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


  9.5) Testing of camera shutter speed

If you suspect shutter speed problems, there are several easy ways to
measure this for your camera.  The most accurate require some test
equipment but you can get a pretty good idea with little or no equipment
beyond a stopwatch (for slow shutter speeds - above 1/2 to 1 second and a
TV (for fast shutter speeds - below about 1/60 of a second (NTSC 525/60).

Some of these approaches assume that you have access to the film
plane of the camera - this may be tough with many highly automated
compact cameras which will be unhappy unless a roll of film is
properly loaded with the back door closed.

Note that the behavior of focal plane and leaf (in-the-lens) shutters
is significantly different at high shutter speeds and this affects the
the interpretation of measurements.

Some simple homemade equipment will enable testing of the intermediate
shutter speeds.

1. Testing slow to medium shutter speeds - the use of a stopwatch is self
   evident for really long times (greater than .5 second or so).  However,
   viewing or photographing the sweep hand of a mechanical stop watch
   or a homemade motor driven rotating white spot or LED can provide quite
   accurate results.  Accurate timing motors are inexpensive and readily
   available.  Mount a black disk with a single small white spot at its
   edge on the motor shaft and mark some graduations around its perimeter
   on a stationary back board.  For a high tech look, use an LED instead.
   Use your creativity.

   Making measurements from the photographic images of the arcs formed
   by the spot as it rotates while the shutter is open should result in
   accuracies better than 1 or 2% for shutter speeds comparable to or slower
   than the rotation frequency of the motor.  In other words, shutter speeds
   down to 1/10th of a second for a 600 rpm (10 rps) motor or down to 1/60th
   of a second for a 3600 rpm (60 rps) motor.

   At these speeds, focal plane and leaf shutters should result in similar
   results since the open and close times are small compared to the total
   exposure time.

2. Testing fast shutter speeds - view a TV (B/W is fine) screen on a piece
   of ground glass at the focal plane or take a series of snapshots of a
   TV screen (a well adjusted B/W TV is best as the individual scan lines
   will be visible).

   Note: If your camera has a focal plane shutter (e.g., 35 mm SLRs), orient
   the camera so that the shutter curtain travels across - horizontally
   (rather than up or down).

   If you are photographing the screen, take a few shots at each speed
   in case the timing of your trigger finger is not quite precise and
   you cross the vertical blanking period with some of them.  This will
   also allow you to identify and quantify any variations in shutter
   speed that may be present from shot-to-shot.

   For a focal plane shutter, you will see a bright diagonal bar.  (The
   angle of the bar can be used to estimate the speed of the shutter
   curtain's traversal.)

   For a leaf (in-the-lens) shutter, you will see a bright horizontal
   bar. but the start and end of the exposure (top and bottom of the bar)
   will be somewhat fuzzy due to the non-zero time it takes to open and
   close the shutter leaves.  You will have to estimate the locations of
   the 'full width half maximum' for each speed.

   In both cases, there will some additional smearing at the bottom of the
   bar due to the persistence of the CRT phosphors.

   The effective exposure time can then be calculated by multiplying the
   number of scan lines in the bar at any given horizontal position by
   63.5 uS (the NTSC horizontal scan time).

   If you cannot resolve individual scan lines, figure that a typical over-
   scanned (NTSC) TV screen has 420-440 visible lines.  If you can adjust
   your TV (remember this can be an old B/W set when knobs were knobs!) for
   underscan, about 488 or so active video lines will be visible.

If you have an oscilloscope or electronic counter/timer, fairly accurate
measurements can be made at all shutter speeds using a bright light and
a photodetector circuit.

3. Using an electronic counter/timer or oscilloscope.  A gated 24 bit counter
   clocked at 1 MHz would permit (ideally) testing shutter speeds from 1/2000th
   second to 16 seconds with an accuracy of better than .2 percent.  Of course
   in practice, the finite size of any photodiode and/or the finite open/close
   time of any shutter will limit this at high shutter speeds.  Any resonably
   well calibrated oscilloscope will be accurate enough for shutter speed

   Construct the IR detector circuit described in the document: "Notes on the
   Troubleshooting and Repair of Hand Held Remote Controls".  (Note that the
   fact that it is called an IR detector is irrelevant since the typical
   photodiode is sensitive to visible wavelengths of light as well.)  Connect
   its output to the minus gate of your counter or the vertical input of your
   scope.  Put a diffuse light source (i.e., light bulb) close to the lens so
   that it is not in focus.  Position the detector photodiode in the center of
   the focal plane - mount it on a little piece of cardboard that fits on the
   film guide rails.  Using this setup, it should be a simple matter to measure
   the shutter timing.  Take multiple 'exposures' to identify and quantify
   any variations in shutter speed that may be present from shot-to-shot.

   For a focal plane shutter, the time response will be the convolution
   of the photodetector area and the slit in the shutter curtain.  The
   smaller the aperture of the photodiode, the less this will be a factor.
   Masking it with black tape may be desirable when testing fast shutter
   speeds.  In simple terms, make the photodiode aperture narrow.

   For between-the-lens shutters, the finite open and close times of the
   leaves will show up on the oscilloscope in the rise and fall times of
   the trace.  The measurement on the electronic timer will be affected
   by its trigger level setting for this reason.  However, since this
   photodetector is not linearly calibrated, the open and close times
   cannot be accurately determined from the waveform.

  9.6) Electronic flash fundamentals

All modern electronic flash units (often called photographic strobes) are
based on the same principles of operation whether of the subminiature
variety in a disposable pocket camers or high quality 35 mm camera, compact
separate hot shoe mounted unit, or the high power high performance unit
found in a photo studio 'speed light'.  All of these use the triggered
discharge of an energy storage capacitor through a special flash tube
filled with Xenon gas at low pressure to produce a very short burst of
high intensity white light.

The typical electronic flash consists of four parts: (1) power supply,
(2) energy storage capacitor, (3) trigger circuit, and  (4) flash tube.

An electronic flash works as follows:

1. The energy storage capacitor connected across the flash tube is charged
   from a 300V (typical) power supply.  This is either a battery or AC adapter
   operated inverter (pocket cameras and compact strobes) or an AC line
   operated supply using a power transformer or voltage doubler or tripler
   (high performance studio 'speed' lights).  These are large electrolytic
   capacitors (200-1000+ uF at 300+ V) designed specifically for the rapid
   discharge needs of photoflash applications.

2. A 'ready light' indicates when the capacitor is fully charged.  Most
   monitor the voltage on the energy storage capacitor.  However, some
   detect that the inverter or power supply load has decreased indicating
   full charge.

3. Normally, the flash tube remains non-conductive even when the capacitor
   is fully charged.

4. A separate small capacitor (e.g., .1 uF) is charged from the same power
   supply to generate a trigger pulse.

5. Contacts on the camera's shutter close at the instant the shutter is
   fully open.  These cause the charge on the trigger capacitor to be
   dumped into the primary of a pulse transformer whose secondary is
   connected to a wire, strip, or the metal reflector in close proximity
   to the flash tube.

6. The pulse generated by this trigger (typically around 10 KV) is enough to
   ionize the Xenon gas inside the flash tube.

7. The Xenon gas suddenly becomes a low resistance and the energy storage
   capacitor discharges through the flash tube resulting in a short
   duration brilliant white light.

The energy of each flash is roughly equal to 1/2*C*V^^2 in watt-seconds
(W-s) where V is the value of the energy storage capacitor's voltage and
C is its capacitance in.  Not quite all of the energy in the capacitor is
used but it is very close.  This energy storage capacitor for pocket cameras
is typically 200-300 uF at 330 V (charged to 300 V) with a typical flash
energy of 10 W-s.  For high power strobes, 1000s of uF at higher voltages
are common with maximum flash energies of 100 W-s or more.  Another important
difference is in the cycle time.  For pocket cameras it may be several
seconds - or much longer as the batteries run down.  For a studio 'speed
light', fractional second cycle times are common.

Typical flash duration is much less than a millisecond resulting in crystal
clear stop action photographs of almost any moving subject.

On cheap cameras (and probably some expensive ones as well) physical contacts
on the shutter close the trigger circuit precisely when the shutter is wide
open.  Better designs use an SCR or other electronic switch so that no high
voltage appears at the shutter contacts (or hot shoe connector of the flash
unit) and contact deterioration due to high voltage sparking is avoided.

Note that for cameras with focal plane shutters, the maximum shutter speed
setting that can be used is typically limited to 1/60-1/120 of a second.
The reason is that for higher shutter speeds, the entire picture is not
exposed simultaneously by the moving curtains of the focal plane mechanism.
Rather, a slit with a width determined the by the effective shutter speed
moves in front of the film plane.  For example, with a shutter speed setting
of 1/1000 of a second, a horizontally moving slit would need to be about
1/10 of an inch wide for a total travel time of 1/60 of a second to cover the
entire 1.5 inch wide 35 mm frame.  Since the flash duration is extremely
short and much much less than the focal plane curtain travel time, only the
film behind the slit would be exposed by an electronic flash.  For shutter
speed settings longer than the travel time, the entire frame is uncovered
when the flash is triggered.

See the section: "Photoflash circuit from pocket camera" for the schematic
of a typical small battery powered strobe.

Red-eye reduction provides a means of providing a flash twice in rapid
succession.  The idea is that the pupils of the subjects' eyes close somewhat
due to the first flash resulting in less red-eye - imaging of the inside of
the eyeball - in the actual photograph.

This may be done by using the main flash but many cameras use a small, bright
incandescent bulb to 'blind' the eyes when the shutter is pressed to meter,
then it goes off and the flash preserves the 'closed' pupils.  This approach
works.  Using the main flash would require sub-second recycle time which is
not a problem if an energy conserving flash is used (see the document: "Notes
on the Troubleshooting and Repair of Electronic Flash Units and Strobe
Lights".  However, it would add significant additional expense otherwise (as
is the case with most cameras with built in electronic flash).  A separate
little bulb is effective and much cheaper.

Automatic electronic flashes provide an optical feedback mechanism to sense
the amount of light actually reaching the subject.  The flash is then aborted
in mid stride once the proper exposure has been made.  Inexpensive units
just short across the flash tube with an SCR or even a gas discharge tube
that is triggered by a photosensor once the proper amount of light has been
detected.  With these units, the same amount of energy is used regardless
of how far away the subject is and thus low and high intensity flashes drain
the battery by the same amount and require the same cycle time.  The excess
energy is wasted as heat.  More sophisticated units use something like a gate
turnoff thyristor to actually interrupt the flash discharge at the proper
instant.  These use only as much energy as needed and the batteries last
much longer since most flash photographs do not require maximum power.

Failure of red-eye reduction or the automatic exposure control circuits
will probably require a schematic to troubleshoot unless tests for bad
connections or shorted or open components identify specific problems.  It is
also possible for that extra red-eye incandescent light bulb to be burnt out
but good luck replacing it!

Remotely triggered 'fill flashes' use a photocell or photodiode to trigger
an SCR - or a light activated SCR (LASCR) - which emulates the camera shutter
switch closure for the flash unit being controlled.  There is little to go
wrong with these devices.

  9.7) Electronic flash problems

A variety of failures are possible with electronic flash units.  Much
of the circuitry is similar for battery/AC adapter and line powered
units but the power supplies in particular do differ substantially.

Most common problems are likely to be failures of the power supply, bad
connections, dried of or deformed energy storage or other electrolytic
capacitor(s) and physical damage to the to the flashtube.

  9.8) Problems unique to battery or AC adapter powered electronic flash units

* Power source - dead or weak batteries or defective charging circuit,
  incorrect or bad AC adapter, worn power switch, or bad connections.

  Symptoms: unit is totally dead, intermittent, or excessively long
  cycle time.

  Test and/or replace batteries.  Determine if batteries are being charged.
  Check continuity of power switch or interlock and inspect for corroded
  battery contacts and bad connections or cold solder joints on the circuit

* Power inverter - blown chopper transistor, bad transformer, other
  defective components.

  Symptoms: unit is totally dead or loads down power source when switched
  on (or at all times with some compact cameras).  No high pitched audible
  whine when charging the capacitor.  Regulator failure may result in
  excess voltage on the flash tube and spontaneous triggering or failure
  of the energy storage capacitor or other components.

  Test main chopper transistor for shorts and opens.  This is the most
  likely failure.  There is no easy way to test the transformer and the
  other components rarely fail.   Check for bad connections.

  9.9) Problems unique to AC line powered electronic flash units

WARNING: Line powered units often do not include a power transformer.
Therefore, none of the circuitry is isolated from the AC line.  Read,
understand, and follow the safety guidelines for working on line powered
equipment.  Use an isolation transformer while troubleshooting.  However,
realize that this will NOT protect you from the charge on the large high
voltage power supply and energy storage capacitors.  Take all appropriate

* Power source - dead outlet or incorrect line voltage.

  Symptoms: unit is totally dead, operates poorly, catches fire, or blows up.
  Spontaneous triggering may be the result of a regulator failure or running
  on a too high line voltage (if the unit survives).

  Test outlet with a lamp or circuit tester.  Check line voltage
  setting on flash unit (if it is not too late!).

* Power supply - bad line cord or power switch, blown fuse, defective
  rectifiers or capacitors in voltage doubler, defective components, or
  bad connections.

  Symptoms: unit is totally dead or fuse blows.  Excessive cycle time.

  Test fuse.  If blown check for shorted components like rectifiers and
  capacitors in the power supply.  If fuse is ok, test continuity of line
  cord, power switch, and other input components and wiring.  Check
  rectifiers for opens and the capacitors for opens or reduced value.

  9.10) Problems common to all electronic flash units

WARNING: the amount of charge contained in the energy storage capacitor
may be enough to kill - especially with larger AC line powered flash units
and high power studio equipment.  Read and follow all safety guidelines
with respect to high voltage high power equipment.  Discharge the energy
storage capacitors fully (see the document: "Capacitors: Testing with a
Multimeter and Safe Discharging") and then measure to double check that they
are totally flat before touching anything.  Don't assume that triggering
a flash does this for you!  For added insurance, clip a wire across the
capacitor terminals while doing any work inside the unit.

* Energy storage capacitor - dried up or shorted, leaky or needs to be

  Symptoms: reduced light output and unusually short cycle time may 
  indicate a dried up capacitor.  Heavy loading of power source with
  low frequency or weak audible whine may indicate a shorted capacitor.
  Excessively long cycle time may mean that the capacitor has too much
  leakage or needs to be formed. 

  Test for shorts and value.  Substitute another capacitor of similar
  or smaller uF rating and at least equal voltage rating if available.

  Cycling the unit at full power several times should reform a capacitor
  that has deteriorated due to lack of use.  If the flash intensity and
  cycle time do not return to normal after a dozen or so full intensity
  flashes, the capacitor may need to be replaced or there may be some
  other problem with the power supply.

* Trigger circuit - bad trigger capacitor, trigger transformer, SCR (if
  used), or other components.

  Symptoms: energy storage capacitor charges as indicated by the audible
  inverter whine changing frequency increasing in pitch until ready light
  comes on (if it does) but pressing shutter release or manual test button
  has no effect.  Spontaneous triggering may be a result of a component
  breaking down or an intermittent short circuit.

  Test for voltage on the trigger capacitor and continuity of the trigger
  transformer windings.  Confirm that the energy storage capacitor is
  indeed fully charged with a voltmeter.

* Ready light - bad LED or neon bulb, resistor, zener, or bad connections.

  Symptoms: flash works normally but no indication from ready light.  Or,
  ready light on all the time or prematurely.

  Test for voltage on the LED or neon bulb and work backwards to its voltage
  supply - either the trigger or energy storage capacitor or inverter trans-
  former.  In the latter case (where load detection is used instead of simple
  voltage monitoring) there may be AC across the lamp so a DC measurement may
  be deceptive.)

* Trigger initiator - shutter contacts or cable.

  Symptoms: manual test button will fire flash but shutter release has
  no effect.

  Test for shutter contact closure, clean hot shoe contacts (if relevant),
  inspect and test for bad connections, test or swap cable, clean shutter
  contacts (right, good luck).  Try an alternate way of triggering the
  flash like a cable instead of a the hot shoe.

* Xenon tube - broken or leaky.

  Symptoms: energy storage and trigger capacitors charges to proper
  voltage but the manual test button does not fire the flash even though
  you can hear the tick that indicates that the trigger circuit is

  Inspect the flash tube for physical damage.  Substitute another similar
  or somewhat larger (but not smaller) flash tube.  A neon bulb can be put
  across the trigger transformer output and ground to see if it flashes when
  you press the manual test button shutter release.  This won't determine
  if the trigger voltage high enough but will provide an indication that
  most of the trigger circuitry is operating.

  9.11) Electronic flash dead after long time in storage

The unit may be totally dead or take so long to charge that you give up.

For rechargeable units, try charging for the recommended time (24 hours if
you don't know what it is).  Then, check the battery voltage.  If it does
not indicate full charge (roughly 1.2 x n for NiCds, 2 x n for lead-acid where
n is the number of cells), then the battery is likely expired and will need
to be replaced.

Even for testing, don't just remove the bad rechargeable batteries - replace
them.  They may be required to provide filtering for the power supply even
when running off the AC line or adapter.

For units with disposable batteries, of course try a fresh set but first
thoroughly clean the battery contacts.

See the Chapter: "Batteries".

The energy storage capacitor will tend to 'deform' resulting in high leakage
and reduced capacity after long non-use.  However, I would still expect to
be able to hear sound of the inverter while it is attempting to charge.

Where the unit shows no sign of life on batteries or AC, check for dirty
switch contacts and bad internal connections.  Electrolytic capacitors
in the power supply and inverter may have deteriorated as well.

If the unit simply takes a long time to charge, cycling it a dozen times
should restore an energy storage capacitor that is has deformed but is
salvageable.  This is probably safe for the energy storage capacitor as
the power source is current limited.  However, there is no way of telling
if continuous operation with the excessive load of the leaky energy storage
capacitor will overheat power supply or inverter components.

  9.12) Photoflash circuit from pocket camera

This schematic was traced from an electronic flash unit removed from an
inexpensive pocket camera, a Keystone model XR308.  Errors in transcription
are possible.

Note that the ready light is not in the usual place monitoring the energy
storage capacitor voltage.  It operates on the principle that once nearly
full charge is reached and the inverter is not being heavily loaded, enough
drive voltage is available from an auxiliary winding on the inverter
transformer to light the LED.  It is also interesting that the trigger
circuit dumps charge into the trigger capacitor instead of the other way
around but the effect is the same.

           Inverter                                                  Flashtube
      |       1 K     Ready LED      |            S1 Power |  |        |   |
      |   +--/\/\-----+--|<|-----+   |           ______ On |  +-+ T2 +-+   |
 BT1  _   |    R1     |  IL1     |   |          |      \___|     )||(      |
 3 V ___  | || +------|--/\/\/---+   | C1       |    __ Off      )||(     +|FL1
2-AA  _   | ||(2 .4   |  R2 10       | Energy   |   |            )||(     _|_
     ___  | || +-------------+       | Storage  |   +-------+---+ ||(    | | |
      |   | ||(5 .2   |      |      +|  280 uF  |           |     ||(   ||   |
      +---+ || +------+      |     __|__ 330 V  | S2 Fire -|      ||(   ||   |
      |     ||(1      |      |     _____        | (Shutter) |        +--||   |
      +---+ ||(       |  C3  |       |          |     +-----+   Trigger ||   |
          3)||( 142  -|47 uF |      -|          |     |     |           || _ |
       <.1 )||(      _|_ 6.3 |       |          |  R1 \    _|_  C2       |_|_|
           )||(      ___  V  |       |          |  1M /    ___ .02 uF      |
        +-+ || +-+    |      |       |          |     \     |   400 V     -|
       C| 4 T1 6 |   +|      |       |          |     /     |              |
     B|/         |    |      |  D1   |          |     |     |              |
   +--| 2SD879   +--------------|<|--+----------------+-----+--------------+
   |  |\  Q1          |      | HV Rect.         |
   |   E|             |      |                  |
   |    +-------------+------|------------------+
   |                         |


1. The inverter boosts the battery voltage to about 300 V.  This is rectified
   by D1 and charges the energy storage capacitor, C1.

2. The LED, IL1, signals ready by once C1 is nearly fully charged.

3. Pressing the shutter closes S2 which charges C2 from C1 through T2
   generating a high voltage pulse (8-10KV) which ionizes the Xenon
   gas in the flashlamp, FL1.

4. The energy storage capacitor discharges through the flashlamp.


1. The inverter transformer winding resistances measured with a Radio Shack
   DMM.  Primary resistance was below .1 ohms.

       |                                     |
    ---+--- are connected;    ---|--- and ------- are NOT connected.
       |                                     |

  9.13) Darkroom timers

Developing timers only provide a display or clock face (possibly with an
alarm) while enlarging timers include a pair of switched outlets - one for
the enlarger and the other for the safe light.  These are usually self
resetting to permit multiple prints to be made at the same exposure time

Where the device plugged into a controlled outlet does not come on, first
make sure these units are operational (i.e., the bulbs of the enlarger
and/or safelight are not burned out and that their power switches are
in the 'on' position.  The problem could also be that one of these devices is
defective as well.

Two types of designs are common:

1. Electromechanical - using an AC timing motor and gear train with cam
   operated switches controlling the output circuits directly or via relays.

   If the hands fail to move or it does not reset properly, the timing motor
   or other mechanical parts may require cleaning and lubrication.  The
   motor may be inoperative due to open or shorted windings.  See the section:
   "Small motors in consumer electronic equipment".  Where the timer appears
   to work but the controlled outlets (e.g., enlarger and safe light) do not
   go on oroff, check for a loose cam or bent linkages and dirty or worn switch
   or relay contacts.  If the dial fails to reset after the cycle completes,
   it may be binding or require cleaning and lubrication or a spring may
   have come loose or broken.

2. Electronic - digital countdown circuits and logic controlling mechanical
   or solid state relays or triacs.

   Where the unit appears dead, test as with AC line powered digital clocks
   (see the section: "AC powered digital clock problems").  If the buttons
   have the proper effect and the digits count properly but the external
   circuits are not switching, then test for problems in the power control
   circuits.  If the unit is erratic or does not properly count or reset,
   there could be power supply or logic problems.

  9.14) Weird exposure meter problem of the year

Here is one for the photo album:

"Ever since I bought the Mamiya 645 Pro 2 months ago, I've had exposure
 problems. I usually bring any new eqpt up to Twin Peaks (in SF) to test
 for lens sharpness, and overall function. Well my first shots from there
 were 2 stops overexposed, and the meter was reading wrong, so I returned
 the camera for repair, assuming it was broken out of the box. Mamiya went
 over it with a fine tooth comb, and could find nothing wrong with it. I
 got it back on Monday, and went up to Twin Peaks again. Same problem as
 before! The meter read 2 stops over! I cursed the techies at Mamiya, I
 cursed the product, I cursed MF, and then I decided to get scientific
 about it. So I took the camera off the tripod, and pointed it around at
 various things: all normal readings...
 I pointed the camera back at the scene I had just metered on the
 tripod...normal reading. I remounted the camera on the tripod ... 2 stops
 over. I removed the camera ... normal reading. I remounted the camera ...
 2 stops over. Unbelievable. So that's when I started thinking about the RF
 and TV signals being transmitted from the big tower there, and how the
 tripod might act as an antenna, and cause a small current to enter through
 the ground socket and perhaps change the ground reference voltage. But it's
 a carbon fiber tripod! Still, I was on a quest.
 So I borrowed another 645 Pro from the store, and I took my 3 tripods up
 the hill. They were the Gitzo 1228, a Slik U212, and a Tiltall. All 3
 tripods and both cameras exhibited this phenomenon, but to varying
 degrees. The Gitzo was off the most, anywhere from 1-3 stops. The other 2
 did not affect the meter as much, at the most 1-2 stops. Funny thing is,
 the cameras did not even have to *touch* the tripod to have their readings
 affected! As I moved the camera closer, the meter would start overexposing
 by up to a stop, then jump even more once mounted.
 As a control, I then went halfway down the hill, and repeated the test.
 The effect was less, with the Gitzo giving 1-2 stops. I then went
 downtown, and tested again. No difference between on/off camera. I tested
 again when I got home. Again, no difference."

What you have described could indeed be due to RF interference.  Metal and
carbon fiber are both conductors so the construction of the tripods may not
make that much difference.

How is it happening?  This is anyone's guess but enough of a current could
be induced in the sensitive electronic circuitry to throw off the meter.
The ICs are full of diode junctions which can be rectifying (detecting) the
relatively weak RF signal resulting in a DC offset.  If this were the case
and you happened to adjust the tripod height to be around 1/4 wavelength of
one of the transmitters you *would* know it! :-)

Chapter 10) AC Adapters

  10.1) 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: "AC adapter testing".

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 inverter.  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 document: "Notes on the
   Troubleshooting and Repair of Small Switchmode Power Supplies" for more
   info, especially on safety while servicing these units.

Also see the chaper on "Equipment Power Supplies".

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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]