Rejuvenating Old Meters
February 1943 QST Article
article with instructions relating to subjects like overthrow,
balance, friction, and cleaning could very well be about a
country's revolutionary struggles. In this case, it is an article
about how to rejuvenate a persnickety or inaccurate mechanical (aka
analog) meter movement. W.R. Triplett, relative (I assume) of meter
Triplett, is the author (Triplett is now owned by
There are a lot of analog meters around in labs, workshops, and
garages. Unless they have been burnt out, most probably still work
like new. Occasionally, however, the movements get sticky because of
accumulations of dirt and dust, bug filth, or even from corrosion.
This article offers some great tips for making them serviceable
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. As time permits, I will
be glad to scan articles for you. All copyrights (if any) are hereby acknowledged.
I have a Micronta (Radio Shack) model 22-208 FET-VOM
that I bought back in the late 1970s. I put a new 9V battery in it
every 5 years or so and it keeps going like the Energizer bunny (in
fact it now uses an Energizer battery). Analog meters are more
useful than digital meters, IMHO, when making adjustments where you
are looking for a peak reading or if you are watching a
slow-changing voltage. They are also really good for troubleshooting
low frequency circuits because you can see an AC component on a DC
signal that might be missed or misinterpreted with a DMM.
Here is a brief history of the
Triplett company, which was started in 1904 when Ray was 19
years old and sold in 2007. Yes, he built that!
See all available vintage
Rejuvenating Old Meters
Practical hints for Servicing D.C. and A.C. Instrument
W. R. Triplett, W80WW
If that meter with the stationary pointer isn't actually
burned out, there's a chance that it can be put back into operating condition
with a little careful work. Here's how to go about it.
It is hardly necessary to say that at present, and probably for the duration,
amateurs will be unable to buy new meters - or get old ones repaired - without
top priorities. So there is no alternative but to make use of what we have.
This article has been prepared for the amateur who needs meters,
and who has some which may be inoperative but can be fixed up to be serviceable.
But let not false hopes arise; the majority of damaged meters are beyond repair
by the amateur. Nevertheless, if there is nothing seriously wrong it should
not be difficult to put many of them back in operating condition. Consideration
will be given only to small moving-coil d.c, and moving-iron a.c, meters,
since these are the most common types.
For those not familiar with the terminology, some of the terms
used will be explained.
Sticky meter - As the term implies, a sticky meter is one in which
the pointer stops at some point along the scale when the applied current is
gradually increased or decreased. The cause of a sticky meter usually is lint,
dirt or metal chips which interfere with coil movement in d.c. meters or movement
of the vane in a.c. meters. If the meter has been uncased and exposed to the
average debris around the shack, it will probably be sticky.
Fig. 1 - Iron or steel chips clinging to the magnet will prevent
free movement of the coil assembly and cause the pointer to stick.
Fig. 2 - Converting a paper clip into a tool for removing chips.
- A meter is said to have friction when, after gradual application of current
to cause the pointer to advance slowly to a specified point, tapping the meter
gently causes the pointer to show an increase in reading. For most commercial
meters the change in reading caused by tapping should not exceed 1/2 per cent.
However, the amateur can allow considerable leeway depending on the particular
application. Friction is caused by dirty points and jewels, dull pivots, cracked
jewels, or lint. If the meter has been handled roughly it may have excessive
Balance - Theoretically the pointer should
remain on zero (with no current, of course) no matter in what position the
meter is held. If this is not the case, the meter is said to be off balance.
Practical limits permit one degree deviation from zero. The movement is balanced
by small adjustable weights, or else by a flexible "tail weight" which is
bent until balance is obtained. Another method is to use small amounts of
quick-drying paint or shellac, though this is not recommended because of changes
in balance due to humidity and temperature.
Overthrow - This
term applies to the distance the pointer can move beyond full scale or below
zero. The amount of overthrow should be at least 3 per cent of the total scale
and can be adjusted by moving the pointer stops, which frequently are porcelain
beads mounted on wire.
Accuracy - Commercial tolerances
permit variations from the true reading of ± 2 per cent. This is understood
to mean ± 2 per cent of full-scale deflection.
In repairing any meter it is advisable to proceed
as follows: On a clean, well-lighted table place a clean white piece of glazed
paper. Using a small paint brush, clean off any metal chips that may be on
the tools you use. Do not use a cloth since the lint will float around and
eventually get in the meter.
Carefully unease the meter, but do not
unsolder shunts or springs. No attempt should be made to remove the coil and
movement from the magnet.
Fig. 3 - A heating device for burning lint in close
A quick check will indicate whether further labor is worthwhile. If the
springs or coil are burned, the meter is beyond repair by the amateur. If
the case or glass is broken, it is a sure bet that the pivots are dull, causing
excessive friction. However, considerable friction may be tolerated in some
applications. The amateur should no try to replace or sharpen the pivots.
If the coil and springs appear satisfactory, set up a battery or power
supply and potentiometer so the pointer can be slowly run up and down scale.
Then check for stickiness and friction.
Stickiness - Stickiness
is usually caused by chips (see (Fig. 1). These can be seen by looking through
the pole pieces against the white paper. Bend a steel paper clip and file
it as shown in Fig. 2. Brush off the filings before using. Carefully insert
the straightened end between the pole piece and the core, being careful not
to touch the springs or the coil. The chip will be attracted to the steel
clip and can usually be pulled out. A few tries may be necessary until you
get the knack of it.
Stickiness is also caused by lint touching
the coil or pointer. Look for this with a magnifying glass or eye loop. The
least amount of lint can cause erratic readings, so examine thoroughly all
possible places where lint may interfere with a moving part. Lint can sometimes
be removed with tweezers, but frequently must be burned out with a heater
unit as is shown in Fig. 3. If the heater is used, care must be exercised
not to burn the springs or coil wire.
If stickiness is caused simply
by the pointer touching the dial, straighten the pointer with tweezers. If
you chip the paint, a little India ink will fix it up.
- If there is excessive friction, look for fuzz or lint and remove as explained
above. If the friction is not caused by lint, probably the pivots are dull
or the jewel is cracked. Neither of these can be fixed at home.
the bearings are too tight. Try loosening the jewel screw a half revolution
or so. Meters with excessive friction may be used where accuracy is not too
Balance - Before rebalancing the meter, be sure the pointer is perfectly
straight and that any retouching where paint was chipped off is completed.
Fig. 4 - The three steps in balancing a meter. (A) Set pointer on zero by
means of zero adjustment screw while holding meter with plane of dial in
horizontal position. (B) Adjust tail weight until pointer is on zero while
holding meter with plane of dial in vertical position. (C) Adjust side weight
until pointer is on zero while holding meter with plane of dial in vertical
The method of balancing will be readily ascertained from an examination
of the meter. Perhaps a special tool or tweezers will have to be made to move
screw-type weights. The design of such tools must be left to individual ingenuity,
depending upon the particular construction.
The balancing procedure
is indicated in Fig. 4. After completing the process, repeat it for checking
and making final adjustments. As little pressure as possible should be used
in adjusting the weights because the pivots can easily be damaged in this
operation. Also be careful not to touch the springs. After finishing with
the balancing, check for any fuzz or lint that may have been left on the weights.
Follow a similar procedure if a flexible tail weight or shellac is
used for balancing.
Overthrow - If the meter has pointer
stops, these can be adjusted to get an overthrow of a few divisions above
full scale and behind zero. Make certain the pointer hits the stop before
the moving element hits in order to prevent sticking at end scale.
Cleaning - Dial marks can be removed with a rubber eraser. Clean
the case with the paint brush; again take care not to use a cloth rag.
Put the meter back in its case, being careful not to break the tip on
the zero adjusting screw which is mounted in the cover.
- If the springs have not been damaged and if the internal shunt or resistance
wire has not been unsoldered, the meter should be fairly accurate. However,
age or proximity to transformers and leads carrying heavy currents may have
weakened the magnet. If the shunt or series resistance wire has been unsoldered,
errors may be caused by resoldering at a different point.
If no other
meter is available to check the accuracy of the repaired meter, a multimeter
can be used with fair results. Perhaps the local service man will loan his.
Using the potentiometer set-up mentioned before, check the calibration
using the multimeter or other instrument as the standard. If the accuracy
is not satisfactory, remove the cover and make red pencil marks for the points
or paste on & new paper dial and mark off a complete scale. When making
pencil marks, be sure not to touch the pointer since it may bend and thus
upset the meter balance.
It is well to note that the reading
of a d.c, meter will decrease when the instrument is mounted in a steel panel.
The amount of decrease depends upon the particular meter and the thickness
of the panel, If the meter is to be used in a steel panel, it would be well
to check the accuracy in the same panel. A.c. meters are not affected appreciably
by steel panels.
Repairing A.C. Meters
The same procedure should be followed and similar adjustments made in the
case of a.c, moving iron type meters. A few additional words are in order,
Usually there will be no metal chips in an a.c. meter
because there is no magnet to hold them there.
Most a.c. meters employ
a fan swinging in a closely fitted chamber to obtain damping. Dirt or fuzz
in this chamber will cause stickiness or excessive friction.
It is important not to bend the soft iron vane (either movable or stationary)
since the meter accuracy is dependent upon the proper placing of these vanes.
The same holds true to an extent for the pointer on a.c, meters. Also, changing
the position of the coil around the vanes will affect the accuracy.
Extending Meter Ranges
The formulas for
extending the ranges of d.c. voltmeters and milliammeters are given in The
Radio Amateur's Handbook. These apply to a.c, meters as well, if the resistors
are non-inductive and the value of meter resistance used is the a.c. resistance.
Since the resistors mayor may not be non-inductive, and the a.c. resistance
of the meter mayor may not be close to the d.c. value, it will probably be
advisable to check the calibration.
Best accuracy is of course obtained
with precision wire-wound resistors. Lacking these, carbon resistors will
have to suffice. Probably this type will not be obtainable in the correct
resistance values, so the advice is, use what you have and mark the dial accordingly.