mechanical zero on a small meter

On 2/18/2010 3:00 PM klem kedidelhopper spake thus:

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.

Which one of these?

The only one that has both an LCD and meter is the 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.

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.

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 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:

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.

No advice. Just two questions:

1. What's the model number?
2. Are both meters exhibiting the same zeroing problem?

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

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

klem kedidelhopper schrieb:

Hi Lenny,

use something like this:

HTH

Reinhard

Getting to the meter terminals is a bit awkward but not impossible. See photos at:

lly

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:

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.

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.

After setting zero I moved my finger over the meter face first thing in the morning and the needle moved up scale about 10 percent. I let it sit on the bench for about 24 hours without touching it. When I looked at it again it was back to zero. So the trick is to set it, leave it for a day let it discharge, and then tweak it again. Clumsy procedure but it works. Glad this is my own equipment and not a customer repair job....Lenny

Nice workaround. So it is static. Is this NORMAL for a small panel meter? We don't have much of a static problem on the left coast, so I don't see much of that. I waved my hand around various small panel meters around the shop. No deflection.

Well if it were Summer this would be a different story. It does get pretty humid here in New Hampshire at times and I'm sure that during those months static would pose no problem at all. During the Winter the wood stove is going here constantly and it is very dry. We walk around discharging ourselves on handrails, metal cabinets, doorknobs, etc. I once wiped out the front end of a portable two way by walking across a room holding it in my hand and carelessly touching the rubber duck to ground. I did that only once....Lenny

The easiest and best solution is to eliminate the static charge by rubbing the meter face with a clothes dryer anti-cling sheet. Alternatively, buy a bottle of anti-cling spray and give the meter a ligh misting. That will dissipate the static charge and you can handle the meter as you wish without upsetting the zero setting.

This all sounds terribly bizarre - Its a moving coil meter, it might go "whack" on getting a static charge, but a steady reading? - must be an INCREDIBLY sensitive meter movement- femtoamps? - have a look at the output of the op amp, preferably with an analogue meter - (as suggested) (no signal, 0 output) and the bias resistors around it. Especially the high value (>100k ones) (any Tantalum capacitors, replace them) (sorry Dave - a view from the trenches)

Andrew VK3BFA.

PS - its axiomatic that the thing causing the fault is in the least accessible section of the cct. A variation of Murphys Law.

It is NOT current flow from a static discharge that creates the reading. It is a static (as in unchanging) charge on the case attracting or repelling some other part of the movement.

- must be

I think the static charge thing refers to a force produced directly by a static charge on the plastic meter face acting against the needle, not a current flowing through the coil.