testing of flyback
TRANSCRIPT
1
Testing of
Flyback (LOPT)
Transformers
2
Contents
Chapter 1) About the Author & CopyrightChapter 2) Introduction2.1) Scope of this documentChapter 3) Flyback (LOPT) Transformers3.1) What does the flyback (LOPT) transformer do?3.2) How is a flyback transformer different than a regular transformer?3.3) The origin of the term, 'flyback'3.4) A little history3.5) Why is the deflection and high voltage combined?3.6) Flyback construction3.7) Why you don't want to fabricate your own flyback or rebuild a bad one3.8) Why do flyback (LOPT) transformers fail?3.9) How do flyback transformers fail?3.10) Basic testing3.11) The process of elimination3.12) Advanced testing3.13) Method 13.14) Method 23.15) Identifying the high voltage return on a flyback3.16) Method 2 testing procedure3.17) Flyback testing equipment3.18) Quickie in-circuit flyback tests3.19) Why do all flyback (LOPT) transformers seem to be unique?3.20) Typical flyback schematic3.21) Replacement flyback transformers3.22) OEM flyback transformers
3
Chapter 1) About the Author & CopyrightTesting of Flyback (LOPT) Transformers
Copyright (c) 1994, 1995, 1996, 1997, 1998All Rights ReservedReproduction 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.
Chapter 2) IntroductionSafety------
WARNING: Read, understand, and follow the recommendations in the document:"Safety Guidelines for High Voltage and/or Line Powered Equipment" beforeattempting any TV or monitor repairs.
2.1) Scope of this documentWhen problems develop in the horizontal deflection/high voltage subsystemsof TVs or monitors (or even modern oscilloscopes and other CRT displays), theflyback transformer (or line output transformer for those on the other sideofthe Lake) is often a suspected cause. This is due in part to the fact thatitis usually the most expensive and hard to find replacement and becauseflybacksare 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 testthem. For more information on horizontal deflection systems, see thedocument: "TV and Monitor Deflection Systems".
Chapter 3) Flyback (LOPT) Transformers
3.1) What does the flyback (LOPT) transformer do?The typical flyback or Line OutPut Transformer (LOPT) consists of two parts:
1. A special transformer which in conjunction with the horizontal output
4
transistor/deflection circuits boosts the B+ (120 V typical for a TV) ofthe 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 CRTcapacitance 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.
3.2) How is a flyback transformer different than a regulartransformer?While the following is not always strictly true for TV and monitor flybacktransformers, it is a nice overview:
(From: Sivasankar Chander ([email protected])).
The main difference between a flyback transformer and a regular transformeris that a flyback transformer is designed to store energy in its magneticcircuit, i.e., it functions like a pure inductor, whereas a regulartransformer is designed to transfer energy from its primary to secondary andto minimize stored energy.
Secondly, a flyback transformer in its simplest form has current flowingeither 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 fortransistors and diodes, need for snubber circuits, etc).
Thirdly, the reluctance of the magnetic circuit of a flyback transformeris usually much higher than that of a regular transformer. This is becauseof a carefully calculated air-gap for storing energy (it's an inductor).
Fourthly, the voltages applied to a flyback transformer on the primaryside are almost always rectangular (pulsed) whereas regular transformersusually have sinusoidal voltages applied to them.
5
Fifthly, the currents flowing through either side of a flyback transformerare either increasing or decreasing linear sawtooths, whereas a regulartransformer usually has sinusoidal currents.
Finally, due to the properties of core materials, flyback transformersare most conveniently operated in the range from 10^3 to 10^6 Hz, whereasregular 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 bestrecourse for you would be to read any introductory textbook on switchingpower supplies for a more comprehensive picture.
3.3) The origin of the term, 'flyback'In the U.S. (possibly all of North America), the transformer that generatesthe high voltage in a TV, monitor, or other CRT based equipment, is calledthe '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 thatcharges the CRT capacitance is generated by the collapse of the magneticfield in the core of the transformer during the short retrace period - whenthe 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 switchedin polarity by the HOT turning off and damper diode turning on during thisflyback or retrace period.
Many off-line switchmode power supplies and DC-DC converters are also ofthe 'flyback' type with energy transferred to their output circuits mainlyduring the same time in the cycle - but there is no CRT involved and theirhigh frequency transformers are not generally called flyback transformers.
LOPT and LOT derive from the fact that it is the line scan circuit that isinvolved 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 ([email protected])).
When I was about 12 I touched the wire coming from the FBT on the picturetube, this was a BIG floor model TV, and I flew about five feet backwards. Iknow this isn't the real history for the name but for many years I believedit was.
3.4) A little historySo, how far back does the use of a flyback based high voltage go?
6
(From: Henry van Cleef ([email protected])).
A flyback HV supply was a feature of the 1946 RCA 630 and GE 801 sets. Theyused 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 standardwas approved in 1941) generally used a 60 Hz. transformer and 2X2 similar tocircuits used in RCA and Dumont oscilloscopes of the 1930's.
Zworykin/Morton "Television" (Wiley, 1940) has schematics and a projecthome-brew TV set using an 81 tube for the HV off a standard powertransformer.Of course, to follow your way around this book, you have to know vacuum tubetheory and a lot of physics reasonably well, but it is an historical goldmine.
(From: Brad Thompson ([email protected])).
Some of the early TV sets used an RF oscillator to generate the high voltagefor electrostatic-deflection CRTs: a typical tube lineup might include a 6V6oscillator and 1B3 (or 1X2) rectifier.
3.5) Why is the deflection and high voltage combined?One of the main reasons that TVs and many monitors are designed withhorizontaldeflection driven flybacks is simply economics - it provides a cheap way toget the high voltage and many or most of the other voltages for the set withminimal hardware. (High quality computer monitors sometimes use a separatehigh voltage supply so that the horizontal deflection is then used just fordeflection to reduce interactions between changing scan rates and the HV.) Aside benefit is that if the horizontal deflection dies, the power supplyvoltage goe with it and prevents the CRT phosphors from burning do toundeflected high intensity beam.
The use of the horizontal frequency rather than the AC line frequency of 50or 60 Hz allows the power supply components to be small and light compared toa line operated power transformer and filter capacitors.
3.6) Flyback constructionWhile details can vary somewhat, all flybacks consist of a set of windingson a gapped ferrite core. High voltage diodes and resistive dividers (oftenwith 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 hundredturns
7
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 orcould 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. Thedrive winding and auxiliary windings will be wound and separately insulatedunder the high voltage winding. The high voltage winding will consist ofmany layers which have insulating material (i.e., mylar) between them.
The other components will be mounted in a separate part of the assembly andthe entire unit is then potted in an Epoxy type filler. Part of the coreis generally accessible - often one entire leg.
A flyback is not an ordinary transformer. The ferrite core contains agap. Energy is stored in the magnetic field of the core during scan asthe current is ramping up. Energy is also coupled to certain secondaryoutputs during scan. However, energy for the high voltage (HV) is coupled tothe its secondary windings almost entirely when the primary current is shutoff at the end of the scan (probably the source of the name flyback becauseitis during the retrace of the electron beam).
Which type of coupling is in effect depends on the direction of therectifiers on the secondary side of the flyback:
8
_ _ \/ _/\_ 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 scanrectifier 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 usedto calculate expected output voltages since the rate of collapse of themagnetic field (determined by the design of the horizontal output circuit)affects this.
The gap is critical to the proper operation and is usually determined bysome plastic spacers. CAUTION: mark each one and replace them in exactlythe same position if you disassemble the core for any reason.
3.7) Why you don't want to fabricate your own flyback orrebuild a bad oneDisassemble a flyback and you will understand why I don't recommend thisunless the entire future of the explored *and* unexplored universe depends onthe effort! You need specialized equipment to just wind the high voltagecoil.
This isn't something you can do by hand in your basement and the only problemisn't the several thousand turns of nearly invisible wire used in a typicalflyback. To sustain the high voltages without arcing and to minimize theinterwinding capacitance, the high voltage winding is constructed as manyindividual layers - perhaps 50 layers in all - of 50 turns each using superfine wire (#40 typical - thinner than a human hair). Each layer must bewoundperfectly flat with all wires side-by-side and then individually insulatedwith mylar tape. Just breathing on such wire will practically break it letalone wrapping several thousand turns in perfect order!
The other parts: drive and low voltage windings, focus and screen dividernetwork, and high voltage rectifiers must be assembled with the high voltagewinding 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!
9
3.8) Why do flyback (LOPT) transformers fail?While flyback transformers can on occasion be blown due to a failureelsewherein 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 reallyreally thin insulation. This entire assembly is potted with an Epoxy resinwhich 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 theinsulation.Any imperfections, nicks, or scratches in the insulation or trapped airbubbles and impurities in the Epoxy fill material contribute to failure.Temperature cycles and manufacturing defects result in fine cracks in theEpoxy potting material reducing the insulation breakdown particularly in thearea of the high voltage windings, rectifiers, and focus/screen dividernetwork. They also physically vibrate to some extent. A whole bunch ofother 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.
3.9) How do flyback transformers fail?Flybacks fail in several ways:
1. Overheating leading to cracks in the plastic and external arcing. Ifthere 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 currentlyin 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 electricaltape (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 ([email protected]))
For sealing flyback transformers, I have found that silicone sealer has worked very well. I used the clear variety, though others will probably
10
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 toperform 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, temporaryrepair, at least, is sometimes possible for failures (1) and (2). Forfailures (3) to (5) replacement is usually the only alternative.
3.10) Basic testingFirst, perform a careful visual inspection with power off. Look for cracks,bulging or melted plastic, and discoloration, Look for bad solderconnections at the pins of the flyback as well. If the TV or monitor can bepowered safely, check for arcing or corona around the flyback and in itsvicinity,
Next, perform ohmmeter tests for obvious short circuits between windings,much reduced winding resistances, and open windings. Don't neglect tocheck 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 expectedDC resistance (Sams' Photofact, for example). Sometimes, this will changeenough 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 tomeasure the DC resistance of the HV winding since the rectifiers are usuallybuilt in. The value is not published either.
Caution: make sure you have the TV or monitor unplugged and confirm thatthe main filter capacitor is discharged before touching anything as theflyback is usually connected to this point, perhaps directly! If you aregoing to remove or touch the CRT HV, focus, or screen wires, discharge the HVfirst using a well insulated high value resistor (e.g., several M ohms, 5 W)to the CRT ground strap (NOT signal ground).
11
Measurements that are much less than the published values likely indicate apartially shorted winding. However, a difference of 10% may not be at allsignificant. Higher than normal readings might simply indicate that a designchange was made you. Yes, I know, hard to believe they would not haveinformed you of this! For example, various versions of the flyback used inthe Apple MAC Plus - 157-0042A,B,C - are functionally similar but have minorvariations in winding parameters. It is not known what effects this wouldhave 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) andsometimes shorts in the focus/screen divider will drastically lower the Qand increase the load the flyback puts on its driving source with no outputsconnected. It is these types of failures, not detectable by simple ohmmetertests or visual inspection, which the techniques described in the sectionsunder "Advanced testing" address.
While less common, I have seen shorts between the CRT HV connector and thelow voltage windings on the base of the flyback. This implies a breakdown ofthe Epoxy potting material probably due to thermally induced microcracks orpoor quality manufacturing. Once a small arc develops, it rapidly carbonizesthe material around it further reducing the resistance. These rarely healthemselves and thus show up as obviously low resistance readings using anohmmeter. It is an easy test and can be performed without removing theflyback. Discharge the CRT HV (though this will probably be dead) and justremove the connector from the CRT.
It is also possible that various types of flyback faults can damage othercircuitry (beyond taking out the horizontal output transistor and itsassociated parts). For example, a sudden short between the CRT HV connectorand a low voltage winding or a short between two low voltage windings couldconceivably blow solid state components powered from the flyback. Thisdamage will generally not be apparent until the flyback is replaced.Therefore, if shorts are detected in the flyback, it may be worth testingsome of the components in the vicinity though such tests will likely notprove conclusive.
3.11) The process of eliminationBefore attempting the more advanced tests suggested below, there may be waysof being more certain that your flyback is the problem component. Thefollowing assumes that running the TV or monitor with the suspect flybackresults in an excessive load on the low voltage (B+) power supply blowinga 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 onthe actual TV or monitor and mode) to some low value like 25 VDC whenmeasured 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 thepulse nature of the waveform and is not recommended - especially wheneverything is working properly - 1,500 V pulses!).
* Disconnect all the secondary loads from the suspect flyback including the
12
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.
3.12) Advanced testingAlso see the section: "Flyback testing equipment".
There are several ways of testing flybacks (assuming you do not actuallyhave 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.
3.13) Method 1The following technique works for flybacks, chopper transformers, motors,mains transformers, deflection yoke windings, VCR video and other magneticheads, and other transformers, coils, or inductors.
(Portions from: Gabe ([email protected])).
This is called a 'ring test' and is the method often used by commercialflyback (or other coil/transformer) testers. The theory is that a faultyflyback (which cannot be found by simple resistance measurements) willhave shorted turns in one of the coils. In such a case, the 'Q' of thetransformer is greatly reduced. If excited by an impulse, a faultytransformer will resonate with a highly damped oscillation while a goodone will decay gradually.
13
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: "Flybacktesting equipment".
3) Look at the waveform across the resonant circuit with a 'scope. A goodunit 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 2complete cycles. Experience and/or comparison with a known good device willtell 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 ([email protected])).
I tried the Q evaluation method using the 100 volt CAL voltage pulse from aTektronix scope. It worked best when I used a series 200 pF capacitor. Igot maybe 100 pulses before it decayed to zero. If I shorted two of theprimary pins, the decaying pulse train went to zero almost immediately. Soit works!
I thought of another method. The Q of a resonant circuit is equal to thecenter frequency divided by the half power bandwidth. I applied an audiogenerator through a 22k resistor, found the peak frequency, then went offthat frequency to .707 of that amplitude. Double this would be thebandwidth. I got Q's of 26 and 16 for two I tried. (Editor's note: Thisappears to be a valid approach.)
14
3.14) Method 2The circuit below excites the flyback in much the same way as in normaloperation. The only caution is that this tester probably does not putenough stress on the flyback to find an intermittent that fails only underfull operating conditions. However, most flyback failures are solid - oncea 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 wellinsulated with plastic electrical tape. Connect the center taps directly tothe coils - do not bring out a loop of wire. Make sure all the turns of eachcoil are wound in the same direction. Wind the feedback coil directly on topof the drive coil. The secondary of this core is a 10 turn well insulatedcoil similar to the other two wound on the opposite side of the ferrite core.
15
You will need to remove the suspect flyback from the TV or monitor. Another10 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 oldflyback core. Use a wire nut or twist together securely. Provide an easyway 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 asthe return for the arc. Otherwise, you may puncture the insulation whenthe high voltage finds it own path to ground.
3.15) Identifying the high voltage return on a flybackIt 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 circuitryany 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 atthe 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.
16
3.16) Method 2 testing procedureOnce 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 willbe 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 resultingin HV arcing internally to the low voltage windings.
I have used this 'tester' on a dozen or so flybacks. It has never beenwrong (though I have opted not to believe it and gotten screwed).
3.17) Flyback testing equipmentSencore and others sell test equipment that includes the 'ring test' orsimilarcapabilities built in. For the professional, these are well worth theexpense.
However, the hobbyist could probably purchase lifetime TV replacements forthecost of once of these fancy gadgets. Various electronics magazines havepublished construction articles for various types of simplified versions ofthese devices. Here is a pointer to one such article:
(Portions from: Tony Duell ([email protected])).
The February 1998 issue of 'Television' magazine, has a simple circuit for anLOPT (Line Output Transformer - flyback transformer) tester.
It uses a TBA920 chip as an oscillator, driving a BUT11AF which supplies theprimary of the LOPT. The voltage developed across this winding (the back EMFwhen the transistor is turned off) is shown on a DMM. There's also a 'scopepoint 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.
17
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 drivetransistor.
3.18) Quickie in-circuit flyback testsNote: Larry has 'beta tested' Bob Parker's (of ESR meter fame) soon to beintroduced flyback tester.
(From: Larry Sabo ([email protected])).
Checking out flybacks can be frustrating and very time consuming without agood tester.
Now, it just takes me a second to check for ringing on the HOT collector. Noringing? Check the HOT with a DVM for shorts. No shorts? Unsolder allflyback 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,orbad HV diodes, seems about right. And an LC102 (tester) won't catch theseeither :-). I've found that about half of these show up with a lowresistancemeasurement between the EHT cap and ground.
Sometimes scoping the output at the EHT cap shows unrectified ringing butstray capacitance probably accounts for that. Other times, it's clearlyrectified, so go figure. As a last resort, I resort to Sam's chopper towrestle the hold-outs to the ground, but it takes a bit of time to remove theflyback and put 10-15 turns around the core. The ringer has also helped meisolate 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 hadtheopportunity to test it--and keep the prototype! :-)
3.19) Why do all flyback (LOPT) transformers seem to beunique?(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 aunique part. This is not so much due to the high voltage winding and/or HVmultipler but rather related to its usual function as the source of multiplesecondary power supply voltages used by various tuner, deflection, video, and
18
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 havea 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 reallycreative. After all, specifying the flyback windings gives them completefreedom to pick the number and types of secondary voltages! Your chancesof picking up something off the street so-to-speak and expecting it to fitanything 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 specifingHOT's too).
Besides specifying auxiliary secondaries you can also specify an overturn onthe primary (for deflection coils which would otherwise require a >1500 VHOT)and influence the tuning of the EHT secondary, to determine the EHT internalimpedance. And finally you might specify a built-in EHT capacitor or bleederresistor and various types of clicked-on potmeter modules (perhaps with asecond focus voltage for DAF).
3.20) Typical flyback schematicThis diagram shows a typical flyback that might be found in a directview color television or computer monitor. Resistances are included forillustrative purposes only and may be quite different on your flyback!
The high voltage section on the right may actually be constructed as avoltage multiplier rather than a single winding with multiple HV diodes.The rectifiers or multiplier, and/or focus/screen divider may be externalto the flyback transformer in some models.
Flyback transformers used in black-and-white TVs and monochrome computermonitors do not have a focus and screen divider network. Older ones donot include a high voltage rectifier either - it is external.
The ferrite core of a flyback transformer is constructed with a precisiongap usually formed by some plastic spacers or pieces of tape. Don't losethem if you need to disassemble the core. The ferrite core is alsorelatively 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 notregister at all on your multimeter. The high voltage rectifiers (CR1to CR3 on this diagram) are composed of many silicon diodes in seriesand will read open on a typical VOM or DMM.
Note that there is no standardization to the color code. However, the fat
19
wire to the CRT is most often red but could also be black. Of course, youcannot 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 flyingleads.
|:| +--|>|-----------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 +----------------+
3.21) Replacement flyback transformersUnfortunately, you cannot walk into Radio Shack and expect to locate aflybackfor your TV or monitor. It is unlikely the carrots at the counter will evenknow what a flyback is or recognize one if it hit them over the head(whereverthat would be on a carrot). They will probably attempt to sell you a 6.3 Vpower 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
20
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: [email protected].
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. Extraconnections 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 carefulmeasurements there is no way of knowing whether the problem is due to the new flyback or
21
a fault that still exists elsewhere.
Here is one example of such a situation:
(From: Michael Caplan ([email protected])).
"The FBTs that I tried (three samples in two generic brands available herein Canada) all seem to be missing the required internal voltage divider.This was confirmed by comparison with a new oem Sony part. The OEM partexhibits 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.
3.22) OEM flyback transformersElim Technology has an on-line resource with their OEM flyback transformers(and other components including deflection yokes). These include completespecifications and pinouts so this site may be useful to get an idea oftypicalflyback characteristics:
* http://www.elim.com/ (Elim Technology)* http://www.elim.com/product/dy.html (Flyback specifications)