yep. MFJ probably saved about 5 cents on the cost of the case by removing the meter adjust holes. However, meters should not require zero adjustment unless affected by a nearby magnetic field. With two meters that close together, there's going to be some magnetic interaction. Perhaps MFJ adjusts it at the factory, and doesn't want anyone changing it as the procedure might be a bit convoluted. Dunno and I'm guessing. That brings about another possibility that all the jarring around in the service truck cause the meter magnet to break loose. Shake it. Hear anything rattling? I did this and opened the small meter and adjusted mechanical zero. This is a simple procedure however the problem is static electricity. Depending on how I hold this meter, touch it, rub my finger or even a piece of cardboard near it, mechanical zero may shift. At times I think that I have it zeroed and I close the case up only to find that zero has shifted. Then I touch it wirth my finger and zero will sometimes shift again. So the problem is that I don't know if after I touch this thing if It is totally discharged and it is really at zero or if static electricity has influenced the position of the needle. This simple job is really driving me nuts. It is very dry in the house and I'm sure that this may have something to do with it, but I just don't know how to compensate for this. I don't think it's static electricity. If it were, BOTH meters would be affected. Since you didn't mention that there were two meters or that the other meter was affected, my guess is that only the SWR meter is giving you problems. Also, the meter plastic front is not large enough to collect a very large static charge. Schematic: http://www.dk3red.homepage.t-online.de/en/s8a.htm Hmmm.... SWR meter has a 100K pot in series to the output of the OP amp. Not exactly low impedance. So, the meter is essentially running with no dampening. Eliminating static electricity and a broken shunt, that leaves mechanical damage. Obviously it is important that my adjustment is correct as there is no access to it once the unit is re assembled. Can anyone offer any advice? Thanks, Lenny. No advice. Just two questions: 1. What's the model number? 2. Are both meters exhibiting the same zeroing problem? -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com # http://www.LearnByDestroying.com AE6KS |
#5
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mechanical zero on a small meter
klem kedidelhopper wrote in message
... I have an old MFJ SWR meter with a built in counter. It has an LCD display to read frequency and it uses a small 2 inch korean made plastic analog meter to read SWR. I keep this unit in my service truck wrapped up, however it may have gotten jarred because the unit still works fine but all of a sudden mechanical zero is up about ten percent of full scale. There is no zero adjustment accessable through the front of the instrument. This is a nice little unit however to adjust mechanical zero you have remove the analog meter and open the case. This requires complete disassembly of the entire instrument and it is a real pain. I did this and opened the small meter and adjusted mechanical zero. This is a simple procedure however the problem is static electricity. Depending on how I hold this meter, touch it, rub my finger or even a piece of cardboard near it, mechanical zero may shift. At times I think that I have it zeroed and I close the case up only to find that zero has shifted. Then I touch it wirth my finger and zero will sometimes shift again. So the problem is that I don't know if after I touch this thing if It is totally discharged and it is really at zero or if static electricity has influenced the position of the needle. This simple job is really driving me nuts. It is very dry in the house and I'm sure that this may have something to do with it, but I just don't know how to compensate for this. Obviously it is important that my adjustment is correct as there is no access to it once the unit is re assembled. Can anyone offer any advice? Thanks, Lenny. Mask off the scale area, coil section ,bearings and hair spring, and bulb section and spray all else inside and out with nickel spray? or paint with real or faux aquadag (PVA and graphite). With painted-in conductive tails inside and out to chassis -- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on http://diverse.4mg.com/index.htm |
#6
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mechanical zero on a small meter
klem kedidelhopper wrote in message
... I have an old MFJ SWR meter with a built in counter. It has an LCD display to read frequency and it uses a small 2 inch korean made plastic analog meter to read SWR. I keep this unit in my service truck wrapped up, however it may have gotten jarred because the unit still works fine but all of a sudden mechanical zero is up about ten percent of full scale. There is no zero adjustment accessable through the front of the instrument. This is a nice little unit however to adjust mechanical zero you have remove the analog meter and open the case. This requires complete disassembly of the entire instrument and it is a real pain. I did this and opened the small meter and adjusted mechanical zero. This is a simple procedure however the problem is static electricity. Depending on how I hold this meter, touch it, rub my finger or even a piece of cardboard near it, mechanical zero may shift. At times I think that I have it zeroed and I close the case up only to find that zero has shifted. Then I touch it wirth my finger and zero will sometimes shift again. So the problem is that I don't know if after I touch this thing if It is totally discharged and it is really at zero or if static electricity has influenced the position of the needle. This simple job is really driving me nuts. It is very dry in the house and I'm sure that this may have something to do with it, but I just don't know how to compensate for this. Obviously it is important that my adjustment is correct as there is no access to it once the unit is re assembled. Can anyone offer any advice? Thanks, Lenny. Are you sure it is not erroneous electronics doing the sensing and actually putting out some wavering DC instead of zero, rather than static electricity. Monitoring terminals of meter with a DVM say |
#7
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mechanical zero on a small meter
On Fri, 19 Feb 2010 10:48:21 -0000, "N_Cook"
wrote: Are you sure it is not erroneous electronics doing the sensing and actually putting out some wavering DC instead of zero, rather than static electricity. Monitoring terminals of meter with a DVM say Getting to the meter terminals is a bit awkward but not impossible. See photos at: http://jan.kepic.cz/accessories/MFJ-259b/index.htm http://fs.textcube.com/blog/0/281/attach/Xc2ili2zpJ.jpg -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#8
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mechanical zero on a small meter
On Feb 19, 11:06*am, Jeff Liebermann wrote:
On Fri, 19 Feb 2010 10:48:21 -0000, "N_Cook" wrote: Are you sure it is not erroneous electronics doing the sensing and actually putting out some wavering DC instead of zero, rather than static electricity. Monitoring terminals of meter with a DVM say Getting to the meter terminals is a bit awkward but not impossible. See photos at: http://jan.kepic.cz/accessories/MFJ-259b/index.htm http://fs.textcube.com/blog/0/281/attach/Xc2ili2zpJ.jpg -- Jeff Liebermann * * 150 Felker St #D * *http://www.LearnByDestroying.com Santa Cruz CA 95060http://802.11junk.com Skype: JeffLiebermann * * AE6KS * *831-336-2558 It is static electricity. Try rubbing your friendly neighborhood cat or dog and then put your hand on the meter face. I'd bet my pension that you can move the needle from its normal zero point! |
#10
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mechanical zero on a small meter
On Feb 20, 11:35*am, Jeff Liebermann wrote:
On Fri, 19 Feb 2010 19:20:06 -0800 (PST), "hr(bob) " wrote: It is static electricity. *Try rubbing your friendly neighborhood cat or dog and then put your hand on the meter face. *I'd bet my pension that you can move the needle from its normal zero point! Well Ok. *I'll concede that it might be static electricity. Currently, the humidity here is between 75-100% depending on who's home wx station I want to believe: http://www.wunderground.com/US/CA/Santa_Cruz.html I could rub the overly friendly cat or worthless dog until my hand tires and still not get a spark. *However, the author indicates that his house is quite dry, but doesn't indicate if he's throwing lightning bolts (a sure sign of static electricity). *I've seen static electricity affect meters, but only on larger panel meters. *The smaller meters don't seem to have enough insulating plastic to store a big enough charge. *I have a nearby friend that has an MFJ-259B and will do some tinkering as time permits. -- Jeff Liebermann * * 150 Felker St #D * *http://www.LearnByDestroying.com Santa Cruz CA 95060http://802.11junk.com Skype: JeffLiebermann * * AE6KS * *831-336-2558 I can put my hand (here in the Chicago area) on the plastic face of my multimeter, slide my index finger around in circles on the clear plastic that is the faceplate of the meter, and get the no-input / meter zero to go up or down 1/10 of the full-scale deflection. Of course, it is about freezing and relative humidity is low enough that when I slide out of my car I make sure to touch the car with my car keys firmly grasped when I remember. Otherwise, I get a pretty good shock. |
#11
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mechanical zero on a small meter
klem kedidelhopper schrieb:
[...] So the problem is that I don't know if after I touch this thing if It is totally discharged and it is really at zero or if static electricity has influenced the position of the needle. This simple job is really driving me nuts. It is very dry in the house and I'm sure that this may have something to do with it, but I just don't know how to compensate for this. Obviously it is important that my adjustment is correct as there is no access to it once the unit is re assembled. Can anyone offer any advice? Thanks, Lenny. Hi Lenny, use something like this: http://www.youtube.com/watch?v=l5_8lN_2aKs HTH Reinhard |
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