Notes on the Troubleshooting and Repair of Audio Equipment and Other Miscellaneous Stuff


  12.8) Care and feeding of NiCds

Here are six guidelines to follow which will hopefully avoid voltage
depression or the memory effect or whatever:

(Portions of the following guidelines are
 from the NiCd FAQ written by: Ken A. Nishimura (KO6AF))

1. DON'T deliberately discharge the batteries to avoid memory.  You risk
   reverse charging one or more cell which is a sure way of killing them.

2. DO let the cells discharge to 1.0V/cell on occasion through normal use.

3. DON'T leave the cells on trickle charge for long times, unless voltage
   depression can be tolerated.

4. DO protect the cells from high temperature both in charging and storage.

5. DON'T overcharge the cells.  Use a good charging technique.  With most
   inexpensive equipment, the charging circuits are not intelligent and will
   not terminate properly - only charge for as long as recommended in the
   user manual.

6. DO choose cells wisely.  Sponge/foam plates will not tolerate high
   charge/discharge currents as well as sintered plate.  Of course, it
   is rare that this choice exists.

Author's note: I refuse to get involved in the flame wars with respect
to NiCd battery myths and legends --- sam.

  12.9) Why there will never actually be closure on this topic

(From: Mark Kinsler (kinsler@froggy.frognet.net)).

All of which tends to support my basic operating theory about the charging of
nickel-cadmium batteries:

1) Man is born in sin and must somehow arrange for the salvation of his
   immortal soul. 

2) All nickel-cadmium batteries must be recharged.

3) There is no proper method of performing either task (1) or task (2) to the
   satisfaction of anyone.  

  12.10) NiCd battery pack will not hold a charge

This applies if the pack appears to charge normally and the terminal
voltage immediately after charging is at least 1.2 x n where n is the
number of cells in the pack but after a couple of days, the terminal
voltage has dropped drastically.  For example, a 12 V pack reads only 6 V
48 hours after charging without being used.

What is most likely happening is that several of the NiCd cells have
high leakage current and drain themselves quite rapidly.  If they are
bad enough, then a substantial fraction of the charging current itself is
being wasted so that even right after charging, their capacity is less
than expected.  However, in many cases, the pack will deliver close to
rated capacity if used immediately after charging.

If the pack is old and unused or abused (especially, it seems, if it
is a fast recharge type of pack), this is quite possible.  The cause
is the growth of fine metallic whiskers called dendrites that partially
shorts the cell(s).  If severe enough, a dead short is created and no
charge at all is possible.

Sometimes this can be repaired temporarily at least by 'zapping' using
a large charged capacitor to blow out the whiskers or densrites that
are causing the leakage (on a cell-by-cell basis) but my success on
these types of larger or high charge rate packs such as used in laptop
computers or camcorders has been less than spectacular.

Is my battery charging?

If you are trying to substitute a battery of a different type, all bets may
be off.  For example, NiCd and lead-acid are quite different in operation
and termination conditions.  Thus, your charger may not be fully charging the
new pack for some reason or one or more of the cells may be defective.

If you can, monitor both current and voltage into the battery during charging.
The voltage should top out somewhat over the marked ratings.  The current
should work out to around 1.5 times the A-h rating over the charging period.
If this is the case, put a load on the battery and see if you get something
near the A-h rating out.

  12.11) What is this thing in my NiCd battery pack?

In addition to the NiCd cells, you will often find one or more small parts
that are generally unrecognizable.  Normally, you won't see these until
you have a problem and, ignoring all warnings, open the pack.

If it is a little rectangular silver box in series with one of the positive
or negative terminals of the pack, it is probably a thermostat and is there
to shut down the charging or discharging if the temperature of the pack
rises too high.  If it tests open at room temperature, it is bad.  With
care, you can safely substitute a low value resistor or auto tail light
bulb and see if the original problem goes away or at least the behavior
changes.  However, if there is a dead short somewhere, that device may
have sacrificed its life to protect your equipment or charger and going
beyond this (like shorting it out entirely) should be done with extreme

If it looks like a small diode or resistor, it could be a temperature
sensing thermistor which is used by the charger to determine that the
cells are heating which in its simple minded way means the cells are being
overcharged and it is should quit charging them.  You can try using a
resistor in place of the thermistor to see if the charger will now
cooperate.  Try a variety of values while monitoring the current or
charge indicators.  However, the problem may actually be in the charger
controller and not the thermistor.  The best approach is to try another pack.

It could be any of a number of other possible components but they all serve
a protective and/or charge related function.

Of course, the part may be bad due to a fault in the charger not shutting
down or not properly limiting the current as well.

  12.12) Zapping NiCds to clear shorted cells

Nickel-Cadmium batteries that have shorted cells can sometimes be rejuvenated -
at least temporarily - by a procedure affectionately called 'zapping'.

The cause of these bad NiCd cells is the formation of conductive filaments
called whiskers or dendrites that pierce the separator and short the positive
and negative electrodes of the cell.  The result is either a cell that will
not take a charge at all or which self discharges in a very short time.  A
high current pulse can sometimes vaporize the filament and clear the short.

The result may be reliable particularly if the battery is under constant
charge (float service) and/or is never discharged fully.  Since there are
still holes in the separator, repeated shorts are quite likely especially if
the battery is discharged fully which seems to promote filament formation,

I have used zapping with long term reliability (with the restrictions
identified above) on NiCds for shavers, Dust Busters, portable phones,
and calculators.

WARNING:  There is some danger in the following procedures as heat is
generated.  The cell may explode!  Take appropriate precautions and don't
overdo it.  If the first few attempts do not work, dump the battery pack.


You will need a DC power supply and a large capacitor - one of those 70,000
uF 40 V types used for filtering in multimegawatt geek type automotive audio
systems, for example.  A smaller capacitor can be tried as well.

Alternatively, a you can use a 50-100 A 5 volt power supply that doesn't mind
(or is protected against) being overloaded or shorted.

Some people recommend the use of a car battery for NiCd zapping.  DO NOT be
tempted - there is nearly unlimited current available and you could end with
a disaster including the possible destruction of that battery, your NiCd,
you, and anything else that is in the vicinity.

OK, you have read the warnings:

Remove the battery pack from the equipment.  Gain access to the shorted
cell(s) by removing the outer covering or case of the battery pack and
test the individual cells with a multimeter.  Since you likely tried
charging the pack, the good cells will be around 1.2 V and the shorted
cells will be exactly 0 V.  You must perform the zapping directly across
each shorted cell for best results.

Connect a pair of heavy duty clip leads - #12 wire would be fine - directly
across the first shorted cell.  Clip your multimeter across the cell
as well to monitor the operation.  Put it on a high enough scale such
that the full voltage of your power supply or capacitor won't cause any
damage to the multimeter.


1. Using the large capacitor:

   Charge the capacitor from a current limited 12-24 V DC power supply.

   Momentarily touch the leads connected across the shorted cell to the
   charged capacitor.  There will be sparks.  The voltage on the cell may
   spike to a high value - up to the charged voltage level on the capacitor.
   The capacitor will discharge almost instantly.

2. Using the high current power supply:

   Turn on the supply.

   Momentarily touch the leads connected across the shorted cell to the
   power supply output.  There will be sparks.  DO NOT maintain contact
   for more than a couple of seconds.  The NiCd may get warm!  While
   the power supply is connected, the voltage on the cell may rise to
   anywhere up to the supply voltage.

Now check the voltage on the (hopefully previously) shorted cell.

If the filaments have blown, the voltage on the cell should have jumped
to anywhere from a few hundred millivolts to the normal 1 V of a
charged NiCd cell.  If there is no change or if the voltage almost
immediately decays back to zero, you can try zapping couple more times
but beyond this is probably not productive.

If the voltage has increased and is relatively stable, immediately
continue charging the repaired cell at the maximum SAFE rate specified
for the battery pack.  Note: if the other cells of the battery pack are
fully charged as is likely if you had attempted to charge the pack, don't
put the entire pack on high current charge as this will damage the other
cells through overcharging.

One easy way is to use your power supply with a current limiting resistor
connected just to the cell you just zapped.  A 1/4 C rate should be
safe and effective but avoid overcharging.  Then trickle charge at
the 1/10 C rate for several hours.  (C here is the amp-hour capacity of
the cell.  Therefore, a 1/10 C rate for a 600 mA NiCd is 50 mA.)

This works better on small cells like AAs than on C or D cells since the
zapping current requirement is lower.  Also, it seems to be more difficult
to reliably restore the quick charge type battery packs in portable tools
and laptop computers that have developed shorted cells (though there are
some success stories).

My experience has been that if you then maintain the battery pack in
float service (on a trickle charger) and/or make sure it never discharges
completely, there is a good chance it will last.  However, allow the
bad cells to discharge to near 0 volts and those mischievous dendrites
will make their may through the separator again and short out the cell(s).

  12.13) Identifying technology of umarked battery packs

Since the nominal (rated) voltages for the common battery technologies
differ, it is often possible to identify which type is inside a pack
by the total output voltage:

NiCd packs will be a multiple of 1.2 V.
Lead-acid packs will be a multiple of 2.0 V.
Alkaline packs will be a multiple of 1.5.

Note that these are open circuit voltages and may be very slightly higher
when fully charged or new.

Therefore, it is generally easy to tell what kind of technology is
inside a pack even if the type is not marked as long as the voltage
is.  Of course, there are some - like 6 V that will be ambiguous.

  12.14) Problems with battery operated equipment

For primary batteries like Alkalines, first try a fresh set.  For NiCds,
test across the battery pack after charging overnight (or as recommended
by the manufacturer of the equipment).  The voltage should be 1.2 x n V
where n is the number of cells in the pack.  If it is much lower - off by
a multiple of 1.2 V, one or more cells is shorted and will need to be
replaced or you can attempt zapping it to restore the shorted cells.  See
the section: "Zapping NiCds to clear shorted cells".  Attempt at your own

If the voltage drops when the device is turned on or the batteries are
installed - and the batteries are known to be good - then an overload
may be pulling the voltage down.

Assuming the battery is putting out the proper voltage, then a number of
causes are possible:

1. Corroded contacts or bad connections in the battery holder.

2. Bad connections or broken wires inside the device.

3. Faulty regulator in the internal power supply circuits.  Test
   semiconductors and IC regulators.

4. Faulty DC-DC inverter components.  Test semiconductors and other

5. Defective on/off switch (!!) or logic problem in power control.

6. Other problems in the internal circuitry.

  12.15) Battery juice and corroded contacts

Unless you have just arrived from the other side of the galaxy (where such
problems do not exist), you know that so-called 'leak-proof' batteries (even
those with fancy warranties and high budget advertising) sometimes leak.
This is a lot less common with modern technologies than with the carbon-zinc
cells of the good old days, but still can happen.  It is always good advice
to remove batteries from equipment when not being used for an extended period
of time.  Dead batteries also seem to be more prone to leakage than fresh ones
(in some cases because the casing material is depleted in the chemical reaction
which generates electricity and thus gets thinner or develops actual holes).

In most cases, the actual stuff that leaks from a battery is not 'battery
acid' but rather some other chemical.  For example, alkaline batteries
are so called because their electrolyte is an alkaline material - just the
opposite in reactivity from an acid.  Usually it is not particularly
reactive (but isn't something you would want to eat).

One exception is the lead-acid type where the liquid inside is sulfuric acid
of varying degrees of strength depending on charge.  This is nasty and should
be neutralized with an alkaline material like baking soda before being
cleaned up.  Fortunately, these sealed lead-acid battery packs rarely
leak (though I did find one with a scary looking bulging case, probably
due to overcharging - got rid of that in a hurry).
Nickel Cadmium cells contain so-called heavy metal compounds which are also
bad for you if you feast on them but can be safely cleaned up without harm.

Scrape dried up battery juice from the battery compartment and contacts
with a plastic or wooden stick and/or wipe any liquid up first with a dry
paper towel.  Then use a damp paper towel to pick up as much residue as
possible.  Dispose of the dirty towels promptly.

If the contacts are corroded, use fine sandpaper or a small file to remove
the corrosion and brighten the metal.  Do not use an emery board, emery
paper, or steel wool as any of these will leave conductive particles behind
which will be difficult to remove.  If the contacts are eaten through entirely,
you will have to improvise alternative contacts or obtain replacements.

Sometimes the corrosion extends to the solder and circuit board traces as
well and some additional repairs may be needed - possible requiring
disassembly to gain access to the wiring.

Don't forget that many batteries do come with explicit or implicit warranties
against leakage (and resulting damage) which cover the equipment they are in
as well.  Thus, you may be able to obtain a replacement device from the battery
manufacturer for at most shipping charges.  I don't know if this extends to
expensive products like palmtop computers :-).

  12.16) Automotive power

While it is tempting to want to use your car's battery as a power source for
small portable appliances, audio equipment, and laptop computers, beware: the
power available from your car's electrical system is not pretty.  The voltage
can vary from 9 (0 for a dead battery) to 15 V under normal conditions and
much higher spikes or excursions are possible when loads like the radiator fan
or air conditioner are switched on or off.

Unless the equipment is designed specifically for such power, you are
taking a serious risk that it will be damaged or blown away.

Furthermore, there is essentially unlimited current available from the battery
(cigarette lighter) and 20 A or more without blowing a fuse.  This will
instantly turn your expensive CD player to toast should you get the connections
wrong.  No amount of internal protection can protect equipment from fools.

My recommendation for laptop computers is to use a commercially available
DC-AC inverter with the laptop's normal AC power pack.  This is not the most
efficient but is the safest and should maintain the laptop's warranty should
something go wrong.  For CD players and other audio equipment, only use
approved automotive adapters.

For something like a CD player that runs on a 9VDC wall adapter, even if the
droid at Radio Shack says it will work without dropping the voltage, proceed
with caution.

The 3 V difference isn't the only problem - you might get away with that
though I would recommend against it (measure the open circuit voltage out of
your AC adapter - it is probably closer to 12 V or more anyhow).  It is the
other nastiness of the automotive power.

Putting 4 diodes (e.g., 1N4002) in series with the power would drop the
voltage to be closer to 9 V but the spikes will sail right through

If it were mine, I would probably add some filtering to the 12 V - maybe
10,000 uF, 35 V, and then use a 7809 or LM317 regulator to drop it to 9 V.
This isn't a guarantee but is much better than ignoring the issues entirely.
See the section: "Adding an IC regulator to a wall adapter or battery".

However, there is a one more minor problem - when starting, the voltage can
easily drop to 9 V or less.  With the regulator, the output would be closer to
7 V which may or may not be enough.  So, the player may quit while starting
but I suppose there are more important things to worry about!

As with a laptop, another option is to use a small 12 VDC to 115 VAC inverter,
perhaps $25.  This would definitely protect the player (assuming the adapter
doesn't mind the squarewave it puts out) but would not be very efficient.

I received a dead CD player with an auto adapter included.  It was supposed
to run on 3 V.  Guess what?  There was no circuitry in the adapter!  That
was probably a Radio Shack recommendation as well :-).  Just because the
plugs match doesn't mean it will work and not blow up!

  12.17) How do those on-battery or on-the-package battery testers work?

There is a graded width resistance element that gets connected when you pinch
those two points.  It heats up - substantially, BTW.  Some sort of liquid
crystal or other heat sensitive material changes from dark to clear or yellow
at a fairly well defined temperature.

Incidentally, since the current is significant, repeated 'testing' will drain
the batteries - as with any proper under-load battery test!  This isn't an
issue for occasional testing but if the kids figure how to do this....

Personally, I would rather use a $3 battery checker instead of paying for
throw-away frills!

Chapter 13) Motors and Relays

  13.1) Small motors in consumer electronic equipment

A variety of motor types are found in audio and other electronic equipment.
For the additional information on the specific types of motors used in
VCRs and CD players, see the documents: "Notes on the Troubleshooting and
Repair of Video Cassette Recorders" and "Notes on the Troubleshooting and
Repair of Compact Disc Players and CDROM Drives".

Types of motors:

1. Small brush-type permanent magnet (PM) DC motors similar to those found
   in battery operated appliances.  Such motors are used in cassette decks
   and boomboxes, answering machines, motorized toys, CD players and CDROM
   drives, and VCRs.  Where speed is critical, these may include an internal
   mechanical governor or electronic regulator.  In some cases there will be
   an auxiliary tachometer winding for speed control feedback.

   These are usually quite reliable but can develop shorted or open windings,
   a dirty commutator, gummed up lubrication, or dry or worn bearings.
   Replacement is best but mechanical repair (lubrication, cleaning) is
   sometimes possible. 

   Also see the section: "General tape speed problems - slow, fast, or dead".

   Additional info on these types of motors can be found in "Notes on
   the Troubleshooting and Repair of Compact Disc Players and CDROM Drives".

2. A low profile or 'pancake' brushless DC motor may provide power for a
   in some Walkman type tape players, direct drive capstans and general
   power in VCRs or tape decks.   Since these are electronically controlled,
   any non-mechanical failures are difficult to diagnose.  In some cases,
   electronic component malfunction can be identified and remedied.

3. AC induction motors - shaded pole or synchronous type used in inexpensive
   turntables.  These motors are extremely reliable and are easy to
   disassemble, clean, and lubricate.  Just do not lose any of the
   spacer washers on each end of the shaft and make notes to assure
   proper reassembly.

4. Miniature synchronous motors used in mechanical clock drives as found
   in older clock radios or electric clocks powered from the AC line,
   appliance controllers, and refrigerator defrost timers.  These assemblies
   include a gear train either sealed inside the motor or external to it.
   If the motor does not start up, it is probably due to dried gummed up
   lubrication.  Getting inside can be a joy but it is usually possible to
   pop the cover and get at the rotor shaft (which is usually where the
   lubrication is needed).  However, the tiny pinion gear may need to be
   removed to get at both ends of the rotor shaft and bearings.

  13.2) Motor noise in audio equipment

Of course you expect your audio equipment to be absolutely silent unless
told to perform.  Motor noise should not be objectionable.  However, what
if it is?  There are several kinds of noise: rotating noise, vibration,
and electrical interference:

If the noise is related to the rotating motor shaft, try lubricating the motor
(or other suspect) bearings - a single drop of electric motor oil, sewing
machine oil, or other light oil (NOT WD40 - it is not a suitable lubricant),
to the bearings (at each end for the motor).  This may help at least as a
temporary fix.  In some cases, using a slightly heavier oil will help with
a worn bearing.  See the section: "Lubrication of electronic equipment".

For AC motors and transformers, steel laminations or the motor's mounting
may be loose resulting in a buzz or hum.  Tightening a screw or two
may quiet it down.  Painting the laminations with varnish suitable for
electrical equipment may be needed in extreme cases.  Sometimes, the
noise may actually be a result of a nearby metal shield or other
chassis hardware that is being vibrated by the motor's magnetic field.
A strategically placed shim or piece of masking tape may work wonders.

If the noise - a buzz or whine - is actually coming from the audio output
but only occurs with the motor running, the interference filter on the
motor power supply may have failed.  This is often just a capacitor across
the motor terminals and it may be defective or there may be a bad connection.

  13.3) Finding a replacement motor

In many cases, motors are fairly standardized and you may be able
to find a generic replacement much more cheaply than the original
manufacturer's part.  However, the replacement must match the following:

1. Mechanical - you must be able to mount it.  In most cases, this
   really does mean an exact drop-in.  Sometimes, a slightly longer
   shaft or mounting hole out of place can be tolerated.  The pulley
   or other drive bushing, if any, must be able to be mounted on the new
   motor's shaft.  If this is a press fit on the old motor, take extreme
   care so as not to damage this part when removing it (even if this means
   destroying the old motor in the process - it is garbage anyway).

2. Electrical - the voltage and current ratings must be similar.

3. Rotation direction - with conventional DC motors, this may be
   reversible by changing polarity of the voltage source.  With AC motors,
   turning the stator around with respect to the rotor will reverse rotation
   direction.  However, some motors have a fixed direction of rotation
   which cannot be altered.

4. Speed - for tape players and turntables - this may not be feedback
   controlled.  With a little care you should be able to determine the 
   normal rpms of the motor.  For example, with a cassette deck, knowing
   the tape speed (1-7/8" inches per second is standard), it is
   straightforward calculate the motor shaft speed based on simple
   measurements of pulley and capstan diameter ratios.

MCM Electronics, Dalbani, and Premium Parts stock a variety of generic
replacement motors for tape decks, Walkmen, boomboxes, and CD players.

  13.4) Relay basics

The ubiquitous electromechanical relay is a device that is used in a large
variety of applications to switch power as well as signals in electrical
and electronic equipment.  Operation is quite simple:  An electromagnet
powered by an AC or DC coil pulls on an armature having a set of moving
contacts which make or break a connection with a set of stationary contacts.

Most common relays can be characterized by three sets of parameters:

1. Coil - voltage; resistance, current, or power consumption; and whether it
   is AC or DC.  For AC coils only, the VA (volt-amps) rating may be used
   instead of or in addition to power consumption due to the inductive coil.
   Typical coil voltages range from 5 V to 480 V (AC or DC) - and beyond.
   Current and power consumption depend on the size of the relay.

2. Contact configuration - number of sets of contacts and whether they
   are their type.  The designation will be something like SPST-NO, DPDT,
   4PST-NC, 6PDT, etc.  The first two letters refers to the number of sets
   of simultaneously activated contacts (S=1, D=2, numbers are usually
   used for more than 2 sets of contacts).  The second two letters refers
   to the contact configuration (ST=NO or NC but no common terminal, DT
   will have a common - there will be both an NO and NC terminal).  Where
   contacts are ST, the last two letters indicate NO or NC.  An almost
   unlimited number of variations are possible.  Typical relays have
   anywhere from 1 to 6 or more separate sets of ST or DT contacts or a
   mixture of the two.

3. Contact ratings - this may be specified for a number of types of
   applications.  For example: in amperes at a particular voltage for DC
   resistive loads, or in horsepower at various voltages for AC inductive
   loads.  Like fuse ratings, these are maximum ratings and lower values
   are almost always acceptable.  Small relays may be able to switch
   only a few hundred mA at 32 V while large industrial contactors can
   switch 1000s of A at 1000s of V.  Even the contactor in your automobile's
   starter must control hundreds of amps to the starter motor.

The common (C) contacts connect to the normally closed (NC) contacts when
the coil is unpowered and to the normally open (NO) contacts when the coil
is powered.

Miniature and subminiature relays are used to switch phone line signals
in modems, fax machines, and telephone answering machines; audio amplifier
speaker protection circuits; multiscan monitor deflection components; and
many other places.

Small relays control power in lighting equipment, TVs and other home
appliances, automotive systems and accessories, and the like.

Large relays (often called contactors) are used for the control of central
air conditioning systems (compressor and blower motors), all types and sizes
of industrial machinery - as well as in the starter of your automobile.

  13.5) Relay identification

A relay without a pin connection diagram can usually be identified with
a multimeter and variable power supply - or by eye.  Many have the critical
information printed on the cover.  However, for detailed specifications,
referring to the manufacturer's databook (or WEB page) really is best!

(The following assumes a subminiature (DIP) relay.  Lower coil resistances,
higher coil voltages, and other variations may exist for larger relays.)

1. If the case of the relay is transparent or you can pop the top, examine
   the pole piece of the electromagnet.  If there is a (copper) ring around
   half the pole piece, the relay coil is designed for AC (usually line
   frequency - 50 or 60 Hz) operation.  An AC relay operated on DC will
   overheat very quickly but can be tested on DC.

2. Determine the coil pins.  Use your eyeball if possible or your multimeter
   on the low resistance scale.  For a small relay, the coil will most likely
   be a few hundred ohms.  All other combinations of pins will be zero or
   infinity.  If the resistance is under, say, 100 ohms, you may have an AC
   coil rather than a DC coil.

3. Power the relay from a variable DC supply (I am assuming it has a DC
   coil which is likely for a DIP relay.  You can still do this with an
   AC coil but it will heat up quickly).  Start at zero and increase the
   voltage until you hear the contacts close.  This will probably be at
   around 3 volts (for a 5 V coil) or 8 volts for a 12 V coil - this will be
   roughly 60% of nominal coil voltage.  If you do not hear anything,
   reverse the polarity of the coil and try again - you may have a latching
   relay.  Alternatively, put your multimeter on the resistance scale
   across one of the pairs of pins that measured zero ohms as it is likely
   to be a NC set of contacts.  This will change to infinity ohms when
   the relay switches.

4. Now that you can switch the relay on and off, you can use your multimeter
   on the resistance scale to determine which contacts are normally open
   (NO) and which contacts are normally closed (NC).  (Normally here means

5, The power rating of the contacts can be estimated by their diameter (if
   they are visible).  Rough current estimates (resistive loads): 20 A - 5 mm,
   10 A - 3 mm, 5 A - 2 mm, 1 A - 1 mm.  These must be derated substantially
   for inductive loads.

For latching relays, the polarity of the coil voltage determines whether the
relay is switched on or off.  In other words, to switch to the opposite
state requires the polarity of the voltage to the coil to be reversed.  Other
types are possible but not very common.

  13.6) Relay testing and repair

If the relay is totally inoperative, test for voltage to the coil.  If the
voltage is correct, the relay may have an open coil.  If the voltage is low
or zero, the coil may be shorted or the driving circuit may be defective.
If the relay makes a normal switching sound but does not correctly control
its output connections, the contacts may be corroded, dirty, worn, welded
closed, binding, or there may be other mechanical problems.

Remove the relay from the circuit (if possible) and measure the coil
resistance.  Compare your reading with the marked or specified value
and/or compare with a known working relay of the same type.  An open
coil is obviously defective but sometimes the break is right at the
terminal connections and can be repaired easily.  If you can gain access
by removing the cover, a visual examination will confirm this.  If the
resistance is too low, some of the windings are probably shorted.  This
will result in overheating as well as no or erratic operation.  Replacement
will be required.

Relay contacts start out bright and shiny.  As they are used, arcing,
dirt, and wear take their toll.  A sealed relay used at well below its
rated current with a resistive load may work reliably for millions of cycles.
However, this will be significantly reduced when switching high currents -
especially with inductive loads which results in contact arcing.  One speck
of dirt can prevent a contact from closing so cleanliness is important.
Excessive arcing can result in the contacts getting welded together as well.

The resistance of closed contacts on a relay that is in good condition
should be very low - probably below the measurable limits on a typical
multimeter - a few milliohms.  If you measure significant or erratic
resistance for the closed contacts as the relay is switched or if very
gentle tapping results in erratic resistance changes, the contacts are
probably dirty, corroded, or worn.  If you can get at the contacts, the
use of contact cleaner first and a piece of paper pulled back and forth
through the closed contacts may help. Superfine sandpaper may be used as
a last resort but this is only a short term fix.  The relay will most likely
need to be replaced if the contacts are switching any substantial power.

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]