Magnetized meter

[Mike Muderick]

I have a battery tester meter (Mallory) that came from a retail store. The
meter is magnetized and now sits center scale, instead of over to the left.
Any solutions to return it to it's normal state. I tried reverse polarity-
it goes to the left, but then returns to center.

thanks

 

[HankG]

I have a battery tester meter (Mallory) that came from a retail store. The
meter is magnetized and now sits center scale, instead of over to the left.
Any solutions to return it to it's normal state. I tried reverse polarity-
it goes to the left, but then returns to center.

thanks

Just wondering--is there a zero-adjust screw or tang (for want of a better
word) on the meter?

HankG

 

[Just Another Theremin Fan]

The meter is magnetized and now sits center scale,

Cobblers and rubbish. You've probably dropped it.

 

[Michael Ware]

Like Hank suggested, should be a flathead screw adjustment on the front of
the meter, at the bottom.

 

[Dave D]

I have a battery tester meter (Mallory) that came from a retail store. The
meter is magnetized and now sits center scale, instead of over to the
left.
Any solutions to return it to it's normal state. I tried reverse
polarity-
it goes to the left, but then returns to center.

thanks

Sounds more like some kind of mechanical failure, I've never heard of
magnetism causing that margin of error.

Dave

 

[Reinhard Zwirner]

I have a battery tester meter (Mallory) that came from a retail store. The
meter is magnetized and now sits center scale, instead of over to the left.
Any solutions to return it to it's normal state. I tried reverse polarity-
it goes to the left, but then returns to center.

I've seen such an effect when the transparent plastic cover of the scale
has
been wiped intensively for cleaning purposes generating electrostatic
charge.
Try to breathe on the plastik to discharge it.

HTH

Reinhard

 

[Sam Goldwasser]

Sounds more like some kind of mechanical failure, I've never heard of
magnetism causing that margin of error.

Magentism won't affect a normal D'Arsonval meter movement as far as
zero position is concerned.

If you examine it closely, there will probably be an obvious cause like
a bent needle or misadjusted zero screw.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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[Mike Muderick]

Tried all of the above. There is no zero-adjust screw. Took the plastic
cover off to try and discharge any static electicity. Nothing is bent-
meter wasn't dropped- It just won't go back to zero.
mm

 

[Mark D. Zacharias]

Tried all of the above. There is no zero-adjust screw. Took the
plastic cover off to try and discharge any static electicity.
Nothing is bent- meter wasn't dropped- It just won't go back to zero.
mm

Maybe the internal magnet shifted.

Mark Z.

 

[Ken Weitzel]

Tried all of the above. There is no zero-adjust screw. Took the plastic
cover off to try and discharge any static electicity. Nothing is bent-
meter wasn't dropped- It just won't go back to zero.
mm

Hi...

It's just a shot in the dark, but doesn't cost anything so perhaps
worth a try... ?

Dampen a clean cloth in a bit of diluted fabric softener; wipe the
whole thing down with it. See what happens.

Ken

 

[Sam Goldwasser]

Maybe the internal magnet shifted.

Mark Z.

Huh?

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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[DaveM]

Well, since you've already had the cover off, try this.
Open the meter again and gently try to move the needle back toward zero with a
fingertip or a toothpick. If it doesn't move freely, or moves but stays in the
position it was moved to, your meter is damaged. Most likely cause is a broken
hairspring or pivot. In either case, the meter is unrepairable by most. The
only course of action is to buy a new tester.
You can, of course, get some small tools and gingerly disassemble the meter to
get at the innards, but if the meter is damaged as I described, you're just
chasing a rainbow.

If the meter needle does move freely but just returns to mid scale, you might be
able to remedy that. Open the meter and remove the screws that hold the meter's
mechanism in the housing. You might have to fiddle with the wires that connect
the meter mechanism to the terminals on the rear of the housing. Keep them out
of the way, or at least keep the strain off those wires because they attach to a
delicate mechanism. On the rear of the movement is another pivot adjustment,
normally where one of the connecting wires is soldered. Gently move this pivot
arm and see if the needle responds. If so, see if there is enough range in
movement to return the needle to normal zero position. If so, you're home free.
If not, you're back to square one, looking for a new tester.


--
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!!

 

[Bill Janssen]

Well, since you've already had the cover off, try this.
Open the meter again and gently try to move the needle back toward zero with a
fingertip or a toothpick. If it doesn't move freely, or moves but stays in the
position it was moved to, your meter is damaged. Most likely cause is a broken
hairspring or pivot. In either case, the meter is unrepairable by most. The
only course of action is to buy a new tester.
You can, of course, get some small tools and gingerly disassemble the meter to
get at the innards, but if the meter is damaged as I described, you're just
chasing a rainbow.

If the meter needle does move freely but just returns to mid scale, you might be
able to remedy that. Open the meter and remove the screws that hold the meter's
mechanism in the housing. You might have to fiddle with the wires that connect
the meter mechanism to the terminals on the rear of the housing. Keep them out
of the way, or at least keep the strain off those wires because they attach to a
delicate mechanism. On the rear of the movement is another pivot adjustment,
normally where one of the connecting wires is soldered. Gently move this pivot
arm and see if the needle responds. If so, see if there is enough range in
movement to return the needle to normal zero position. If so, you're home free.
If not, you're back to square one, looking for a new tester.

Before you change the rear spring setting you should inspect the spiral
springs. I have seen cases
where the spiral turns got tangled (probably by too much current). I
used a tooth pick or small
wire to untangle the loops of the spring. Also look at the pivots. If
your meter coil and pointer
are supported by a "taught band" then all of the advice I gave doesn't
apply.

Bill K7NOM

 

[Mark D. Zacharias]

Huh?

--- sam | Sci.Electronics.Repair FAQ:

Well, the coil has to react with something. It's been a long time since I've
taken a meter movement apart, but clearly at the heart of the meter movement
is a magnet. If the magnet is normally stationary, while the meter coil and
pointer move with voltage applied, if the magnet were to physically rotate,
this would affect the zero center of the coil / pointer, yes?

Mark Z.

 

[Mark D. Zacharias]

Well, the coil has to react with something. It's been a long time since
I've taken a meter movement apart, but clearly at the heart of the meter
movement is a magnet. If the magnet is normally stationary, while the
meter coil and pointer move with voltage applied, if the magnet were to
physically rotate, this would affect the zero center of the coil /
pointer, yes?

Mark Z.

Actually, just realized my error. If the coil were magnetized, my point
might have been valid, otherwise not.

"never mind"


Mark Z.

 

[Sam Goldwasser]

Actually, just realized my error. If the coil were magnetized, my point
might have been valid, otherwise not.

"never mind"

Right.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header above is
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[Bob AZ]

Mike

a ½ scale thing like this is usually a hair spring failure or possible
a shock to the spring has caused a coil to interfere with another.
Bob AZ

 

[Bob AZ]

Mike

a ½ scale thing like this is usually a hair spring failure or possible
a shock to the spring has caused a coil to interfere with another.
Bob AZ

 

[Just Another Theremin Fan]

Mike

a ? scale thing like this is usually a hair spring failure or possible
a shock to the spring has caused a coil to interfere with another.
Bob AZ

I think you might find that these Mallory cheapo testers actually
place a load across the cell under test. The meter may not be a
hair spring device at all but more like a car Amp meter.

 

[Wild Bill]

Chances are good that the meter movement has suffered physical damage,
or has had a mechanical part failure.

Damage can be related to harsh handling or misuse. Checking a battery
voltage that exceeds the selected range/type can cause irrepairable
damage. In many cases, this will open the moving coil windings (fuse
action).

I suspect that since the meter will deflect down-scale (as well as up),
that the problem is a damaged hairspring. There are almost always 2
hairsprings, which are wound in different directions, at each end of
the armature.
As each hairspring tension opposes the other, if one were removed or
weakened, the other would be stronger, moving the pointer "harder" in
that direction.

If the meter movement isn't the moving coil type, then a hairspring or
the band has possibly been damaged (weakened or distorted) by
overvoltage or misuse.

If the meter wouldn't deflect down-scale, I'd suggest that the problem
might be a small bit of debris inside the meter movement that was
stopping the coil from returning to it's zero position. Sometimes a
small magnet flake or other material will cause this.

Resetting hairsprings is (not can be) a very delicate procedure. Meter
manufacturers used to actually list hairsprings as replacment parts.
The tension needs to be precisely balanced for a precision meter
application. Many armatures also utilize counterweights fitted on pins
that extend outward radially.

If it's a separate panel meter (not built into the tester's case), and
you can see some numbers on the meter, you may be able to find a
suitable replacement meter assembly. Many meter movements have a full
scale current value printed on the meter faceplate/scale plate,
something similar to 100uA DC F.S. (full scale deflection at 100
microamps).
For using meters of different ratings as substitute replacements, refer
to books covering meter shunt resistors and similar topics.

Cheers
WB
.................