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AC adapter basics:
It seems that the world now revolves around AC Adapters or ‘Wall Warts’ as they tend to be called. There are several basic types. Despite the fact that the plugs to the equipment may be identical THESE CAN GENERALLY NOT BE INTERCHANGED. The type (AC or DC), voltage, current capacity, and polarity are all critical to proper operation of the equipment. Use of an improper adapter or even just reverse polarity can permanently damage or destroy the device. Most equipment is protected against stupidity to a greater or lessor degree but don’t count on it. The most common problems are due to failure of the output cable due to flexing at either the adapter or output plug end. See section below on repair procedure.

1. AC Transformer. All wall warts are often called transformers. However, only if the output is stated to be ‘AC’ is the device simply a transformer. These typically put out anywhere from 3 to 20 VAC or more at 50 mA to 3 A or more. The most common range from 6-15 VAC at less than an Amp.

Typically, the regulation is very poor so that an adapter rated at 12 VAC will typically put out 14 VAC with no load and drop to less than 12 VAC at rated load. To gain agency approval, these need to be protected internally so that there is no fire hazard even if the output is shorted. There may be a fuse or thermal fuse internally located (and inaccessible).

If the output tested inside the adapter (assuming that you can get it open without total destruction – it is secured with screws and is not glued or you are skilled with a hacksaw – measures 0 or very low with no load but plugged into a live outlet, either the transformer has failed or the internal fuse had blown. In either case, it is probably easier to just buy a new adapter but sometimes these can be repaired.

Occasionally, it will be as simple as a bad connection inside the adapter. Check the fine wires connected to the AC plug as well as the output connections. There may be a thermal fuse buried under the outer layers of the transformer which may have blown. These can be replaced but locating one may prove quite a challenge.

2. DC Power Pack. In addition to a step down transformer, these include at the very least a rectifier and filter capacitor. There may be additional regulation but most often there is none. Thus, while the output is DC, the powered equipment will almost always include an electronic regulation.

As above, you may find bad connections or a blown fuse or thermal fuse inside the adapter but the most common problems are with the cable.

3. Switching Power Supply. These are complete low power AC-DC converters using a high frequency invertor. Most common applications are laptop computers and camcorders. The output(s) will be fairly well regulated and these will often accept universal power – 90-250 V AC or DC.

Again, cable problems predominate but failures of the switching power supply components are also possible. If the output is dead and you have eliminated the cable as a possible problem or the output is cycling on and off at approximately a 1 second rate, then some part of the switching power supply may be bad. In the first case, it could be a blown fuse, bad startup resistor, shorted/open semiconductors, bad controller, or other components. If the output is cycling, it could be a shorted diode or capacitor, or a bad controller. See the “Notes on the Diagnosis and Repair of Small Switchmode Power Supplies” for more info, especially on safety while servicing these units.

AC adapter testing:
AC adapters that are not the switching type (1 and 2 above) can easily be tested with a VOM or DMM. The voltage you measure (AC or DC) will probably be 10-25% higher than the label specification. If you get no reading, wiggle, squeeze, squish, and otherwise abuse the cord both at the wall wart end and at the device end. You may be able to get it to make momentary contact and confirm that the adapter itself is functioning.

The most common problem is one or both conductors breaking internally at one of the ends due to continuous bending and stretching.
Make sure the outlet is live – try a lamp.
Make sure any voltage selector switch is set to the correct position. Move it back and forth a couple of times to make sure the contacts are clean.
If the voltage readings check out for now, then wiggle the cord as above in any case to make sure the internal wiring is intact – it may be intermittent.

Although it is possible for the adapter to fail in peculiar ways, a satisfactory voltage test should indicate that the adapter is functioning correctly.

AC adapter repair:
Although the cost of a new adapter is usually modest, repair is often so easy that it makes sense in any case. The most common problem (and the only one we will deal with here) is the case of a broken wire internal to the cable at either the wall wart or device end due to excessive flexing of the cable.

Usually, the point of the break is just at the end of the rubber cable guard. If you flex the cable, you will probably see that it bends more easily here than elsewhere due to the broken inner conductor. If you are reasonably dextrous, you can cut the cable at this point, strip the wires back far enough to get to the good copper, and solder the ends together. Insulate completely with several layers of electrical tape. Make sure you do not interchange the two wires for DC output adapters! (They are usually marked somehow either with a stripe on the insulator, a thread inside with one of the conductors, or copper and silver colored conductors. Before you cut, make a note of the proper hookup just to be sure. Verify polarity after the repair with a voltmeter.

The same procedure can be followed if the break is at the device plug end but you may be able to buy a replacement plug which has solder or screw terminals rather than attempting to salvage the old one. Once the repair is complete, test for correct voltage and polarity before connecting the powered equipment. This repair may not be pretty, but it will work fine, is safe, and will last a long time if done carefully.

If the adapter can be opened – it is assembled with screws rather than being glued together – then you can run the good part of the cable inside and solder directly to the internal terminals. Again, verify the polarity before you plug in your expensive equipment.

Warning: If this is a switching power supply type of adapter, there are dangerous voltages present inside in addition to the actual line connections. Do not touch any parts of the internal circuitry when plugged in and make sure the large filter capacitor is discharged (test with a voltmeter) before touching or doing any work on the circuit board. For more info on switching power supply repair, refer to the Notes on the Diagnosis and Repair of Small Switchmode Power Supplies.

If it is a normal adapter, then the only danger when open are direct connections to the AC plug. Stay clear when it is plugged in.

AC adapter substitution and equipment damage:
Those voltage and current ratings are there for a reason. You may get away with a lower voltage or current adapter without permanent damage but using a higher voltage adapter is playing Russian Roulette. Even using an adapter from a different device – even with similar ratings, may be risky because there is no real standard. A 12 V adapter from one manufacturer may put out 12 V at all times whereas one from another manufacturer may put out 20 V or more when unloaded.

A variety of types of protection are often incorporated into adapter powered equipment. Sometimes these actually will save the day. Unfortunately, designers cannot anticipate all the creative techniques people use to prove they really do not have a clue of what they are doing.

The worst seems to be where an attempt is made to operate portable devices off of an automotive electrical system. Fireworks are often the result, see below and the section on: “Automotive power”.

If you tried an incorrect adapter and the device now does not work there are several possibilities (assuming the adapter survived and this is not the problem):
1. An internal fuse blew. This would be the easiest to repair.
2. A protection diode sacrificed itself. This is usually reverse biased across the input and is supposed to short out the adapter if the polarity is reversed. However, it may have failed shorted particularly if you used a high current adapter (or automotive power).
3. Some really expensive hard to obtain parts blew up. Unfortunately, this outcome is all too common.

I inherited a Sony Discman from a guy who thought he would save a few bucks and make an adapter cord to use it in his car. Not only was the 12-15 volts from the car battery too high but he got it backwards! Blew the DC-DC converter transistor in two despite the built in reverse voltage protection and fried the microcontroller. Needless to say, the player was a loss but the cigarette lighter fuse was happy as a clam!

Moral: those voltage, current, and polarity ratings marked on portable equipment are there for a reason.

Voltage rating should not be exceeded, though using a slightly lower voltage adapter will probably cause no harm though performance may suffer. The current rating of the adapter should be at least equal to the printed rating. The polarity, of course, must be correct. If connected backwards with a current limited adapter, there may be no immediate damage depending on the design of the protective circuits. But don’t take chances – double check that the polarities match – with a voltmeter if necessary – before you plug it in! Note that even some identically marked adapters put out widely different open circuit voltages. If the unloaded voltage reading is more than 25-30% higher than the marked value, I would be cautious about using the adapter without confirmation that it is acceptable for your equipment. Needless to say, if you experience any strange or unexpected behavior with a new adapter, if any part gets unusually warm, or if there is any unusual odor, unplug it immediately and attempt to identify the cause of the problem.

Loudspeaker anatomy:
In this document, we use the terms ‘loudspeaker’ or ‘speaker system’ to denote a unit consisting of one or more drivers in an acoustic enclosure perhaps along with a frequency selective crossover, tone controls and switches, fuses or circuit breakers. Connections to the amplifier or receiver are via terminals on the rear.

The front is covered with an (optically) opaque or semitransparent grille which provides protection and improves the appearance (depending on your point of view).

A ‘driver’ is the actual unit that converts electrical energy into sound energy. Most drivers use voice coil technology: a very low mass coil wound on a light rigid tube is suspended within a powerful magnetic field and attached to a paper, plastic, or composite cone. The audio signal causes the coil to move back and forth and this motion causes the cone to move which causes the air to move which we perceive as sound.

The typical driver consists of several parts:
* Frame – a rigid steel or composite structure on which the driver is constructed. The frame holds the magnet and core, cone suspension, and connection terminals.

* Magnet – this includes a powerful (usually ceramic, AlNiCo, or rare earth) magnet including a core structure provide a very narrow cylindrical air gap. This accounts for most of the mass of a driver.

* Voice coil – a one or two layer coil of fine wire wound on a light rigid cardboard, plastic, or composite tube suspended within the air gap of the magnet and connected via flexible wires to the electrical terminals.

* Cone – a roughly cone shaped very light and rigid structure that does the actual work of moving air molecules. The cone in a woofer may be 12 or more inches across. The cone in a tweeter may only be an inch in diameter. This is the part of the driver you actually see from the front of the speaker system with the grille removed. The center is usually protected with a small plastic dome.

* Suspension – a corrugated flexible mounting for the voice coil called a ‘spider’ and outer ring of very soft plastic or foam. Together, these allow the voice coil/cone combination to move readily in and out as a unit without tilting or or rubbing. For most designs, there is a certain amount of springiness to this suspension. Acoustic suspension loudspeaker, however, use the trapped air in a totally sealed speaker enclosure to provide the restoring force.
Inexpensive ‘LoFi’ devices like portable and clock radios, many TVs, intercoms, and so forth use a single, cheap driver. Some have a coaxial pair of cones but this does little to improve the frequency response.

HiFi speakers systems will divide the audio frequency spectrum into several bands and use drivers optimized for each. The reason is that it is not possible to design a single driver that has a uniform response for the entire audio frequency spectrum. A ‘woofer’ is large and massive and handles the low base notes.

A ‘tweeter’ has a very low mass structure and is used for the high frequencies. A ‘mid-range’ handles the mid frequencies. There may also be ‘sub-woofers’ for the very very low notes that we feel more than hear. Some systems may include ‘super-tweeters’ for the very highest frequencies (which few people can hear. This may make for some impressive specifications but perhaps little else.)

A ‘crossover’ network – a set of inductors and capacitors – implements a set of filters to direct the electrical signal (mostly) to the proper drivers.

Various controls or switches may be provided to allow for the adjustment of low, mid, and high frequency response to match the room acoustics more faithfully or to taste. Fuses or circuit breakers may be included to protect the speaker system from intentional (high volume levels) or accidental (amplifier output stage blows) abuse.

Loudspeaker problems:
If you have a high quality and expensive set of loudspeaker, then the cost of professional repair may be justified. However, if the problem is with speaker systems you might not write home about, then read on. Playing your music system at very high volume levels, especially CDs which may have peaks that way exceed the ratings of your loudspeakers is asking for trouble – but you knew that! CDs can be deceiving because the noise floor is so low that you are tempted to turn up the volume. A peak comes along and your speaker cones are clear across the county (remember the movie ‘Back to the Future’?). Loudspeaker systems are generally pretty robust but continuous abuse can take its toll.

Problems with loudspeakers:
1. An entire speaker system is dead.
Verify that the connections both at the speaker system and at the source are secure. Check circuit breakers or fuses in the speaker system. Reset or replace as needed. Make sure it is not the amplifier or other source that is defective by swapping channels if that is possible. Alternatively, test for output using a speaker from another system or even a set of headphones (but keep the volume turned way down). Assuming that these tests confirm that the speaker system is indeed not responding, you will need to get inside.

It would take quite a blast of power to kill an entire speaker system. Therefore, it is likely that there is a simple bad connection inside, perhaps right at the terminal block. You should be able to easily trace the circuitry – this is not a missile guidance system after all – to locate the bad connection. If nothing is found, then proceed to test the individual drivers as outlined below.

2. One or more drivers (the name for the individual speakers in a loudspeaker enclosure) is dead – no sound at all even when you place you ear right up to it. The cause may be a bad driver, a bad component or bad connection in the crossover network. Test these components as outlined below.

3. One or more drivers produces distorted or weak sound. Distorted may mean fuzzy, buzzing, or scratchy a various volume levels. Most likely this is due to a bad driver but it could also be a defective component in the crossover – a capacitor for example or even a marginal connection.

Getting inside a speaker system usually means removing the decorative grille if it snaps off or unscrewing the backpanel and/or terminal block. Use your judgement. With the grille removed, you will be able to unscrew the individual drivers one at a time. With the back off, you will have access to all the internal components. If sealing putty is used, don’t lose it or expect to obtain some replacement putty (non-hardening window caulking like Mortite is suitable).

Test the components in the crossover network with a multimeter. These are simple parts like capacitors, inductors, and potentiometers or reostats. Confirm that any circuit breakers or fuse holders have continuity.

Test the drivers on the low ohms scale of your multimeter. Disconnect one wire so that the crossover components will not influence the reading. Woofers and midrange drivers should measure a few ohms. If their impedance is marked, the reading you get will probably be somewhat lower but not 0. If possible compare your readings with the same driver in the good speaker system (if this is a stereo setup). Some tweeters (very small high frequency drivers) may have a series capacitor built in which will result in an infinite ohms measurement. Other than these, a high reading indicates an open voice coil which means a bad driver. In a comparison with an identical unit, a very low reading would mean a partially or totally shorted voice coil, again meaning a bad driver. Except for expensive systems with removable voice coil assemblies, either of these usually mean that a replacement will be required for the entire driver. Sometimes an open voice coil can be repaired if the break can be found.

To confirm these tests, use an audio source to power just the suspect driver. Your stereo system, a small amplifier attached to an audio source, or even a pocket radio (use its speaker output if the headphone output does not have enough power) will suffice. The resulting sound will not be of high quality because you do not have the enclosure sealed and it is only one of the drivers in the system, but it should give you some idea of its condition. Again, comparing with an identical unit would be another confirmation.

Repairing loudspeaker drivers:
As noted above, if you are dealing with a high quality system, leave these repairs to professionals or obtain an entire replacement as some reduction in audio quality may result from the abuse you are about to inflict on the poor defenseless driver.

Minor damage to the cone can be repaired using a flexible adhesive like weatherstrip cement and a piece of thick paper to reinforce the seam or hole if necessary. Since this will not totally perfect match with the original paper cone, there could be audible distortion at certain frequencies particularly at higher volume levels. However, such a repair will be better than nothing. Cut the paper in a shape and size to just overlap both sides of the torn area or completely cover the puncture. Use just the smallest amount of adhesive to fasten your ‘splint’ to the cone. The less material you add, the more likely that the audio effects will be minimal.

An open driver can sometimes be rescued by tracing the input wires through the cone and under the center protective dome. The most likely places for these wires to break are right at the place where they pass through the cone and just after they pass under the dome. Note: some drivers have replaceable voice coil units. If this is the case, you should probably just replace the entire unit.

First, scrape away the insulating varnish on the front of the cone where the wires emerge and head toward the center. Use your ohmmeter to test for continuity here. If you find that you now are measuring a reasonable resistance – a few ohms, then trace back to determine which of the two wires is broken or has had the solder connection come loose. If it is still infinite, you will have to go under the dome.

Use an Xacto knife to carefully remove the dome. Use a shallow angle and cut as near the edge as you can. Take care not to puncture the paper cone which may continue under the dome as the voice coil may be of a smaller diameter than the dome. The shallow cut will also provide a base to reattach the dome if you are successful. Carefully scrape off a bit of the enamel insulation as near to the voice coil as possible and test with your ohmmeter once again. If the resistance is still infinite, there is nothing more you can do but salvage the magnet for fun experiments or erasing floppy disks. There is essentially no way to replace just the voice coil unless your driver has a removable voice coil unit (in which case you would not be reading this).

If the resistance now measures normal – a few ohms, trace back to determine which wire is broken and use some fine (e.g., #30 gauge) wire to bridge the break. You will have to scrape off the enamel insulation to permit the solder to adhere. Make sure it is secure mechanically first – a speaker cone is a rather violent environment for soldered connections. Finally, use some flexible adhesive to protect and reinforce the solder connections, to glue down your new wire along its entire length, to protect and reinforce the place where the wire passes through the cone, and finally, to reattach the central dome. Let the adhesive dry thoroughly before playing the 1812 Overture.

Loudspeakers – repair or replace?:
Assuming that the cabinet is in reasonable condition, the question arises: is it worth replacing broken, damaged, or worn out drivers or faulty crossover components that are not repairable rather than just dumping the speaker systems?

It is very straightforward to swap drivers as long as you get ones with similar characteristics. It all depends on what you want out of a loudspeaker. If you are basically happy with them, then it will be a lot cheaper than replacing the entire speaker system(s). However, speaker system quality has improved considerably in the last 15 years so now may be the time to upgrade.

As far as crossover components are concerned, these are basically common electronic parts and replacement is probably worthwhile.

However, if one driver has a deteriorated suspension, it is likely that its mate does as well and that other drivers may not far behind. Replacing **all** the internal components of a loudspeaker may not be worth it.

Read more the explanation how to repair and test audio power amplifier by downloading this PDF file