Newsgroups: sci.electronics.repair,comp.sys.ibm.pc.hardware.video 
From: sam@colossus.stdavids.picker.com (Sam Goldwasser) 
Date: Thu, 2 Mar 1995 23:53:14 GMT 
 
    ************************************************************************ 
    * NOTES ON THE DIAGNOSIS AND REPAIR OF SMALL SWITCHMODE POWER SUPPLIES * 
    *                                                                      * 
    *                       **** Version 1.01 ****                         * 
    *                                                                      * 
    *                       Copyright (C) 1994,1995                        * 
    *                        Samuel M. Goldwasser                          * 
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Safety: 
------ 
 
The primary danger to you is from the input side of the supply which is 
directly connected to the AC line and will have large electrolytic capacitors 
with 300 V or greater DC when powered (often, even if the supply does not work 
correctly) and for some time after being unplugged. 
 
There is also risk of instantly destroying expensive parts of the supply 
(like the switchmode power transistor) if your probe should slip and short 
something. 
 
General Safety Guidelines when working on line powered equipment including: 
        TVs, monitors, and microwave ovens. 
 
These guidelines are to protect you from potentially deadly electrical shock 
hazards as well as the equipment from accidental damage. 
 
Note that the danger to you is not only in your body providing a conducting 
path, particularly through your heart.  Any involuntary muscle contractions 
caused by a shock, while perhaps harmless in themselves, may cause collateral 
damage - there are many sharp edges inside this type of equipment as well as 
other electrically live parts you may contact accidentally. 
 
The purpose of this set of guidelines is not to frighten you but rather to 
make you aware of the appropriate precautions.  Repair of TVs, monitors, 
microwave ovens, and other consumer and industrial equipment can be both 
rewarding and economical.  Just be sure that it is also safe! 
 
* Don't work alone - in the event of an emergency another person's presence 
  may be essential. 
 
* Always keep one hand in your pocket when anywhere around a powered 
  line-connected or high voltage system. 
 
* Wear rubber bottom shoes or sneakers. 
 
* Don't wear any jewelry or other articles that could accidentally contact 
  circuitry and conduct current, or get caught in moving parts. 
 
* Set up your work area away from possible grounds that you may accidentally 
  contact. 
 
* Know your equipment: TVs and monitors may use parts of the metal chassis 
  as ground return yet the chassis may be electrically live with respect to the 
  earth ground of the AC line.  Microwave ovens use the chassis as ground 
  return for the high voltage.  In addition, do not assume that the chassis 
  is a suitable ground for your test equipment! 
 
* If circuit boards need to be removed from their mountings, put insulating 
  material between the boards and anything they may short to.  Hold them in 
  place with string or electrical tape.  Prop them up with insulation sticks - 
  plastic or wood. 
 
* If you need to probe, solder, or otherwise touch circuits with power off, 
  discharge (across) large power supply filter capacitors with a 2 W or greater 
  resistor of 100-500 ohms/V approximate value (e.g., for a 200 V capacitor, 
  use a 20K-100K ohm resistor).  Monitor while discharging and/or verify that 
  there is no residual charge with a suitable voltmeter.  In a TV or monitor, 
  if you are removing the high voltage connection to the CRT (to replace the 
  flyback transformer for example) first discharge the CRT contact (under the 
  insulating cup at the end of the fat red wire).  Use a 1M-10M ohm 1W or 
  greater wattage resistor on the end of an insulating stick or the probe 
  of a high voltage meter.  Discharge to the metal frame which is connected 
  to the outside of the CRT. 
 
* For TVs and monitors in particular, there is the additional danger of 
  CRT implosion - take care not to bang the CRT envelope with your tools. 
  An implosion will scatter shards of glass at high velocity in every direction. 
  There is several tons of force attempting to crush the typical CRT.  Always 
  wear eye protection. 
 
* Connect/disconnect any test leads with the equipment unpowered and 
  unplugged. Use clip leads or solder temporary wires to reach cramped 
  locations or difficult to access locations. 
 
* If you must probe live, put electrical tape over all but the last 1/16" 
  of the test probes to avoid the possibility of an accidental short which 
  could cause damage to various components.  Clip the reference end of the 
  meter or scope to the appropriate ground return so that you need to only 
  probe with one hand. 
 
* Perform as many tests as possible with power off and the equipment unplugged. 
  For example, the semiconductors in the power supply section of a TV or 
  monitor can be tested for short circuits with an ohmmeter. 
 
* Use an isolation transformer if there is any chance of contacting line 
  connected circuits.  A Variac(tm) is not an isolation transformer! 
  The use of GFCI (Ground Fault Circuit Interrupter) protected outlet is a 
  good idea but will not protect you from shock from many points in a line 
  connected TV or monitor, or the high voltage side of a microwave oven, for 
  example.  A circuit breaker is too slow and insensitive to provide any 
  protection for you or in many cases, your equipment.  The GFCI may protect 
  your scope probe from smoking if you accidentally connect its ground to 
  a live chassis. 
 
* Don't attempt repair work when you are tired.  Not only will you be more 
  careless, but your primary diagnostic tool - deductive reasoning - will 
  not be operating at full capacity. 
 
* Finally, never assume anything without checking it out for yourself! 
  Don't take shortcuts! 
 
Power Supply Fundamentals: 
------------------------- 
 
A typical line connected power supply must perform the following functions: 
 
* Voltage conversion - changing the 115/230 VAC line voltage into one 
  or more other voltages as determined by application. 
 
* Rectification - turning the AC into DC. 
 
* Filtering - smoothing the ripple of the rectified voltage(s). 
 
* Regulation - making the output voltage(s) independent of line 
  and load variations. 
 
* Isolation - separating the supply outputs from any direct connection to 
  the AC line. 
 
A Linear Power Supply (LPS) such you would find in most audio or precision 
analog equipment includes a power transformer which converts the 115/230 VAC 
to other (usually lower) voltages (now that most equipment has done away 
with vacuum tubes except for CRTs, more on that later).  The power 
transformer also provides the isolation 
between the load and the line.  The outputs are rectified by a diode 
bridge or other solid state configuration.   Filtering is accomplished with 
electrolytic capacitors and sometimes inductors or resistors arranged as a low 
pass filter C-L-C or C-R-C or other configuration. 
 
Where regulation is important - that is, it is desirable for the 
output voltage to be relatively independent of line or load variations, 
a regulator stage is added.  This may take the form of a Zener diode if 
the current requirements are modest, discrete 
transistor circuit, or an integrated 3 terminal regulator like an LM317 
(variable), 7805 (+5), or 7912 (-12).  There are many more as well as 
linear regulators for higher voltages such as 115 VDC or 125 VDC for 
 
TV power supplies and multiple output hybrid regulators for VCRs. 
The regulator circuit essentially compares the output with a 
reference and adjusts the current flow to make the output as 
nearly equal to the desired voltage as possible.  However, a significant 
amount of power may be lost in the regulator especially under high line 
voltage/high load conditions.  Therefore, the efficiency of linear power 
supplies is usually quite low - under 50%. 
 
Notable characteristics of LPSs are excellent regulation and low output noise. 
 
What is a Switch Mode Power Supply? 
---------------------------------- 
 
Also called switching power supplies and sometimes chopper controlled 
power supplies, SMPSs use high frequency (relative to 50/60 Hz) switching 
devices such as Bipolar Junction Transistors (BJTs), MOSFETs, Insulated 
Gate Bipolar Transistors (IGBTs), or Thyristers (SCRs or Triacs) to take 
directly rectified line voltage and convert it to a pulsed waveform. 
The input to the switches is usually either 150-160 VDC after rectification 
of 115 VAC, or 300-320 VDC after doubling of 115 VAC or rectification of 
230 VAC.  Up to this point, there is no line isolation as there is no 
power transformer. 
 
A high frequency transformer converts the pulsed waveform into one or 
more output voltages which are then rectified and filtered using 
electrolytic capacitors and small inductors in a 'pi' configuration 
C-L-C, or in less critical applications, just a capacitor. 
 
This high frequency transformer provides the isolation barrier. 
 
Feedback is accomplished across the isolation barrier by either a small 
pulse transformer or opto-isolator.  The feedback controls the pulse 
width or pulse frequency of the switching devices to maintain the 
output constant.  Since the feedback is usually only from the "primary" 
output, regulation of the other outputs, if any, is usually worse than 
for the primary output.  Also, because of the nature of the switching 
designs, the regulation even of the primary output is usually not nearly 
as good both statically and dynamically as a decent linear supply. 
 
Probably the most common topology for small switchers is the 
flyback circuit shown below: 
 
             CR1                              CR2           L 
   o---------|>|---+----+-------_  T1  _------|>|-----+---UUUUU---+---+----o V+ 
       line rect.  |    |       _) || (_             _|_         _|_  | 
                   |    \ R1    _) || (_           C ---       C ---  | 
  AC               |    /       _) || (_______________|___________|__ | ___o V- 
 Line             _|_   \       |                                     | 
  in       filter ---   |     |/       +-------+   +-----------+   +-----+ 
            cap    |    +-----+--------|  PWM  |<--| Isolation |<--| REF | 
                   |       Q1 |\       +-------+   +-----------+   +-----+ 
                   |            | 
   o---------------+------------+ 
 
The input to the supply is the AC line which may have RFI and surge protection 
(not shown).  There may be several Ls and Cs to minimize the input as well 
as the radiation of radio frequency interference.  There may be MOV type 
of surge suppressors across the three input leads (H, N, G). 
 
Rectification is usually via a voltage doubler or bridge.  One common circuit 
uses a bridge rectifier as a doubler or normal bridge by changing a jumper. 
The voltage across the switching transistor is usually designed to be 
around 150-300 V. 
 
When Q1 turns on, current increases linearly in T1 based on the voltage 
applied and the inductance of the transformer.  When Q1 turns off, the 
field callapses and transfers power to the output.  The longer Q1 is on, 
the more energy is stored (until saturation at which point it blows up). 
Thus, controlling the pulse width of the Q1 on time determines the amount 
of power available from the output.  The output rectifier, CR2, must be 
a high efficiency, high frequency unit - a 1N400X will not work.  The pie 
filter on the output smooths the pulses provided by CR2.  Sometimes, a full 
wave configuration is used with a center tapped transformer secondary. 
 
A reference circuit monitors the primary output and controls the duty 
cycle of the switching pulses to maintain a constant output voltage. 
(Secondary outputs are not shown.) 
 
R1 is the startup resistor (some startup circuits are more sophisticated) 
and provides the initial current to the switchmode transistor base.  The 
PWM circuit guarantees that Q1 will not be turned on continuously. 
 
Most small SMPSs use opto-isolators for the feedback.  An opto-isolator 
is simply an LED and a photodiode in a single package.  Typically, a 
circuit on the output side senses the primary output voltage and 
turns on the LED of the opto-isolator when the output voltage exceeds 
the desired value.  The photodiode detects the light from the LED 
and causes the pulse width of the switching waveform to be reduced enough 
to provide just the right amount of output power to maintain the 
output voltage constant.  This circuit may be as simple as putting 
the photodiode across the base drive to the BJT switch thus cutting 
it off when the output voltage exceeds the desired value.  The reference is 
often a TL431 or similar shunt regulator chip monitoring a voltage divided 
version of the primary output.  When the shunt regulator kicks in, the 
opto-isolator LED turns on reducing the switchmode transistor drive. 
There may be an adjustment for the output voltage. 
 
Where additional regulation is needed, small linear regulators may also 
be included at the output(s). 
 
There are many other topologies for switching power supplies but the basic 
principles are similar but the detail differ depending on application. 
The flyback topology described above is one of the most common for small 
multi-output supplies.  However, you may find other types of circuits 
in TVs and monitors. 
 
The advantages of implementing the switch mode operation are with 
respect to size, weight, and efficiency. 
 
Since the transformer and final filter(s) run at a high frequency 
(we are talking about 10KHz to 1 MHz or more), they can be much 
smaller and lighter than the big bulky components needed for 50/60 
Hz operation.  Since the switching devices are (ideally) fully on 
or fully off, there is relatively little power lost so that the 
efficiency can be much higher for SMPSs than for LPSs, especially near 
full load.  Efficiencies can exceed 85% with improvements being made 
in technology continuously.  Since the advent of the laptop computer, 
portable phone, and other portable devices, the importance 
of optimizing power utilization has increased dramatically.  There are 
now many ICs for controlling and implementing SMPSs with relatively 
few external components. 
 
Where are SMPSs uses? 
-------------------- 
 
Switch Mode Power Supplies are commonly used in computer and other digital 
systems as well as consumer electronics - particularly TVs and newer VCRs 
though audio equipment will tend to use linear power supplies due to noise 
considerations.  You will find SMPSs in: 
 
PCs, laptops and their power packs, external peripheral boxes, X-terminals, 
TVs, some VCRs, Camcorder AC adapters, other video equipment. 
 
In additional, you will find DC-DC converters which are SMPSs without 
the AC line connection, internally in an increasing number of consumer 
and industrial applications. 
 
The up side is that they are usually quite reliable, efficient, and cool 
running. 
 
The down side is that when a failure occurs, it may take out many parts 
in the supply, though not usually the equipment being powered unless the 
feedback circuitry screws up and there is no overvoltage protection. 
 
Troubleshooting: 
--------------- 
 
The diagnosis is often difficult due the interdependence of components that 
must function properly for any portion of the power supply to begin to work. 
Depending on design, SMPS may or may not be protected from overload 
conditions and may fail catastrophically under a heavy load even when 
supposedly short circuit proof.  There is particular stress on the 
switching devices (they are often 800 V transistors) which can lead 
to early or unexpected failure.  Also, SMPS may fail upon restoration 
of power after a blackout if there is any kind of power spike since 
turn-on is a very stressful period - some designs take this into account 
and limit turn on surge. 
 
Notes on SMPSs in TVs and Monitors: 
----------------------------------- 
 
Tvs and monitors have at least one SMPS - the horizontal deflection 
flyback circuit and may have an additional SMPS to provide the low 
voltages or the DC for the horizontal output transistor.  Most of the 
basic comments below apply to these as well.  However, manufacturers 
of TVs and monitors tend to be really creative (can you say, obscure?) 
when it comes to these designs so a little more head scratching is 
often necessary to decipher the circuit and get into the mind of the 
designer.  However, the basic failure modes are similar and the same test 
procedures may be used. 
 
SMPS Failure Modes: 
------------------ 
 
SMPS fail in many ways but the following are common: 
 
* Shorted switchmode transistor - may take out additional parts such as 
  fusable flameproof resistors in collector or emitter circuits (or 
  source or drain circuits for MOSFETs).  Main fuse will blow unless 
  protected by fusable resistors. 
 
  Symptoms:  totally dead supply, fuse blows instantly (unless fusable 
  resistor has opened).  Measuring across C-E or D-S of switchmode  
  transistor yields near ohms even when removed from circuit. 
 
* Shorted rectifier diodes in secondary circuits - these are high frequency 
  high efficiency diodes under a fail amount of stress. 
 
  Symptoms:  In a very basic supply without overcurrent protection, 
  the failure of one or more of these diodes may then overload the 
  supply and cause a catastrophic failure of the switchmode power 
  transistor (see above) and related components.  Thus, these should 
  be checked before reapplying power to a supply that had a shorted 
  switchmode transistor. 
 
  On short circuit protected supplies, the symptom may be a periodic 
  tweet-tweet-tweet as the supply attempts to restart and then shuts down. 
 
  Test with an ohmmeter - a low reading in both directions indicates a 
  bad diode.  Sometimes these will test ok but fail under load or at 
  operating voltage.  Easiest to replace with known good diodes to verify 
  diagnosis.  Rectifiers either look like 1N400X type on steroids - 
  cylinders about 1/4" x 1/2" (example: HFR854) or TO220 packages 
  (example: C92M) with dual diodes connected at the cathode for positive 
  supplies or the anode for negative supplies (the package may include a 
  little diagram as well).  These may either be used with a center-tapped 
  transformer, or simply parallel for high current capacity.  If in doubt, 
  remove from the circuit and test with the ohmmeter again.  If not the 
  output used for regulation feedback, try the supply with the rectifier 
  removed.  As noted, a test with an ohmmeter may be misleading as these 
  rectifiers can fail at full voltage.  When in doubt, substitute a known 
  good rectifier (one half of a pair will be good enough for a test). 
 
* Bad startup circuit - initial base (gate) drive is often provided by 
  a high value, high power resistor or resistors from the rectified 
  AC voltage.  These can simply open for no good reason. 
 
  Symptoms:  In this case the supply will appear totally dead but all 
  the semiconductors will check out and no fuses will blow.  Check the 
  startup resistors with an ohmmeter - power resistors in the AC 
  line input section.  There will be full voltage on the main filter 
  capacitor(s) (1x or 2x peak - around 150 or 300 VDC depending on design.) 
 
* Dried up capacitors - either input or output side. 
 
  Symptoms: The main filter capacitor may dry up or open and cause the 
  output to be pulsing at 60 (50) or 120 (100) Hz and all kinds of 
  regulation problems.  Measure across main filter capacitor(s).  If reading 
  is low and drops to a much lower value or 0 instantly upon pulling the 
  plug, then one of these capacitors may be open or dried up. 
 
  Capacitors in the low voltage section may fail causing regulation problems. 
  Sometimes there are slew rate limiting capacitors which feed from the 
  primary output to the regulator controller to limit initial in-rush and 
  overshoot.  A failure of one of these may mess up regulation at the very 
  least. 
 
  In almost all cases, when in doubt parallel a known good capacitor of 
  similar capacitance and at least equal voltage rating. 
 
  For Panasonic (and other) VCR power supplies, it has been suggested that 
  one or more the output filter capacitors commonly fail and replacing all 
  of them, while perhaps a brute force solution, will fix a whining 
  supply or one having bad regulation or noise.  However, check the 
  semiconductors as well before applying power. 
 
* Bad connection/cold solder joints - as with all other mass produced 
  power systems (including TVs and monitors), cracked or defective 
  solder connections are very common especially around the pins of 
  high power components like transformers, power resistors and transistors, 
  and connectors. 
 
  Symptoms: almost any kind of intermittent behavior is possible.  Visually 
  inspect with a bright light and magnifying glass if necessary.  Gently 
  prod or twist the circuit board with an insulating stick to see if the 
  problem can be made to change. 
 
* Regulation problems - outputs high or low. 
 
  Symptoms: voltage has changed and adjustment pot if one exists has no 
  effect or is unable to set voltage to proper value.  Check components 
  in the feedback regulator, particularly the optocoupler and its associated 
  circuitry.  A weak optocoupler may allow for excessive output voltage. 
  A shorted photodiode in the optocoupler may prevent startup.  An open 
  photodiode may lead to a runaway condition.  WARNING: probe these circuits 
  with care both as a result of the safety issues but also since any slip of 
  the probe may lead to a runaway condition and catastrophic failure of 
  the switchmode transistor and its related parts as well as damage to 
  any attached equipment. 
 
Note that the high frequency transformer does not make the top 10 list - 
failure rates for these components are relatively low.  You better hope 
so in any case - replacements are usually only available from the original 
manufacturer at outragious cost. 
 
All other parts are readily available from places like MCM Electronics, 
Dalbani, Premium Parts, and other national distributors. 
 
Also, while it is tempting to suspect any ICs or hybrid controllers, 
these parts are pretty robust unless a catastrophic failure elsewhere 
sent current where it should not have gone. 
 
 
Repair Comments: 
--------------- 
 
Any time the switchmode transistor requires replacement, check all 
semiconductors for shorts and fusable resistors for opens. even if 
you locate 'the' problem early on.  Multiple parts often fail and just 
replacing the transistor may cause it to fail as a result of something 
else still being bad.  It only takes a few more minutes.  However, 
for other problems like an open startup resistor or dried up capacitor, 
this excessive caution is unnecessary as these are usually isolated failures. 
 
It is often helpful to trace the circuit by hand if a service manual is 
not available.  You will gain a better understanding of this supply and 
be able to put the knowledge to use when the next one shows up on your 
bench - there is a lot of similarity even between different manufacturers. 
The only difficult part will be determining how the transformer windings 
are hooked up.  An ohmmeter will help but even if you cannot entirely 
determine this, just make a note.  For most purposes, the exact topology 
of the windings is not critical for diagnostic procedures. 
 
Flameproof Resistors in Switch Mode Power Supplies: 
-------------------------------------------------- 
 
'FR' means 'Flameproof Resistor' or 'Fusable Resistor'. They are the same. 
They will look like power resistors but will be colored blue or gray, or may 
be rectangular ceramic blocks.  They should only be replaced with flameproof 
resistors with identical ratings.  They serve a very important safety function: 
they cannot catch fire when overheated. 
 
These usually serve as fuses in addition to any other fuses that may be 
present (and in addition to their function as a resistor, though this isn't 
always needed).  If an FR type resistor has blown, you probably have shorted 
semiconductors that will need to be replaced as well.  Check 
all the transistors and diodes in the power supply with an ohmmeter. 
You may find that the main switch mode transistor has decided to turn into 
a blob of solder - dead short.  Check everything out even if you find one 
bad part - many components can fail or cause other components to fail 
if you don't locate them all.  Check resistors as well, even if they look ok. 
 
The most common location for these in a small SMPS is in the emitter circuit 
of a bipolar switchmode transistor.  The value will usually be a fraction of 
an ohm.  For testing ONLY, a normal resistor may be substituted but the 
proper replacement MUST be installed before returning the supply to service. 
 
In TVs and monitors, these are often found in the hot supply side to the 
main low voltage power supply and in various secondary supply feeds as 
well.  For the main supply, they will be 5-25 W rectangular ceramic power 
resistors.  For the secondary supplies, they may be the 1/2-2 W blue or 
brown tubular variety. 
 
Unusual Components: 
------------------ 
 
The following are some other types of parts that you may find in a SMPS and 
may not be familiar to you: 
 
* MOVs - Metal Oxide Varisters - look like brightly colored plastic coated 
  disk capacitors but not marked with capacitance.  These are surge 
  suppressors.  A severe surge or lightning strike may obliterate one or 
  more of these.  There will usually be either 1 between the Hot and Neutral 
  or 3 across H, N, and safety ground. 
 
* NTC Resistors - Negative Temperature Coefficient resistors act as inrush 
  surge limiters.  There may be one or two of these in series with the AC 
  input.  These are a high value when cold but drop to a low value once 
  they heat up due to current flow into the supply.  These often look like fat 
  black disk capacitors. 
 
* Coupled Inductors - used as part of the Pi type RFI filter in the AC input 
  circuit.  These look like small transformers but the windings are in series 
  with the AC line.  There are usually 1 or 2 of these on better supplies. 
  Very reliable. 
 
* Bypass Capacitors - high quality plastic dipped or rectangular molded 
  capacitors as part of RFI filter.  Rarely fail. 
 
* High Frequency Transformer - the large transformer which provides line 
  isolation and voltage conversion from the line.  These are usually custom 
  and replacements are only available from the manufacturer.   However, 
  some distributors will stock replacements for a few TVs and computer 
  monitors. 
 
* Optoisolator - either a 4 or 6 pin DIP or a 4 pin cylindrical object. 
  Provides the regulator feedback across the isolation barrier.  Replacements 
  are readily available.  Test by putting 10-20 mA through LED and measuring 
  decrease in resistance of reverse biased photodiode.  However, this will 
  not identify a weak optoisolator. 
 
* TL431 or similar shunt regulator IC - either a TO92 or 8 pin DIP.  Has 
  3 active terminals - A, C, and R.  Current will flow from C to A if R-A is 
  greater than 2.5 V. 
 
* SCRs - small SCRs may be found in the overvoltage protection circuitry of 
  some supplies.  Note that SCR type of crowbars are used across the output 
  as a way to guarantee that an overvoltage condition will kill the output 
  regardless of the reason for the overvoltage condition.  Hopefully, 
  the supply's overcurrent protection will kick in rather than having the 
  supply blow up. 
 
Initial Post-Repair Testing: 
--------------------------- 
 
Once defective parts have been replaced, if possible remove the normal 
load from the supply just in case it decides to put excessive voltage on 
its outputs and replace with a dummy load.  For a multiple output supply, 
the most important output to have a load on is the one that is used for 
regulation but some modest load on all the outputs is preferred.  You should 
be able to determine a suitable value by considering the application. 
For something like a VCR, a few hundred mA on the main output is probably 
enough.  This would require something like a 25 ohm 2 W resistor for a 
5 or 6 volt output or 50 ohm 5 W resistor for a 12 volt output (depending 
on which is the primary output).  For a PC power supply, a couple of amps may 
be needed - a 2 or 3 ohm 15 W resistor on the +5 output.  The minimum 
load is sometimes indicated on the specification sticker.  In the case 
of a TV or monitor, disconnecting the load may not be possible (or at 
least, easy). 
 
If available, use a Variac to bring up the input voltage slowly while 
observing the primary output.  You should see something at about 50% of 
normal input voltage - 50 or 60 V for a normal 115 VAC supply.  With a 
small load, the output should very quickly reach or even exceed its normal 
value.  Regulation at very low line voltage may be far off - this is often 
normal.  If you do not have a Variac, put a lightbulb in series with the 
line (this is desirable in any case).  Use a 100 W bulb for a TV or PC, 
40 W for a VCR typical.  The lightbulb should limit the current to a 
non-destructive value long enough to determine whether everything is ok. 
It may not permit normal operation under full load, however.  When power 
is first applied, the lightbulb will flash briefly but may just barely 
be glowing once the output has stabilized.  If it is fairly bright 
continuously, there is likely still a problem in the supply. 
 
Once you are finished, save your schematic and notes for the future. 
For example, multiple models of VCRs even from different manufacturers 
use the same basic design, maybe even the same supply. 
 
