Analog voltmeter problem

[timelessbeing]

I have an Armaco KR-38 analog panel-mount voltmeter, which I salvaged out of an antique home-made (or possibly kit) power supply. Made in Japan.

I decided to re-purpose it for a project I'm working on, when I noticed a problem. It was reading more than 2V low, which I verified using a DMM with probes put directly to the KR-38 terminals. The zero calibration screw was set properly. I have three more KR-38s, so I decided to just swap it out with another one. I tested the other three, and they were reading fine, so I mounted another one to my project's metal front panel.  Then it was reading low too. I tested the (now liberated) original meter again, and it was reading fine. Sure enough, when I take the nuts off the meter, apply a voltage, and move the meter in and out of the panel, the needle rises and falls.

The metal panel is somehow interfering with the meter reading, but why? Here's the kicker... my project enclosure is the very one which the KR-38 came out of. I reused that too! Surely, these meters were designed with metal panels in mind. I have another analog volt meter (Chinese cheapie from eBay) mounted to the same panel, and it reads fine. As far as I can tell, the sheet metal panel isn't magnetised. I doesn't pick up tiny bits of metal.

Can anyone think of a reason why this is happening?

[dom0]

Moving coil  instruments are _very_ sensitive to external magnetic fields. I have not a single metal screw driver in my workshop that I can use to adjust the zero screw of some of them, they are that sensitive - and almost all of these are un-magnetized tools.

I imagine it is quite possible that the steel sheet of the case picked up a small magnetic field that dampens the meter. I only built these into aluminium panels and noticed no issues with them.

[timelessbeing]

Wow. Not a terribly useful instrument, IMO, if it is that sensitive to fields.

The meter itself has a field strong enough to pick up screws, so I guess that means you cannot have two meters close to each other.

[Dr. Frank]

The cover of this movement is made of plastic,which usually picks up electric charge.The metallic enclosure may act as the counter electrode, causing this effect.
You should  treat this plastic surface with an antistatic foam cleanser,like AFC from Electrolube, for example.

Very probably, that will help.

Frank

[calexanian]

Short the meter inputs together before cleaning it, otherwise the needle might bounce around. somewhere on the meter face should read the current required for full scale deflection as well. You may just have a ridiculously sensitive meter that is being affected by something you otherwise may not be aware of. Knowing what type of movement it has may be helpful. Generally power supply meters will be of either a moving magnet or moving coil type. Rarely are they moving iron or electrostatic, but I have seen it.

[SuzyC]

Your meter simply needs a little tuning. You just need to adjust the angle of the dangle to ensure the meter is more honest than modest in its presentation.

Get a bit of superglue and some ceramic cheapo magnets used to paste memos on fridges. Cut one or more cutesy mags into 1/4-in strips.
Now apply the correct voltage at 1/4/ 1/2, /3/4 and FS and adj the position of these mini-magnets such behind the panel, but proximal  to the meter's movement so as to minimize  error.  Recheck your readings, optimize your calibration to the limits of your patience and extent of your desire for accuracy and then apply a dot of superglue to each to seal the deal.

Be aware that re-orientating the device you've installed the meter in relative to the earth's poles or changing from horiz. to vert. positioning might affect readings.

A meter like this is most comfortable being honest when allowed to be stably positioned in this vast universe of ours..while a gentile wipe of the viewing surface with a drop of lemon juice on a water soaked Q-Tip is also necessary to remove any electrostatic distraction.

[HighVoltage]

I only built these into aluminium panels and noticed no issues with them.

Even in Aluminum cases you can have magnetic currents and I had a similar problem a long time ago.
The only real solution was to use "magnetically shielded" analog meters.

The suggestion made by SuzyC might be the only solution, if you want to stay with theses meters.

[timelessbeing]

I picked up this project again today.

I tried the lemon juice, and the magnets (even strong neodymium ones). It didn't change a thing. At full scale, the meter is off by 2V, which means that even the offset adjustment screw isn't enough to compensate. I might have to switch to a plastic enclosure, which won't be as robust.

[duak]

Is there any chance the needle is dragging on the cover or body?  I've got a few Armaco meters and if memory serves, the body is made of something like bakelite which is usually stable.  However, since the meter is repurposed, it may have been dropped at one time and either the body cracked or the movement itself got knocked out of alignment with the body and the needle then drags slightly.  It can work fine unmounted, but when it's tightened down the trouble appears.

Also, is there any chance one of the mounting screws or nuts is magnetized?  I think they used brass or stainless but one could have been replaced.

BTW, if the reading is consistently low, you might consider paralleling the series resistor within the meter with a high value trim resistor.

Cheers,

[timelessbeing]

No, it happens even if I hold it in lightly with my finger, and it's not cracked.

That's an interesting idea with the parallel pot. Now that I think about it, there might have been a component across the inputs originally, but I'm pretty sure it was just a cap.

[duak]

News Flash!  I ran across this article: http://www.rfcafe.com/references/qst/rejuvenating-old-meters-feb-1943-qst.htm

The 7th to last paragraph talks about steel panels causing DC instruments to read low.  Who knew?  It seems they did in 1943.  When I look at fig 1. of the article, I see that steel could disturb or shunt the magnetic flux in the gap around the moving coil making it less responsive to the current and then read low.

Cheers,

[Kleinstein]

A steel case can disturb the meter internal field. Depending on the construction the reading could go up or down.  An aluminum case is no problem.  For protection there might have been diodes in parallel to the actual meter (without the series resistor to set the voltage range). For extra damping, sometimes a capacitor in parallel is used too.

Electrostatic charge is a different problem. Cheap meters tend to use polystyrene and this can hold static charge for very long. I had an old analog multimeter with such a problem - the needle nearly stuck at about 40% full scale and thus nearly useless, until I finally decided to open it up to remove the charge. Essentially no discharge in more than 1 year. So be careful with wiping with a dry cloth.

[timelessbeing]

BTW, if the reading is consistently low, you might consider paralleling the series resistor within the meter with a high value trim resistor.

It's not exactly consistent. The higher up on the scale, the greater the difference. But it could be linear.

So where can I find the series resistor? I wouldn't mind knowing a bit more about this before I start dissecting this delicate instrument.

[Cyberdragon]

BTW, if the reading is consistently low, you might consider paralleling the series resistor within the meter with a high value trim resistor.

It's not exactly consistent. The higher up on the scale, the greater the difference. But it could be linear.

So where can I find the series resistor? I wouldn't mind knowing a bit more about this before I start dissecting this delicate instrument.

It will most likely be a coil of wire on a little spindle somewhere in the back section near the terminals.

[duak]

If memory serves, these meters use two threaded studs for input terminals that are attached to the movement and passing thru the back of the case.  There should be nuts on the studs.  By removing the nuts, pushing on the studs will push the movement out of the case and allow access to the multiplier resistor.  Leave the transparent front cover in place to protect the movement and also be careful to keep your work area clean.  Any magnetic debris will be drawn into the movement's gap.  If they don't use studs, then there must be a few small screws holding the movement in the case.

Cheers,

[timelessbeing]

It looks like the terminal studs are molded into the case. They don't seem to push through.

There are also two screws recessed in deep holes, and some white compound painted over them. I guess those are it ...

[duak]

Well, my recollections of those meters sure wasn't right.   I found the movement is a lot easier to manipulate if the dial is left on.  If you take the dial off be extra super careful to not bend the pointer or the top suspension. 

Best o' luck