Rejuvenating Old Meters
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 manufacturer
L. 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
February 1943 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights 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!
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
Fig. 2 - Converting a paper clip into a tool for removing chips.
Friction - 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 friction.
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.
Repairing D.C. Meters
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.
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 quarters.
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.
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.
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.
Friction - 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
Sometimes 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 important.
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 position.
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.
the meter back in its case, being careful not to break the tip on
the zero adjusting screw which is mounted in the cover.
Calibration - 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
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, however.
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.
1996 - 2018
BSEE - KB3UON
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