Contents:
Testing of Flyback (LOPT) Transformers
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Safety ------ WARNING: Read, understand, and follow the recommendations in the document: "Safety Guidelines for High Voltage and/or Line Powered Equipment" before attempting any TV or monitor repairs.
When problems develop in the horizontal deflection/high voltage subsystems of TVs or monitors (or even modern oscilloscopes and other CRT displays), the flyback transformer (or line output transformer for those on the other side of the Lake) is often a suspected cause. This is due in part to the fact that it is usually the most expensive and hard to find replacement and because flybacks are often less well understood than other more common components. This document addresses the operation and testing of flyback (LOPT) transformers: What they are, how they fail, why they fail, and how to test them. For more information on horizontal deflection systems, see the document: "TV and Monitor Deflection Systems".
The typical flyback or Line OutPut Transformer (LOPT) consists of two parts: 1. A special transformer which in conjunction with the horizontal output transistor/deflection circuits boosts the B+ (120 V typical for a TV) of the low voltage power supply to the 20 to 30 KV for the CRT as well as provide various secondary lower voltages for other circuits. A HV rectifier turns the high voltage pulses into DC and the CRT capacitance smooths it. The HV may be developed from a single winding with many many turns of wire or a lower voltage winding and a diode-capacitor voltage multiplier. The various secondary voltages power the logic, tuner, video signal, vertical deflection circuits, and CRT filaments. In fact, with many TV designs, the only power not derived from the flyback is for the keep-alive circuitry needed to maintain channel memory and provide startup drive to the horizontal deflection/high voltage system. 2. A voltage divider that provides the focus and screen supplies. The pots are in this divider network - and these things fail resulting poor focus, uncontrolled brightness, or fluctuating focus and/or brightness. A total short could also result in failure of other components like the horizontal output transistor. The focus and screen are generally the top and bottom knobs, respectively. In some TVs, the focus and screen divider and/or controls are external to the flyback and susceptible to dust and problems particularly on damp days.
While the following is not always strictly true for TV and monitor flyback transformers, it is a nice overview: (From: Sivasankar Chander (siva@bond.bocaraton.ibm.com)). The main difference between a flyback transformer and a regular transformer is that a flyback transformer is designed to store energy in its magnetic circuit, i.e., it functions like a pure inductor, whereas a regular transformer is designed to transfer energy from its primary to secondary and to minimize stored energy. Secondly, a flyback transformer in its simplest form has current flowing either in its primary, or in its secondary (but not both at the same time). (This is more complicated in practice because of finite turn-off times for transistors and diodes, need for snubber circuits, etc). Thirdly, the reluctance of the magnetic circuit of a flyback transformer is usually much higher than that of a regular transformer. This is because of a carefully calculated air-gap for storing energy (it's an inductor). Fourthly, the voltages applied to a flyback transformer on the primary side are almost always rectangular (pulsed) whereas regular transformers usually have sinusoidal voltages applied to them. Fifthly, the currents flowing through either side of a flyback transformer are either increasing or decreasing linear sawtooths, whereas a regular transformer usually has sinusoidal currents. Finally, due to the properties of core materials, flyback transformers are most conveniently operated in the range from 10^3 to 10^6 Hz, whereas regular transformers have a much wider range, from a few Hz to 10^12 Hz. I may have succeeded in confusing you beyond redemption, so the best recourse for you would be to read any introductory textbook on switching power supplies for a more comprehensive picture.
In the U.S. (possibly all of North America), the transformer that generates the high voltage in a TV, monitor, or other CRT based equipment, is called the 'flyback' or 'flyback transformer'. Most everywhere else in the world, it is either LOPT (Line OutPut Transformer) or simply LOT. The term 'flyback' probably originated because the high voltage pulse that charges the CRT capacitance is generated by the collapse of the magnetic field in the core of the transformer during the short retrace period - when the electron beam in the CRT 'flies back' to the start of a new scan line. The flux in the core changes slowly during scan and is abruptly switched in polarity by the HOT turning off and damper diode turning on during this flyback or retrace period. Many off-line switchmode power supplies and DC-DC converters are also of the 'flyback' type with energy transferred to their output circuits mainly during the same time in the cycle - but there is no CRT involved and their high frequency transformers are not generally called flyback transformers. LOPT and LOT derive from the fact that it is the line scan circuit that is involved and the transformer is in the output stage. I still think flyback is much more quaint! :-). Of course, others have their own definition: (From: Sam Riner (riner@inet2000.com)). When I was about 12 I touched the wire coming from the FBT on the picture tube, this was a BIG floor model TV, and I flew about five feet backwards. I know this isn't the real history for the name but for many years I believed it was.
So, how far back does the use of a flyback based high voltage go? (From: Henry van Cleef (vancleef@netcom.com)). A flyback HV supply was a feature of the 1946 RCA 630 and GE 801 sets. They used either an 807 or 6BG6 horizontal output tube, 6W4 damper, 1B3 rectifier. The prewar TV's (yes, TV's were made and for sale before the NTSC standard was approved in 1941) generally used a 60 Hz. transformer and 2X2 similar to circuits used in RCA and Dumont oscilloscopes of the 1930's. Zworykin/Morton "Television" (Wiley, 1940) has schematics and a project home-brew TV set using an 81 tube for the HV off a standard power transformer. Of course, to follow your way around this book, you have to know vacuum tube theory and a lot of physics reasonably well, but it is an historical gold mine. (From: Brad Thompson (Brad_Thompson@pop.valley.net)). Some of the early TV sets used an RF oscillator to generate the high voltage for electrostatic-deflection CRTs: a typical tube lineup might include a 6V6 oscillator and 1B3 (or 1X2) rectifier.
One of the main reasons that TVs and many monitors are designed with horizontal deflection driven flybacks is simply economics - it provides a cheap way to get the high voltage and many or most of the other voltages for the set with minimal hardware. (High quality computer monitors sometimes use a separate high voltage supply so that the horizontal deflection is then used just for deflection to reduce interactions between changing scan rates and the HV.) A side benefit is that if the horizontal deflection dies, the power supply voltage goe with it and prevents the CRT phosphors from burning do to undeflected high intensity beam. The use of the horizontal frequency rather than the AC line frequency of 50 or 60 Hz allows the power supply components to be small and light compared to a line operated power transformer and filter capacitors.
While details can vary somewhat, all flybacks consist of a set of windings
on a gapped ferrite core. High voltage diodes and resistive dividers (often
with adjustment pots) for focus and screen (G2) may also be present.
A typical flyback includes the following components:
* Drive winding - for a line powered TV, there will be perhaps a hundred turns
of medium gauge (e.g., AWG #26) wire. This is what is connected in series
with the B+ to the horizontal output transistor in a TV or monitor.
* High voltage winding - several thousand turns. This winding may be split
into several series sections with a high voltage rectifier for each or could
be a single winding. An alternative is provide a lower voltage winding and
use a voltage multiplier (diode-capacitor ladder) to boost this to that
required by the CRT. Very fine wire (e.g., AWG #40) will be used for the
high voltage winding. The high voltage lead to the CRT is fed from the
highest voltage output of the rectifier or multiplier.
Some TV and monitor designs use a physically separate (external - not part
of the flyback transformer) voltage multiplier. In this case, the flyback
high voltage winding will generate 6 to 10 KVAC and the multiplier will
boost this typically 3X or 4X to 20 to 30 KVDC. The focus and screen (G2)
network will generally be part of the multiplier in this case.
* Resistive divider network for focus and screen (G2). This will probably be
fed from only one of the series connected windings (if used). Often, there
are adjustments for focus and screen right on the flyback. The outputs
from this divider may be connected to pins in the base of the flyback or
have their own separate leads which connect to the CRT socket/board.
* Auxiliary windings - anywhere from a couple of turns (for the CRT filament)
to several hundred turns (for a boost source). These supply various
voltages for the typical TV or monitor - CRT filament, logic power, analog
power, boost source (where the flyback does not include its own screen
supply), etc. The gauge of these windings will depend on the current
requirements of each output. They are connected to solder pins at the base
of the flyback.
* Ferrite core - consisting of two C shaped pieces clamped together with
either a spring arrangement or studs and nuts. There will be a gap of a
fraction of a mm provided by a set of spacers between the two C sections.
Most modern flybacks have all the windings on the same leg of the core. The
drive winding and auxiliary windings will be wound and separately insulated
under the high voltage winding. The high voltage winding will consist of
many layers which have insulating material (i.e., mylar) between them.
The other components will be mounted in a separate part of the assembly and
the entire unit is then potted in an Epoxy type filler. Part of the core
is generally accessible - often one entire leg.
A flyback is not an ordinary transformer. The ferrite core contains a
gap. Energy is stored in the magnetic field of the core during scan as
the current is ramping up. Energy is also coupled to certain secondary
outputs during scan. However, energy for the high voltage (HV) is coupled to
the its secondary windings almost entirely when the primary current is shut
off at the end of the scan (probably the source of the name flyback because it
is during the retrace of the electron beam).
Which type of coupling is in effect depends on the direction of the
rectifiers on the secondary side of the flyback:
_ _
\/ _/\_
B+ ------+ +----|>|-----+---o +V1 B+ ------+ +----|>|-----+---o +HV
o )|:|( o Scan | o )|:|( Flyback |
)|:|( Rectifier _|_ )|:|( Rectifier _|_
)|:|( --- )|:|( ---
)|:|( | )|:|( |
_/\_ )|:|( | _/\_ )|:|( o |
HOT ------+ +------------+--+ HOT ------+ +------------+--+
_|_ _|_
- -
Here, V1 is just a typical example of an auxiliary supply derived from a scan
rectifier and HV is the best known example of the use of a flyback rectifier.
Note that the ratio of the number of turns for each winding *cannot* be used
to calculate expected output voltages since the rate of collapse of the
magnetic field (determined by the design of the horizontal output circuit)
affects this.
The gap is critical to the proper operation and is usually determined by
some plastic spacers. CAUTION: mark each one and replace them in exactly
the same position if you disassemble the core for any reason.
Disassemble a flyback and you will understand why I don't recommend this unless the entire future of the explored *and* unexplored universe depends on the effort! You need specialized equipment to just wind the high voltage coil. This isn't something you can do by hand in your basement and the only problem isn't the several thousand turns of nearly invisible wire used in a typical flyback. To sustain the high voltages without arcing and to minimize the interwinding capacitance, the high voltage winding is constructed as many individual layers - perhaps 50 layers in all - of 50 turns each using super fine wire (#40 typical - thinner than a human hair). Each layer must be wound perfectly flat with all wires side-by-side and then individually insulated with mylar tape. Just breathing on such wire will practically break it let alone wrapping several thousand turns in perfect order! The other parts: drive and low voltage windings, focus and screen divider network, and high voltage rectifiers must be assembled with the high voltage winding and CRT leads and then the entire affair is potted in Epoxy. Forget it - you have better things to do than spend a week on a transformer!
While flyback transformers can on occasion be blown due to a failure elsewhere in the TV or monitor's power supply or deflection circuits, in most cases, they simply expire on their own. Why? Flybacks are wound with many layers of really really fine wire with really really thin insulation. This entire assembly is potted with an Epoxy resin which is poured in and allowed to cure. In some ways, these are just short circuits waiting to happen. Flybacks get hot during use and this leads to deterioration of the insulation. Any imperfections, nicks, or scratches in the insulation or trapped air bubbles and impurities in the Epoxy fill material contribute to failure. Temperature cycles and manufacturing defects result in fine cracks in the Epoxy potting material reducing the insulation breakdown particularly in the area of the high voltage windings, rectifiers, and focus/screen divider network. They also physically vibrate to some extent. A whole bunch of other factors are also no doubt important. Once a breakdown - sparking or arcing - develops, it is usually terminal. It is amazing they last as long as they do with the stresses they are under.
Flybacks fail in several ways: 1. Overheating leading to cracks in the plastic and external arcing. If there is no major damage to the windings, repair may be possible. However, arcing from the windings punctures their very thin insulation so that shorted windings may already have developed. Even if the windings are currently in good condition, long term reliability of any such repairs is questionable. Nonetheless, it doesn't hurt to try cleaning and coating with multiple layers of high voltage sealer, corona dope, or even plastic electrical tape (preferably as a temporary repair though I have gotten away with leaving this in place permanently). If possible, moving the point to which the flyback is arcing further away (i.e., a piece of metal or another wire) would also help. (The following from: Tom Riggs (thriggs@mail.netusa1.net)) For sealing flyback transformers, I have found that silicone sealer has worked very well. I used the clear variety, though others will probably work as well. I have heard of burn through with corona dope. (Author's note: make sure you allow ample time for the silicone sealer to setup completely - or else it will breakdown instantly - at least 24 hours. Also, some types (those that smell like vineger - acetic acid - as they cure may result in corroded wiring in the long term). 2. Cracked or otherwise damaged core will effect the flyback characteristics to the point where it may not work correctly or even blow the horizontal output transistor. If the core can be reconstructed so that no gaps are present and clamped and/or glued in place, it should be possible to perform testing without undue risk of circuit damage but consider a replacement flyback as a long term solution. 3. Internal shorts in the FOCUS/SCREEN divider network, if present. One sign of this may be arcover of the FOCUS or SCREEN spark gaps on the PCB on the neck of the CRT. 4. Internal short circuits in the windings. 5. Open windings. More than one of these may apply in any given case. As noted, temporary repair, at least, is sometimes possible for failures (1) and (2). For failures (3) to (5) replacement is usually the only alternative.
First, perform a careful visual inspection with power off. Look for cracks, bulging or melted plastic, and discoloration, Look for bad solder connections at the pins of the flyback as well. If the TV or monitor can be powered safely, check for arcing or corona around the flyback and in its vicinity, Next, perform ohmmeter tests for obvious short circuits between windings, much reduced winding resistances, and open windings. Don't neglect to check between the CRT HV connector (suction cup) and the pins on the base. This should measure infinity. For the low voltage windings, service manuals may provide the expected DC resistance (Sams' Photofact, for example). Sometimes, this will change enough to be detected - if you have an ohmmeter with a low enough scale. These are usually a fraction of an ohm. It is difficult or impossible to measure the DC resistance of the HV winding since the rectifiers are usually built in. The value is not published either. Caution: make sure you have the TV or monitor unplugged and confirm that the main filter capacitor is discharged before touching anything as the flyback is usually connected to this point, perhaps directly! If you are going to remove or touch the CRT HV, focus, or screen wires, discharge the HV first using a well insulated high value resistor (e.g., several M ohms, 5 W) to the CRT ground strap (NOT signal ground). Measurements that are much less than the published values likely indicate a partially shorted winding. However, a difference of 10% may not be at all significant. Higher than normal readings might simply indicate that a design change was made you. Yes, I know, hard to believe they would not have informed you of this! For example, various versions of the flyback used in the Apple MAC Plus - 157-0042A,B,C - are functionally similar but have minor variations in winding parameters. It is not known what effects this would have but they are interchangeable at least for testing. Of course, any continuity between separate windings is definitely a fault. Partially short circuited windings (perhaps, just a couple of turns) and sometimes shorts in the focus/screen divider will drastically lower the Q and increase the load the flyback puts on its driving source with no outputs connected. It is these types of failures, not detectable by simple ohmmeter tests or visual inspection, which the techniques described in the sections under "Advanced testing" address. While less common, I have seen shorts between the CRT HV connector and the low voltage windings on the base of the flyback. This implies a breakdown of the Epoxy potting material probably due to thermally induced microcracks or poor quality manufacturing. Once a small arc develops, it rapidly carbonizes the material around it further reducing the resistance. These rarely heal themselves and thus show up as obviously low resistance readings using an ohmmeter. It is an easy test and can be performed without removing the flyback. Discharge the CRT HV (though this will probably be dead) and just remove the connector from the CRT. It is also possible that various types of flyback faults can damage other circuitry (beyond taking out the horizontal output transistor and its associated parts). For example, a sudden short between the CRT HV connector and a low voltage winding or a short between two low voltage windings could conceivably blow solid state components powered from the flyback. This damage will generally not be apparent until the flyback is replaced. Therefore, if shorts are detected in the flyback, it may be worth testing some of the components in the vicinity though such tests will likely not prove conclusive.
Before attempting the more advanced tests suggested below, there may be ways of being more certain that your flyback is the problem component. The following assumes that running the TV or monitor with the suspect flyback results in an excessive load on the low voltage (B+) power supply blowing a fuse (or attempting to blow a fuse - excessively bright series light bulb). The B+ likely drops from its normal 65 VDC to 140 VDC or more (depending on the actual TV or monitor and mode) to some low value like 25 VDC when measured on the low voltage power supply side of the flyback drive winding. (Measuring at the HOT can result in all sorts of weird readings due to the pulse nature of the waveform and is not recommended - especially when everything is working properly - 1,500 V pulses!). * Disconnect all the secondary loads from the suspect flyback including the CRT. Connect only the drive (B+ and HOT). Power up the TV or monitor (preferably with a series light bulb or on a Variac. If the B+ now climbs to a more normal value, a problem with the HV (CRT short) or one of the secondary loads is indicated. Connect each of these up one a time (or test individual components) to localize the fault. The flyback is likely good. * Remove the suspect flyback and connect just the HOT and B+ to the drive winding of a known good flyback for a similar size TV or similar type of monitor (as appropriate). It may be close enough to keep the drive circuitry happy. Power up the TV or monitor (preferably with a series light bulb or on a Variac. If the B+ now climbs to a more normal value, a problem with the original flyback is indicated. However, more thorough testing may be desirable to be absolutely certain. If you do this regularly, keeping a selection of 'flyback simulators' - just the drive windings and cores may be desirable.
Also see the section: "Flyback testing equipment". There are several ways of testing flybacks (assuming you do not actually have special test equipment for this purpose). Here are two possibilities. The first is easier if you have a scope but the second is more fun.
The following technique works for flybacks, chopper transformers, motors,
mains transformers, deflection yoke windings, VCR video and other magnetic
heads, and other transformers, coils, or inductors.
(Portions from: Gabe (ggabe@mcs.com)).
This is called a 'ring test' and is the method often used by commercial
flyback (or other coil/transformer) testers. The theory is that a faulty
flyback (which cannot be found by simple resistance measurements) will
have shorted turns in one of the coils. In such a case, the 'Q' of the
transformer is greatly reduced. If excited by an impulse, a faulty
transformer will resonate with a highly damped oscillation while a good
one will decay gradually.
1) Connect a high quality capacitor across one winding of the suspect device.
Hope for a resonant frequency of a few kHz. You may need to select the
capacitor value for best results. I have found that a capacitor in the
.001 uF to 1 uF (non-polarized) will usually be satisfactory.
Note that it doesn't matter whether the excitation is applied to the
shorted winding or any other one. However, you should avoid trying to
connect the generator to one of the very small windings like those for the
CRT filament which may only have 2 or 3 turns.
2) Apply a pulse waveform to the parallel resonant circuit. In 1960, most
scopes had a 'sync out' on the timebase that provided a few 10s of volts
at enough current for this. A circuit in "Television" magazine a couple of
years ago used a BU508, a 12 V power supply, and a small oscillator built
from a 4011 chip. A function generator or a 555 timer based circuit will
also make a satisfactory stimulous. Also see the section: "Flyback testing equipment".
3) Look at the waveform across the resonant circuit with a 'scope. A good unit
will give a nicely decaying oscillation, of at least a few cycles, possibly
10's of cycles. If there is a shorted turn *anywhere* in the device, the
oscillations will be seriously damped, and you'd be lucky to see 2 complete
cycles. Experience and/or comparison with a known good device will tell you
what to expect.
Scope
_ o
Pulse or _| |_ | Device under Test
function o---------------------+-----------+ +---
Generator | )|:|(
High Quality _|_ )|:|( All other
Non-polarized --- )|:| +--- windings
Capacitor | )|:| +--- left open
| )|:|(
Ground o---------------------+-----------+ +---
(From: James Elliott (jelliott@stlnet.com)).
I tried the Q evaluation method using the 100 volt CAL voltage pulse from a
Tektronix scope. It worked best when I used a series 200 pF capacitor. I got
maybe 100 pulses before it decayed to zero. If I shorted two of the primary
pins, the decaying pulse train went to zero almost immediately. So it works!
I thought of another method. The Q of a resonant circuit is equal to the
center frequency divided by the half power bandwidth. I applied an audio
generator through a 22k resistor, found the peak frequency, then went off that
frequency to .707 of that amplitude. Double this would be the bandwidth. I
got Q's of 26 and 16 for two I tried. (Editor's note: This appears to be a
valid approach.)
The circuit below excites the flyback in much the same way as in normal
operation. The only caution is that this tester probably does not put
enough stress on the flyback to find an intermittent that fails only under
full operating conditions. However, most flyback failures are solid - once
a short develops, there is a meltdown of sorts and it is there to stay.
You will require a 12 V power source of at least 2 or 3 amps capacity
(regulation is not important - I just use a simple transformer, rectifier,
filter capacitor type of power supply).
The circuit is shown below. None of the component values are critical.
+12 Q1 +----------------+ |:|
o | ) |:|
| B |/ C ) |:| <-- Flyback Under Test -->
| +------| 2N3055 ) |:|
| | |\ E 5T ) |:| +------|>|----------o +HV
| | | ) |:| ( HV Diode(s),
| | -_- ) |:| ( usually built in.
| | ) |:| +-----+ (
+--|-------------------------+ |:|( )|:|(
| | Q2 _-_ ) |:|( 10T )|:|(
| | | ) |:|( each )|:|(
| | B |/ E 5T ) |:|( _ )|:|(
| | +---| 2N3055 ) |:|( _|_ )|:|(
| | | |\ C ) |:| +-- --+ (
| | | | ) |:| Switch (
| | | +----------------+ |:| (
| | | |:| (
| | -----------------------+ |:| +------------------o -HV
| | 2T ) |:|
| | +-----------+ |:| (Numerous other windings not shown.)
| | | 2T ) |:|
| +-------------------------+ |:| Note: |:| denotes ferrite core.
| |
| R1 | R2
+--------/\/\/\--+--/\/\/\---+
110 27 _|_
2W 5W -
Note: if the circuit does not start oscillating at about 5 volts or less,
interchange the two feedback connections to the transistor bases.
The tester is just a chopper feeding the salvaged core from an old flyback
(I removed the inductance control spacers for this core). The drive (5T+5T)
and feedback (2T+2T) coils can be wound from hookup wire (#14-#20) and well
insulated with plastic electrical tape. Connect the center taps directly to
the coils - do not bring out a loop of wire. Make sure all the turns of each
coil are wound in the same direction. Wind the feedback coil directly on top
of the drive coil. The secondary of this core is a 10 turn well insulated
coil similar to the other two wound on the opposite side of the ferrite core.
You will need to remove the suspect flyback from the TV or monitor. Another
10 turn coil is wound on the suspect flyback core anywhere it will fit.
Connect one end of this coil to one end of the 10 turn coil on your old
flyback core. Use a wire nut or twist together securely. Provide an easy
way of connecting the other ends momentarily - a push button comes in handy.
Make sure you locate the HV return lead on the flyback and use that as
the return for the arc. Otherwise, you may puncture the insulation when
the high voltage finds it own path to ground.
It is essential that this be correctly connected or else the high voltage
*will* find a suitable path to ground - and it may not do the other circuitry
any good!
There are several approaches that can be taken - possibly in combination:
* Process of elimination - the HV return will often be an isolated pin on
the flyback not connected to anything else. Therefore, if you test between
all combinations of pins on the flyback (removed from the circuit board)
and find a pin that appears open to all other pins but is connected to a
pad on the circuit board, it is quite likely the HV return.
* Check all connections on the circuit board and identify those that go to
ground. One of these flyback pins will be the HV return. It will do no
harm to connect them all to ground during testing.
* Use a 100 VDC or greater power supply and high value resistor, say 100K.
Connect the power supply negative output through this resistor to the HV
lead on the flyback (suction cup connector):
100K
PS- o--------/\/\--------+--------o CRT (suction cup) connector on flyback
|
o
-
100 VDC Measure voltage here
+
o
|
PS+ o---------------------+--------o Probe to pins on base of flyback
Check each pin on the base of the flyback with the probe. Touching the
return pin will result in the voltage reading dropping to perhaps 50 or 60
volts. This is the forward voltage drop across the high voltage rectifier
stack inside the flyback. All other pins will result in it remaining at the
supply voltage. Note that if you cannot locate the HV return, your flyback
may indeed be defective; it may have an internal bad connection, open HV
rectifier, or burnt out HV winding.
Once everything is wired and double checked, turn on the juice. * If the flyback is good, then with the coils connected there will be several KV at its output - enough to create a small arc (1/8" typical, up to 1/2" for color flybacks). * The load imposed on the oscillator will be modest (the frequency increases in response to load). If there are any shorted windings, then there will be no significant HV output and the load on the oscillator will increase dramatically. * If you get arcing or corona from *under* the flyback - at the pins - either did not locate the correct HV return or there is a short inside resulting in HV arcing internally to the low voltage windings. I have used this 'tester' on a dozen or so flybacks. It has never been wrong (though I have opted not to believe it and gotten screwed).
Sencore and others sell test equipment that includes the 'ring test' or similar capabilities built in. For the professional, these are well worth the expense. However, the hobbyist could probably purchase lifetime TV replacements for the cost of once of these fancy gadgets. Various electronics magazines have published construction articles for various types of simplified versions of these devices. Here is a pointer to one such article: (Portions from: Tony Duell (ard@p850ug1.demon.co.uk)). The February 1998 issue of 'Television' magazine, has a simple circuit for an LOPT (Line Output Transformer - flyback transformer) tester. It uses a TBA920 chip as an oscillator, driving a BUT11AF which supplies the primary of the LOPT. The voltage developed across this winding (the back EMF when the transistor is turned off) is shown on a DMM. There's also a 'scope point to look at the waveform produced. However, there are a few errata in the article: 1. The supply voltage is 12 V as mentioned in the text, not 2 V as shown on the schematic. 2. The peak amplitude given in fig. 3 of 8 V should be after the divider network, not at the transformer itself. 3. There is a capacitor shown from pin 13 (decoupling) which almost certainly should be a bypass to ground, not to the collector of the drive transistor.
Note: Larry has 'beta tested' Bob Parker's (of ESR meter fame) soon to be introduced flyback tester. (From: Larry Sabo (sabo@storm.ca)). Checking out flybacks can be frustrating and very time consuming without a good tester. Now, it just takes me a second to check for ringing on the HOT collector. No ringing? Check the HOT with a DVM for shorts. No shorts? Unsolder all flyback legs except the primary winding and check for rings again. No rings? Shorted turns in the flyback! Bob's estimate that 20% of faulty flybacks have internal leakage or arcing, or bad HV diodes, seems about right. And an LC102 (tester) won't catch these either :-). I've found that about half of these show up with a low resistance measurement between the EHT cap and ground. Sometimes scoping the output at the EHT cap shows unrectified ringing but stray capacitance probably accounts for that. Other times, it's clearly rectified, so go figure. As a last resort, I resort to Sam's chopper to wrestle the hold-outs to the ground, but it takes a bit of time to remove the flyback and put 10-15 turns around the core. The ringer has also helped me isolate a defective yoke, which explained why things wouldn't ring. Anyway, I think Bob's tester is a great little unit and am glad I have had the opportunity to test it--and keep the prototype! :-)
(Most of these comments also apply to SMPS high frequency transformers.) Of all the components in a monitor or TV, the flyback is very likely to be a unique part. This is not so much due to the high voltage winding and/or HV multipler but rather related to its usual function as the source of multiple secondary power supply voltages used by various tuner, deflection, video, and audio subsystems. In addition, inductance, capacitance, pin configuration, and HV, focus, and screen outputs must be compatible. ECG and similar companies do have a line of generic FBTs and should have a catalog/cross reference for these similar to the one for semiconductors. See the section: "Replacement flyback transformers". However, FBTs are where the designers of TVs and monitors can be really creative. After all, specifying the flyback windings gives them complete freedom to pick the number and types of secondary voltages! Your chances of picking up something off the street so-to-speak and expecting it to fit anything you have ever owned - or ever will own - isn't great. (From: an engineer at a TV manufacturer). We have one guy whose mission in life is doing exactly that... (and specifing HOT's too). Besides specifying auxiliary secondaries you can also specify an overturn on the primary (for deflection coils which would otherwise require a >1500 V HOT) and influence the tuning of the EHT secondary, to determine the EHT internal impedance. And finally you might specify a built-in EHT capacitor or bleeder resistor and various types of clicked-on potmeter modules (perhaps with a second focus voltage for DAF).
This diagram shows a typical flyback that might be found in a direct
view color television or computer monitor. Resistances are included for
illustrative purposes only and may be quite different on your flyback!
The high voltage section on the right may actually be constructed as a
voltage multiplier rather than a single winding with multiple HV diodes.
The rectifiers or multiplier, and/or focus/screen divider may be external
to the flyback transformer in some models.
Flyback transformers used in black-and-white TVs and monochrome computer
monitors do not have a focus and screen divider network. Older ones do
not include a high voltage rectifier either - it is external.
The ferrite core of a flyback transformer is constructed with a precision
gap usually formed by some plastic spacers or pieces of tape. Don't lose
them if you need to disassemble the core. The ferrite core is also
relatively fragile, so take care.
The focus and screen divider network uses potentiometers and resistors
(not shown) with values in the 10s to 100s of M ohms so they may not
register at all on your multimeter. The high voltage rectifiers (CR1
to CR3 on this diagram) are composed of many silicon diodes in series
and will read open on a typical VOM or DMM.
Note that there is no standardization to the color code. However, the fat
wire to the CRT is most often red but could also be black. Of course, you
cannot miss it with the suction cup-like insulator at the CRT anode end.
The focus and/or screen connections may also be to pins rather than flying
leads.
|:| +--|>|-----------o HV to CRT
_ 1 |:|( CR1 (25 to 30 KV,
| B+ o-------------+ |:|( suction cup on
Drive | )|:|( fat red wire)
winding < )|:| +-------+
| 1.32 )|:| |
| 2 )|:| +--|>|--+
|_ HOT o-------------+ |:|( CR2
_ 3 |:|(
| 50 o-------------+ |:|(
| )|:| +-------+
| .11 4 )|:| |
| 35 o-------------+ |:| +--|>|--+
Various | )|:|( CR3 |
auxiliary < .28 )|:|( /
windings | 5 )|:|( \<-------o Focus
| 16 o-------------+ |:|( / (3 to 10 KV,
| )|:|( \ orange wire)
| .12 6 )|:|( |
|_ 0 o----------+--+ |:|( 9 |
_ | 7 |:| +--+ /
| H1 o----------)--+ |:| | \<-------o Screen
CRT Heater < .08 | 8 )|:| | / (200 to 800 V,
|_ H2 o----------+--+ |:| | \ brown wire)
| |:| | |
| |:| +----|--------o To CRT DAG
| | ground
+----------------+
Unfortunately, you cannot walk into Radio Shack and expect to locate a flyback
for your TV or monitor. It is unlikely the carrots at the counter will even
know what a flyback is or recognize one if it hit them over the head (wherever
that would be on a carrot). They will probably attempt to sell you a 6.3 V
power transformer :-). Fortunately, there are other options:
* Original manufacturer - most reliable source but most expensive. Older
models may not be available. This may be the only option for many TVs
and monitors - particularly expensive or less popular models.
* Electronics distributors - a number of places including MCM Electronics,
Dalbani, Premium Parts, and Computer Component Source (See the document:
"Notes on the Troubleshooting and Repair of Computer and Video Monitors"
for contact info) sell replacement flybacks. Many of these are actually
original parts and are designated as such. However, there may be no way
of knowing and you may end up with something that isn't quite compatible
(see below). Thus, unless the catalog listing says 'original part', these
may be no better than the sources below.
Here is one apperently just for flybacks:
- Component Technologies, 1-888-FLYBACK or 1-800-878-0540.
email: fbtxformer@aol.com.
and one that is mostly for flybacks:
- CRC Components, 1-800-822-1272.
some others:
- Data Display Ltd, Canadian sub of CCS, 1-800-561-9903.
- EDI (Electro Dynamics, Inc.) NY, 1-800-426-6423.
- Global Semiconductors, 1-800-668-8776, Toronto, http://globalsemi.com.
* Generic replacements - these are sometimes available. ECG, NTE, ASTI, HR
Diemen, for example, offer a line of replacement flybacks. Some of these
sites include a cross reference to their replacement based on TV or monitor
model and/or the part or house number on the flyback:
- NTE (NTE Electronics, Inc), http://www.nteinc.com/
- ECG (Philips), http://www.ecgproducts.com/
- HR (HR Diemen), http://www.hrdiemen.es/
- ASTI (ASTI Mgnetics)
However, these may be of lower quality or not be quite compatible with your
original. In an effort to minimize the number of distinct flyback models,
some corners may be cut and one-size-fits-many may be the rule resulting in
all sorts of problems. Here are a couple of possibilities:
- The number of turns on one or more windings may not quite match your
original meaning there will be lower or higher voltages from certain
outputs and/or drive conditions (current, resonance) may be affected.
- There may even be extra or missing connections - pins on the bottom or
flying leads. It is essential to determine what must be done to make the
flyback work in your equipment *before* applying power. Extra connections
may need to be grounded or connected to some other points in the circuit.
If this is not done, operation may not be correct or other parts may blow
as current from these unconnected pins finds its own way to ground.
- The flyback may simply be defective due to bad quality control, part
number confusion, or mismarking. Internal circuitry such as the focus
and screen(G2) divider could be improperly wired, configured for a
different model, or omitted entirely. Such defects can be very tough to
identify.
Thus, marginal or erratic behavior might result from generic replacements
greatly complicating your troubleshooting since without careful measurements
there is no way of knowing whether the problem is due to the new flyback or
a fault that still exists elsewhere.
Here is one example of such a situation:
(From: Michael Caplan (cy173@freenet.carleton.ca)).
"The FBTs that I tried (three samples in two generic brands available here in
Canada) all seem to be missing the required internal voltage divider. This
was confirmed by comparison with a new oem Sony part. The OEM part exhibits
the proper resistance measurement. It is through this resistance that the
Hold Down voltage is derived. "No resistance = no Hold Down voltage", as
far as I can see."
Disclaimer: I do not know how likely it is to have problems such as these.
In most cases, I would expect the replacement to drop right in and perform
perfectly. However, I have heard of occasionally difficulties. I do not
know which, if any, of the companies listed above sell such incompatible
devices. However, it would be worth checking before buying if possible.
Elim Technology has an on-line resource with their OEM flyback transformers (and other components including deflection yokes). These include complete specifications and pinouts so this site may be useful to get an idea of typical flyback characteristics: * http://www.elim.com/ (Elim Technology) * http://www.elim.com/product/dy.html (Flyback specifications)