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


  10.2) 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

  10.3) Pocket wall adapter tester/polarity checker

This handy low cost device can be built into an old ball point pen case or
something similar to provide a convenient indication of wall adapter type,
operation, and polarity:

            Probe(+) o-----/\/\-----+----|>|----+---o Probe(-)
                         1K, 1/2 W  | Green LED |
                                       Red LED

* The green LED will light up if the polarity of an adapter with a DC output
  agrees with the probe markings.

* The red LED will light up if the polarity of an adapter with a DC output is
  opposite of the probe markings.

* Both LEDs will light up if your adapter puts out AC rather than DC.

* The LED brightness can provide a rough indication of the output voltage.

  10.4) 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 document: "Notes on the
Troubleshooting 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.

  10.5) 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 or IC protector 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.

Some devices are designed in such a way that they will survive almost anything.
A series diode would protect against reverse polarity.  Alternatively, a large
parallel diode with upstream current limiting resistor or PTC thermistor, and
fuses, fusable resistors, or IC protectors would cut off current before the
parallel diode or circuit board traces have time to vaporize.  A crowbar
circuit (zener to trigger an SCR) could be used to protect against reasonable

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.

Or, a more dramatic result of the same principles:

(From: Don Parker (tazman@yournet.com)).

A guy brought a Johnson Messenger CB to my shop a few decades back.  He had
been told it would run on 12 VDC *and* 115 VAC - so he tried it!  I never saw
so many little leads sticking up from any pcb since - that once were capacitors
and top hat transistors.  There was enough fluff from the caps to have the
chassis rated at least R-10 :->).

  10.6) Power reversal - better pray

"That's right, I reversed power and ground on a Sony XR-6000 AM/FM cassette
 car stereo. (12V negative ground).
 The little fellow made a stinky smell, so I assume that at least one
 component is cooked."
If it had not been turned on before you discovered your error, the damage
may have been limited to the display and some filter caps.  Then again...

The problem is that an auto battery has a very high current capacity and
any fuses respond too slowly to be of much value in a situation such as
this.  Any capacitors and solid state components on the 12 V bus at the
time power was applied are likely fried - well done.

"Is there any hope of my repairing it?  (This assumes I show more ability
 than I did when installing it.)  Which part(s) are likely damaged?"

(From: Onat Ahmet (onat@turbine.kuee.kyoto-u.ac.jp)).

Well, based on that last statement ;-) 

BAD: car batteries can provide amps and amps of current (much worse than
     reverse connecting a wall adapter for example.)

GOOD: The stinking might be due to a component getting too hot
      and vaporizing the solder paste/preserver/dust on it, but
      not actually giving up the ghost.

      I would find and check any fuses, or components directly
      in-line with or parallel to the power lines (the latter
      might include the IC's unfortunately...)

GOOD: There might have been a protecting diode somewhere (but why
      did it stink then (^_^)

NEUTR: Did you disassemble it to see if there were any blackened
       areas/components? Smell from a close distance; I can often
       locate a burnt component that way even after a long time.

If not, join the happy crowd, and gut the good old stereo for parts!

  10.7) Replacing batteries with an AC adapter

While most appliances that run off of internal batteries also include a socket
for an wall adapter, this is not always the case.  Just because there is no
hole to plug one in doesn't necessarily mean that you cannot use one.

The type we are considering in this discussion are plug-in wall adapter that
output a DC voltage (not AC transformers).  This would be stated on the

The first major consideration is voltage.  This needs to be matched to the
needs of the equipment.  However, what you provide may also need to be well
regulated for several reasons as the manufacturer may have saved on the cost
of the circuitry by assuming the use of batteries:

* The maximum voltage supplied by a battery is well defined.  For example,
  4 AA cells provide just over 6 V when new.  The design of the device may
  assume that this voltage is never exceeded and include no internal regulator.
  Overheating or failure may result immediately or down the road with a wall
  adapter which supplies more voltage than its nameplate rating (as most do
  especially when lightly loaded).

* Most wall adapters do not include much filtering.  With audio equipment,
  this may mean that there will be unacceptable levels of hum if used direct.
  There are exceptions.  However, there is no way of telling without actually
  testing the adapter under load.

* The load on the power source (batteries or adapter) may vary quite a bit
  depending on what the device is doing.  Fresh batteries can provide quite
  a bit of current without their voltage drooping that much.  This is not
  always the case with wall adapters and the performance of the equipment
  may suffer.

Thus, the typical universal adapter found at Radio Shack and others may not
work satisfactorily.  No-load voltage can be much higher than the voltage at
full load - which in itself may be greater than the marked voltage.  Adding
an external regulator to a somewhat higher voltage wall adapter is best.  See
the section: "Adding an IC regulator to a wall adapter or battery".

The other major consideration is current.  The rating of the was adapter must
be at least equal to the *maximum* current - mA or A - drawn by the device
in any mode which lasts more than a fraction of a second.  The best way to
determine this is to measure it using fresh batteries and checking all modes.
Add a safety factor of 10 to 25 percent to your maximum reading and use this
when selecting an adapter.

For shock and fire safety, any wall adapter you use should be isolated and
have UL approval.

* Isolation means that there is a transformer in the adapter to protect you
  and your equipment from direct connection to the power line.  Most of the
  inexpensive type do have a transformer.  However, if what you have weighs
  almost nothing and is in a tiny case, it may be meant for a specific purpose
  and not be isolated.

* UL (Underwriters Lab) approval means that the adapter has been tested to
  destruction and it is unlikely that a fire would result from any reasonable
  internal fault like a short circuit or external fault like a prolonged
  overload condition.

To wire it in, it is best to obtain a socket like those used on appliances
with external adapter inputs - from something that is lying in your junk-box
or a distributor like MCM Electronics.  Use one with an automatic disconnect
(3 terminals) if possible.  Then, you can retain the optional use of the
battery.  Cut the wire to the battery for the side that will be the outer
ring of the adapter plug and wire it in series with the disconnect (make
sure the disconnected terminal goes to the battery and the other terminal
goes to the equipment).  The common (center) terminal goes to other side of
the battery, adapter, and equipment as shown in the example below.  In this
wiring diagram, it is assumed that the ring is + and the center is -.  Your
adapter could be wired either way.  Don't get it backwards!

                       X V  | (Inserting plug breaks connection at X)
      Battery (+) o-------  |
      Adapter (+) o---------+------------------o Equipment (Ring, +)
      Battery/                            |
      Adapter (-) o-----------------------+----o Equipment (Center, -)

Warning: if you do not use an automatic disconnect socket, remove the battery
holder or otherwise disable it - accidentally using the wall adapter with
the batteries installed could result in leakage or even an explosion!

  10.8) Converting an AC output wall adapter to DC

Where a modest source of DC is required for an appliance or other device,
it may be possible to add a rectifier and filter capacitor (and possibly
a regulator as well) to a wall adapter with an AC output.  While many wall
adapter output DC, some - modems and some phone answering machines, for
example - are just transformers and output low voltage AC.

To convert such an adapter to DC requires the use of:

    * Bridge rectifier - turns AC into pulsating DC.
    * Filter capacitor - smooths the output reducing its ripple.
    * Regulator - produces a nearly constant output voltage.

Depending on your needs, you may find a suitable wall adapter in your junk
box (maybe from that 2400 baud modem that was all the rage a couple of years

The basic circuit is shown below:

            Bridge Rectifier   Filter Capacitor

    AC o-----+----|>|-------+---------+-----o DC (+)
            ~|              |+        |
   In from   +----|<|----+  |       +_|_   Out to powered device
   AC wall               |  |      C ___   or voltage regulator
   Adapter   +----|>|----|--+       - |
             |           |            |
    AC o-----+----|<|----+------------+-----o DC (-)
            ~                -


* An AC input of Vin VRMS will result in a peak output of approximately
  1.4 Vin - 1.4 V.  The first factor of 1.4 results from the fact that the
  peak value of a sinusoid (the power line waveform) is 1.414 (sqrt(2))
  times the RMS value.  The second factor of 1.4 is due to the two diodes
  that are in series as part of the bridge rectifier.  The fact that they
  are both about 1.4 is a total coincidence.

  Therefore, you will need to find an AC wall adapter that produces an output
  voltage which will result in something close to what you need.  However,
  this may be a bit more difficult than it sounds since the nameplate rating
  of many wall adapters is not an accurate indication of what they actually
  produce especially when lightly loaded.  Measuring the output is best.

* Select the filter capacitor to be at least 10,000 uF per 1000 mA of output
  current with a voltage rating of at least 2 x Vin.  This rule of thumb will
  result in a ripple of less than 1 V p-p which will be acceptable for many
  devices or where a voltage regulator is used (but may be inadequate for
  some audio devices resulting in some 120 Hz hum.  Use a larger or additional
  capacitor or a regulator in such a case.

* Suitable components can be purchased at any electronics distributor as well
  as Radio Shack.  The bridge rectifier comes as a single unit or you can put
  one together from 1N400x diodes (the x can be anything from 1 to 7 for these
  low voltage applications).  Observe the polarity for the filter capacitor!

The following examples illustrate some of the possibilities.

* Example 1: A typical modem power pack is rated at 12 VAC but actually
  produces around 14 VAC at modest load (say half the nameplate current
  rating).  This will result in about 17 to 18 VDC at the output of the
  rectifier and filter capacitor.

* Example 2: A cordless VAC battery charger adapter might produce 6 VAC.
  This would result in 6 to 7 VDC at the output of the rectifier and filter

Adding an IC regulator to either of these would permit an output of up to
about 2.5 V less than the filtered DC voltage.

  10.9) Adding an IC regulator to a wall adapter or battery

For many applications, it is desirable to have a well regulated source of
DC power.  This may be the case when running equipment from batteries as
well as from a wall adapter that outputs a DC voltage or the enhanced adapter
described in the section: "Converting an AC output wall adapter to DC".

The following is a very basic introduction to the construction of a circuit
with appropriate modifications will work for outputs in the range of about
1.25 to 35 V and currents up 1 A.  This can also be used as the basis for a
small general purpose power supply for use with electronics experiments.

For an arbitrary voltage between about 1.2 and 35 V what you want is an IC
called an 'adjustable voltage regulator'.  LM317 is one example - Radio Shack
should have it along with a schematic.  The LM317 looks like a power
transistor but is a complete regulator on a chip.

Where the output needs to be a common value like +5 V or -12 V, ICs called
'fixed voltage regulators' are available which are preprogrammed for these.
Typical ICs have designations of 78xx (positive output) and 79xx (negative

For example:

            Positive                   Negative
        Voltage Regulator          Voltage Regulator
     -----------------------    -----------------------
         7805      +5 V             7905      -5 V
         7809      +9 V             7909      -9 V
         7812     +12 V             7912     -12 V
         7815     +15 V             7915     -15 V

and so forth.  Where these will suffice, the circuit below can be simplified
by eliminating the resistors and tying the third terminal to ground.  Note:
pinouts differ between positive and negatve types - check the datasheet!

Here is a sample circuit using the LM317:

                    I +-------+ O                       
    Vin (+) o-----+---| LM317 |---+--------------+-----o Vout (+)
                  |   +-------+   |              |
                  |       | A     /              |
                  |       |       \ R1 = 240     |
                  |       |       /              |           ___
                 _|_ C1   |       |            +_|_ C2      |_0_|  LM317
                 ___ .01  +-------+             ___ 1 uF    |   |  1 - Adjust
                  |  uF   |                    - |          |___|  2 - Output
                  |       \                      |           |||   3 - Input
                  |       / R2                   |           123
                  |       \                      |
                  |       |                      |
    Vin(-) o------+-------+----------------------+-----o Vout (-)

Note: Not all voltage regulator ICs use this pinout.  If you are not using an
LM317, double check its pinout - as well as all the other specifications.

For the LM317:

1. R2 = (192 x Vout) - 240, where R2 in ohms, Vout is in volts and must be at
   between 1.2 V and 35 V.

2. Vin should be at least 2.5V greater than Vout.  Select a wall adapter with
   a voltage at least 2.5 V greater than your regulated output at full load.

   However, note that a typical adapter's voltage may vary quite a bit
   depending on manufacturer and load.  You will have to select one that
   isn't too much greater than what you really want since this will add
   unnecessary wasted power in the device and additional heat dissipation.

3. Maximum output current is 1 A.  Your adapter must be capable of supplying
   the maximum current safely and without its voltage drooping below the
   requirement in (2) above.

4. Additional filter capacitance (across C1) on the adapter's output may help
   (or be required) to reduce its ripple and thus the swing of its input.
   This may allow you to use an adapter with a lower output voltage and reduce
   the power dissipation in the regulator as well.

   Using 10,000 uF per *amp* of output current will result in less than 1 V
   p-p ripple on the input to the regulator.  As long as the input is always
   greater than your desired output voltage plus 2.5 V, the regulator will
   totally remove this ripple resulting in a constant DC output independent
   of line voltage and load current fluctuations.  (For you purists, the
   regulator isn't quite perfect but is good enough for most applications.)

   Make sure you select a capacitor with a voltage rating at least 25% greater
   than the adapter's *unloaded* peak output voltage and observe the polarity!

   Note: wall adapters designed as battery chargers may not have any filter
   capacitors so this will definitely be needed with this type.  Quick check:
   If the voltage on the adapter's output drops to zero as soon as it is pulled
   from the wall - even with no load - it does not have a filter capacitor.

5. The tab of the LM317 is connected to the center pin - keep this in mind
   because the chip will have to be on a heat sink if it will be dissipating
   more than a watt or so.  P = (Vout - Vin) * Iout.  

6. There are other considerations - check the datasheet for the LM317
   particularly if you are running near the limits of 35 V and/or 1 A.

Chapter 11) Equipment Power Supplies

  11.1) AC (plug-in) power supplies

Reread Safety info before tackling any power supply problem in a VCR!

If your equipment uses an AC adapter (wall wart), see the Chapter: "AC Adapters" first.

Consumer electronic equipment typically uses one of four types of power
supplies (There are no doubt others):

1. Power transformer with linear regulator using 78/79XX ICs or discrete
   components.  The power transformer will be large and very near the
   AC line cord.

2. Power transformer with hybrid regulator like STK5481 or any of its
   cousins - multioutput with some outputs switched by power on.  If it
   has one of these, check ECG, SK, or NTE, or post to sci.electronics.repair
   and someone can probably provide the pinout.  Again, the power transformer
   will be large and very near the AC line cord.

3. Small switching power supply.  Most common problems: shorted semiconductors,
   bad capacitors, open fusable resistors.  In this case there is usually
   no large power transformer near the line input but a smaller transformer
   amidships.  This is rare in audio equipment as the switching noise is
   difficult to keep out of the audio circuits.  These are more often found
   found in some VCRs, TVs, monitors, fax machines, and printers.

Some comments for each type:

1. Troubleshooting is quite straightforward as the components are readily
   identified and it is easy to trace through from the power transformer,
   bridge or centertapped full wave rectifiers, regulators, caps, etc.

2. Failures of one or more of the outputs of these hybrid regulators
   are very common.  Use ECG/STK/NTE cross reference to identify the correct
   output voltages.  Test with power switch in both positions.  Any
   discrepancy indicates likely problem.  While an excessive load dragging
   down a voltage is possible, the regulator is the first suspect.
   Replacement cost is usually under $10.

3. Switching supplies.  These are tougher to diagnose, but it is possible
   without service literature by tracing the circuit and checking for
   bad semiconductors with an ohmmeter.  Common problems - dried up
   capacitors, shorted semiconductors, and bad solder joints.   See the
   document: "Notes on the Troubleshooting and Repair of Small Switchmode
   Power Supplies" for more detailed information.

  11.2) Totally dead power supply (non-switching type)

Don't overlook the possibility of bad solder connections or even a
bad line cord or plug.  Maybe Fido was hungry.

First, make sure the outlet is live - try a lamp.  Even a neon circuit
tester is not a 100% guarantee - the outlet may have a high resistance
marginal connection.

Check for blown fuses near the line cord input.  With the unit unplugged,
test for continuity from the AC plug to the fuse, on/off switch and power
transformer.  With the power switch in the 'on' position, testing across
the AC plug should result in a resistance of 1 to 100 ohms depending on
the size of the equipment (see the section: "Internal fuse blew during lightning storm (or elephant hit power pole)" for typical resistance 
readings of transformer primary windings.

If the fuse blew and the readings are too low, the transformer primary may
be partially or totally shorted.  If the resistance is infinite even directly
across the primary of the power transformer, it may be open or there may be
an open thermal fuse underneath the outer layer of insulation wrapping.

If the fuse blew but resistance is reasonable, try a new fuse of the proper
ratings.  If this blows instantly, there is still a fault in the power supply
or one of its loads.  See the section: "About fuses, IC protectors, and circuit breakers".

If these check out, then the problem is likely on the secondary side.
One or more outputs may be low or missing due to bad regulator components.
A secondary winding could be open though is is less common than primary side
failure as the wire (in transistorized equipment at least) is much thicker.

  11.3) Low or missing power supply outputs (non-switching type)

Once the line input and primary circuits have been found to be good (or at
have continuity and a resistance that is reasonable, the problems is
most likely in the secondary side - fuses, rectifiers, filter capacitors,
regulator components, bad connections, excess load due to electronic
problems elsewhere.

Depending on the type of equipment, there may be a single output of several
outputs from the power supply.  A failure of one of these can result in
multiple systems problems depending on what parts of the equipment use what

Check for bad fuses in the secondary circuits - test with an ohmmeter.  (I
once even found an intermittent fuse!)  Try a new fuse of the same ratings.
If this one blows immediately, there is a fault in the power supply or one of
its loads.  See the section: "About fuses, IC protectors, and circuit breakers".  The use of a series current limiting resistor - a low wattage
light bulb, for example - may be useful to allow you to make measurements
without undo risk of damage and an unlimited supply of fuses.

Locate the large electrolytic filter capacitor(s).  These will probably be
near the power transformer connections to the circuit board with the power
supply components.  Test for voltage across each of these with power on.  If
they are in pairs, this may be a dual polarity supply (+/-, very common
in audio equipment).  Sometimes, two or more capacitors are simply used
to provide a higher uF rating.  If you find no voltage on one of these
capacitors, trace back to determine if the problem is a rectifier diode,
bad connection, or bad secondary winding on the power transformer (the
latter is somewhat uncommon as the wire is relatively thick, however).

Dried up electrolytic capacitors will result in excessive ripple leading
to hum or reduced headroom in audio outputs and possible regulation
problems as well.  Test with a scope or multimeter on its AC scale (but
not all multimeters have DC blocking capacitors on its AC input and these
readings may be confused by the DC level).  If ripple is excessive - as a
guideline if it is more than 10 to 20% of the DC level - then substitute
or jumper across with a good capacitor of similar uF rating and at least
the same voltage rating.

If you find voltages that are lower than expected, this could be due to bad
filter capacitors, an open diode or connection (one side of a full wave
rectifier circuit), or excessive load which may be either in the regulator(s),
if any, or driven circuitry.

Disconnect the output of the power supply from its load.  If the voltage
jumps up dramatically (or the fuse now survives or the series light bulb
now goes out or glows dimly), then a short or excess load is likely.

If the behavior does not change substantially, the problem may be in the
regulator(s).  Transistors, zener diodes, resistors, and other discrete
components, and IC regulators like LM317s or 7809s can be tested with an
ohmmeter or by substitution.  The most common failures are shorts for
semiconductors, opens for resistors, and no or low output for ICs.

Where the supply uses a hybrid regulator like an STK5481, confirming
proper input and then testing each output is usually sufficient to
identify a failure.  A defective hybrid regulator will likely provide
no or very low output on one or more outputs.  Confirm by disconnecting
the load.  Test with any on/off (logic) control in both states.

  11.4) Uninterruptible power supplies (UPSs) and power inverters

Caution: reread safety guidelines as portions of these devices can be nasty.

Note: inexpensive UPSs and inverters generate a squarewave output so don't
be surprised at how ugly the waveform appears if you look at it on a scope.
This is probably normal.  More sophisticated and expensive units may use
a modified sinewave - actually a 3 or 5 level discrete approximation to a
sinewave (instead of a 2 level squarewave).  The highest quality units
will generate a true sinewave using high frequency bipolar pulse width 
modulation.  Don't expect to find this in a $100 K-Mart special, however.

A UPS incorporates a battery charger, lead-acid (usually) storage battery,
DC-AC inverter, and control and bypass circuitry.

Note that if finding a UPS that provides surge protection is an important
consideration, look for one that runs the output off of the battery at all
times rather than bypassing the inverter during normal operation.  The battery
will act as a nearly perfect filter in so far as short term line voltage
variations, spikes, and noise, are concerned.

A DC-AC power inverter used to run line powered equipment from an automotive
battery or other low voltage source is similar to the internal inverter in
a UPS.

For a unit that appears dead (and the power has not been off for more than
its rated holdup time and the outlet is live), first, check for a blown
fuse - external or internal.  Perhaps, someone was attempting to run their
microwave oven off of the UPS or inverter!

(See the section on: "Fuse post mortems" to identify likely failure mode.)

If you find one - and it is blown due to a short circuit - then there are
likely internal problems like shorted components.  However, if it is blown
due to a modest overload, the powered equipment may simply be of too high
a wattage for the UPS or inverter - or it may be defective.

Failures of a UPS can be due to:

1. Battery charging circuit - if the battery does not appear to be charging
   even after an extended time, measure across the battery with the unit both
   unplugged and plugged in.  The voltage should jump up some amount with
   power on - when it is supposed to be charging.  Disconnect the battery
   and try again if there is no action - the battery may be shorted totally.
   Check for blown fuses, smoked parts, and bad connections.

2. Battery - deteriorated or abused lead acid batteries are very common.
   If the battery will not charge or hold a charge, battery problems are
   likely.  A UPS (or any kind of lead-acid battery powered equipment) that
   lies idle for a long time (say a year or two) without power to top off
   the battery will likely result in a dead - not salvageable - battery due
   to sulfation.  Symptoms will be: voltage on battery climbs to more than
   2.5 V per cell when first put on charge and even after a long charging
   period, the battery has essentially no capacity.  If the battery voltage
   is at its nominal value - even when the inverter should be running from it
   (and there is no or low output), then there is a problem in the inverter
   or its connections or there is excess load.

3. Inverter - troubleshooting is similar to that required for a switchmode
   power supply.  Common problems: shorted power semiconductors, open fusable
   resistors, dried up electrolytic capacitors, and bad connections.  See the
   document: "Notes on the Troubleshooting and Repair of Small Switchmode Power
   Supplies".  A visual inspection may reveal parts that have exploded or
   lost their smoke.

4. Line bypass circuit (if used) - check for problems in the controller
   or its standby power supply, or power switchover components, and bad

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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]