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. All copyrights (if any) are hereby acknowledged.
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
Ray 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 again.
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
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. Meter Terms
For those not familiar with the terminology, some of the terms used
will be explained.
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.
- 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. 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. 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.
- 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
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
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
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 at home.
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.
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.
- Before rebalancing the meter, be sure the pointer
is perfectly straight and that any retouching where paint was chipped
off is completed.
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
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. 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.
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
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.
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.
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
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.