Topic POWERREP from HARDW FAQ base
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— 299.COMP.IBM.PC.VI (2:5020/299) ———————————————————————— 299.COMP.IBM.PC.VI —
From : sam@colossus.stdavids.picker.com 2:5020/299.100 Fri 03 Mar 95 02:53
Subj : Unknown
————————————————————————————————————————————————————————————————————————————————
In article tparker@netcom.com (Todd Parker)
writes:
> I have a Multisync II (JC1402hma) that the switching power supply will
> not turn on anymore, no smoke, no (obviously) burned components, and the
> fuses and switch :) are good.
> As troubleshooting switching ps's is not somthing I do alot, it will not
> be very easy without a schematic...
> Does someone have one or more of the following:
> 1. a schematic for the ps (and possibly more).
> 2. a hanger queen with a working ps.
> 3. troubleshooting assistance to offer.
If it is totally dead without any sign of action - no tweet, tweet, tweet, no
lights, etc. then it could be that the startup circuitry is faultry. Most
switching power supplies (the flyback qualifies) need some kind of circuit
to get the supply started until the auxiliary windings on the flyback
can provide the necessary voltages for the controller.
Without a schematic, I would suggest tracing the power from the AC line
to the first switching transistor or regulator. There will be a bridge
rectifier (most likely), filter cap, and then it will either go to the
switchmode transistor of a separate switching supply, a regulator, or
the horizontal output transistor collector through a winding on the
flyback.
A quick initial test would be to locate the horizontal output transistor
and determine if there is around 100-150 V on its collector-emitter.
If there is, then the startup circuit is not providing current to turn
this transistor on there is some other fault preventing the transistor
from turning on.
If there is no voltage at the C-E of the horizontal output transistor,
then you need to determine where its voltage comes from.
This should be enough to get started. Get back to us with your findings.
Be careful in there. Those voltages can bite. Be especially careful
as parts of the chassis may be line connected.
Here is some info on switching mode power supplie repair.
-+- sam
************************************************************************
* NOTES ON THE DIAGNOSIS AND REPAIR OF SMALL SWITCHMODE POWER SUPPLIES *
* *
* **** Version 1.01 **** *
* *
* Copyright (C) 1994,1995 *
* Samuel M. Goldwasser *
* Corrections or suggestions to: sam@stdavids.picker.com *
* *
* --- All Rights Reserved --- *
* *
* Reproduction of this document in whole or in part is permitted *
* if both of the following conditions are satisfied: *
* *
* 1. This notice is included in its entirety at the beginning. *
* 2. There is no charge except to cover the costs of copying. *
* *
************************************************************************
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.
> I have scope and all tools/knowledge to do the job, and I /could/ reverse
> eng. the schmatic, but if one is available...
> --
> [todd] Saving Virtual Trees;
> todd
---
* Origin: a kind of gate (2:5020/299.100)