Testing of Flyback (LOPT) Transformers

Testing of Flyback (LOPT) Transformers

Version 1.58 (19-Nov-06)

Copyright ; 1994-2013 Samuel M. Goldwasser

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For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions aresatisfied:

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Table of Contents

PrefaceAuthor and CopyrightDISCLAIMER

IntroductionScope of This Document

Safe Troubleshooting of Flyback Transformers

Flyback (LOPT) TransformersWhat Does the Flyback (LOPT) Transformer Do?How is a Flyback Transformer Different than a Regular Transformer?The Origin of the Term, 'Flyback'A Little HistoryWhy is the Deflection and High Voltage Combined?Flyback ConstructionWhy You Don't Want to Fabricate Your Own Flyback or Rebuild a Bad One

Flyback Failure and TestingWhy Do Flyback Transformers Fail?How Do Flyback Transformers Fail?

Basic TestingInitial Tests Using Your Senses and Perhaps a MultimeterThe Process of Elimination

Advanced TestingWhen the Basic Tests Don't Reveal AnythingMethod 1Method 2

Identifying the High Voltage Return on a FlybackMethod 2 Testing Procedure

Other Flyback Testing Procedures and Comments

Additional Flyback Testing and Service InformationFlyback Testing EquipmentQuickie In-Circuit Flyback TestsTesting for Bad High Voltage DiodesWhy Do All Flyback Transformers Seem to be Unique?Typical Flyback SchematicReplacement Flyback TransformersCheap flybacks - Beware

Back to Flyback Testing Table of Contents.

Preface

Author and Copyright

Author: Samuel M. Goldwasser

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Copyright ; 1994-2013 All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions aresatisfied:

1.This notice is included in its entirety at the beginning. 2.There is no charge except to cover the costs of copying.

DISCLAIMER

We will not be responsible for damage to equipment, your ego, blown parts, county wide power outages,spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury that mayresult from the use of this material.


Introduction

Scope of This Document

When problems develop in the horizontal deflection/high voltage subsystems of TVs or monitors (or evenmodern oscilloscopes and other CRT displays), the flyback transformer (or line output transformer for thoseon the other side of the Lake) is often a suspected cause. This is due in part to the fact that it is usually themost expensive and hard to find replacement part in the unit and because flybacks are often less wellunderstood than other more common components.

This document addresses the operation and testing of flyback (LOPT) transformers: What they are, howthey fail, why they fail, and how to test them. For more information on horizontal deflection systems, see thedocument: TV and Monitor Deflection Systems.


Safe Troubleshooting of Flyback Transformers

WARNING: Read, understand, and follow the recommendations in the document: Safety Guidelines forHigh Voltage and/or Line Powered Equipment before attempting any TV or monitor repairs.

In particular, before touching or probing the flyback or circuitry in its vicinity:

Unplug the equipment!!!

Measure the voltage on B+ feed to the flyback and discharge the main filter capacitors if necessary.

If going near the HV output, focus, or screen connections, discharge the CRT capacitance as well.

For specific information on safety for your equipment, see the documents: Notes on the Troubleshootingand Repair of Computer and Video Monitors or Notes on the Troubleshooting and Repair of TelevisionSets as appropriate.


Flyback (LOPT) Transformers

What Does the Flyback (LOPT) Transformer Do?

The typical flyback or Line OutPut Transformer (LOPT) consists of two parts (you may also encount theterm IHVT - Integrated High Voltage Transformer):

1. A special transformer which in conjunction with the horizontal output transistor/deflection circuitsboosts the B+ (120 V typical for a TV) of the low voltage power supply to the 20 to 30 kV for theCRT as well as provides various secondary lower voltages for other circuits.

A HV rectifier turns the high voltage pulses into DC and the CRT capacitance smooths it. The HVmay be developed from a single winding with many many turns of wire or a lower voltage windingand a diode-capacitor voltage multiplier.

The various secondary voltages power the logic, tuner, video signal, vertical deflection circuits, andCRT filaments. In fact, with many TV designs, the only power not derived from the flyback is for thekeep-alive circuitry needed to maintain channel memory and provide startup drive to the horizontaldeflection/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 in poor focus, uncontrolled brightness, or fluctuating focus and/orbrightness. A total short could also result in failure of other components like the horizontal outputtransistor. 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 andproblems particularly on damp days.

How is a Flyback Transformer Different than a Regular Transformer?

While the following is not always strictly true for TV and monitor flyback transformers, it is a nice overview:

(From: Sivasankar Chander ([email protected]).)

1. The main difference between a flyback transformer and a regular transformer is that a flybacktransformer 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 tominimize stored energy.

2. 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 timesfor transistors and diodes, need for snubber circuits, etc).

3. The reluctance of the magnetic circuit of a flyback transformer is usually much higher than that of aregular transformer. This is because of a carefully calculated air-gap for storing energy (it's aninductor).

4. 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.

5. The currents flowing through either side of a flyback transformer are either increasing or decreasinglinear sawtooths, whereas a regular transformer usually has sinusoidal currents.

6. Finally, due to the properties of core materials, flyback transformers are most conveniently operatedin the range from 10^3 to 10^6 Hz, whereas regular transformers have a much wider range, from afew Hz to 10^12 Hz.

I may have succeeded in confusing you beyond redemption, so the best recourse for you would be to readany introductory textbook on switching power supplies for a more comprehensive picture.

The Origin of the Term, 'Flyback'

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 theworld, it is either LOPT (Line OutPut Transformer) or simply LOT, or as noted IHVT - Integrated HighVoltage Transformer (which is actually the most accurate term for modern units).

The term 'flyback' probably originated because the high voltage pulse that charges the CRT capacitance isgenerated 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 changesslowly during scan and is abruptly switched in polarity by the HOT turning off during the flyback or retraceperiod.

Many off-line switchmode power supplies and DC-DC converters are also of the 'flyback' type with energytransferred to their output circuits mainly during the same time in the cycle - but there is no CRT involved.Indeed, these high frequency ferrite transformers - which generally look like regular transformers often of E-I core construction - may also be referred to as flybacks in transformer company catalogs.

LOPT and LOT derive from the fact that it is the line scan circuit that is involved and the transformer is inthe 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 picture tube, this was a BIG floormodel TV, and I flew about five feet backwards. I know this isn't the real history for the name but for manyyears I believed it was.

A Little History

So, how far back does the use of a flyback based high voltage go?

(From: Henry van Cleef ([email protected]).)

A flyback HV supply was a feature of the 1946 RCA 630 and GE 801 sets. They used either an 807 or6BG6 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 ofthe 1930's.

Zworykin/Morton "Television" (Wiley, 1940) has schematics and a project home-brew TV set using an 81tube 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 ([email protected]).)

Some of the early TV sets used an RF oscillator to generate the high voltage for electrostatic-deflectionCRTs: a typical tube lineup might include a 6V6 oscillator and 1B3 (or 1X2) rectifier.

Why is the Deflection and High Voltage Combined?

One of the main reasons that TVs and many monitors are designed with horizontal deflection driven flybacksis simply economics - it provides a cheap way to get the high voltage and many or most of the othervoltages for the set with minimal hardware. (High quality computer monitors sometimes use a separate highvoltage supply so that the horizontal deflection is then used just for deflection to reduce interactions betweenchanging scan rates and the HV.) A side benefit is that if the horizontal deflection dies, the HV power supplyvoltage 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 powersupply components to be small and light compared to a line operated power transformer and filtercapacitors.

Flyback Construction

While details can vary somewhat, all flybacks consist of a set of windings on a gapped ferrite core. Highvoltage diodes and resistive dividers (often with adjustment pots) for focus and screen (G2) may also bepresent.

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. For low voltage powered, it may only be a dozen turns of thicker wire. This is whatis 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 sectionswith a high voltage rectifier for each or could be a single winding. An alternative is provide a lowervoltage winding and use a voltage multiplier (diode-capacitor ladder) to boost this to that required bythe CRT. Very fine wire (e.g., AWG #40) will be used for the high voltage winding. The high voltagelead 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 flybacktransformer) voltage multiplier. In this case, the flyback high voltage winding will generate 6 to 10kVAC 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 module in this case.

Resistive divider network for focus and screen (G2). This will probably be fed from only one of theseries connected windings (if used). Often, there are adjustments for focus and screen right on theflyback. The outputs from this divider may be connected to pins in the base of the flyback or havetheir 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 areconnected to solder pins at the base of the flyback.

Ferrite core - consisting of two C shaped pieces clamped together with either a spring arrangementor studs and nuts. There will be a gap of a fraction of a mm provided by a set of spacers between thetwo C sections.

Most modern flybacks have all the windings on the same leg of the core. The drive winding and auxiliarywindings will be wound and separately insulated under the high voltage winding. The high voltage windingwill 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 inan 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 magneticfield of the core during scan as the current is ramping up. Energy is also coupled to certain secondaryoutputs during scan. However, energy for the high voltage (HV) is coupled to the its secondary windingsalmost entirely when the primary current is shut off at the end of the scan (probably the source of the nameflyback 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 theflyback:

_ _ \/ _/\_ 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 bestknown 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 outputvoltages since the rate of collapse of the magnetic field (determined by the design of the horizontal outputcircuit) 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.

Why You Don't Want to Fabricate Your Own Flyback or Rebuild a Bad One

Attempt to 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 thousandturns of nearly invisible wire used in a typical flyback. To sustain the high voltages without arcing and tominimize 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 mylartape. Just breathing on such wire will practically break it let alone wrapping several thousand turns in perfectorder!

The other parts: drive and low voltage windings, focus and screen divider network, and high voltagerectifiers must be assembled with the high voltage winding and CRT leads and then the entire affair is pottedin Epoxy.

Forget it - you have better things to do than spend a week on a transformer!


Flyback Failure and Testing

Why do flyback Transformers Fail?

While flyback transformers can on occasion be blown due to a failure elsewhere in the TV or monitor'spower 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 entireassembly 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, orscratches 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 reducingthe insulation breakdown particularly in the area of the high voltage windings, rectifiers, and focus/screendivider network. They also physically vibrate to some extent. A whole bunch of other factors are also nodoubt 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.

How Do Flyback Transformers Fail?

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 thininsulation so that shorted windings may already have developed. Even if the windings are currently ingood 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 gottenaway with leaving this in place permanently). If possible, moving the point to which the flyback isarcing 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 theclear variety, though others will probably work as well. I have heard of burn through with coronadope. (Author's note: make sure you allow ample time for the silicone sealer to setup completely - orelse 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 maynot work correctly or even blow the horizontal output transistor and other expensive parts like thelow voltage regulator or switchmode power supply. If the core can be reconstructed so that no gaps(other than the required ones where the two halves join) are present and clamped and/or glued inplace, it should be possible to perform testing without undue risk of circuit damage but consider areplacement flyback as a long term solution.

3. Internal shorts in the FOCUS/SCREEN divider network, if present. One sign of this may be arcoverof 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 sometimespossible for failures (1) and (2). For failures (3) to (5) replacement is usually the only alternative.


Basic Testing

Initial Tests Using Your Senses and Perhaps a Multimeter

First, perform a careful visual inspection with power off. Look for cracks, bulging or melted plastic, anddiscoloration. Look for bad solder connections at the pins of the flyback as well. If the TV or monitor canbe powered safely, check for arcing or corona around the flyback and in its vicinity, or at the sparkgaps orgas tube protectors on the CRT neck board.

Next, perform ohmmeter tests for obvious short circuits between windings, much reduced windingresistances, 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 lowenough scale. These are usually a fraction of an ohm. It is difficult or impossible to measure the DCresistance of the HV winding since the rectifiers are usually built in. The value is not published either.

WARNING: Make sure you have the TV or monitor unplugged and confirm that the main filter capacitor isdischarged before touching anything as the flyback is usually connected to this point, perhaps directly! If youare going to remove or touch the CRT HV, focus, or screen wires, discharge the HV first using a wellinsulated 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 simplyindicate that a design change was made. Yes, I know, hard to believe they would not have informed you ofthis! For example, various versions of the flyback used in the Apple MAC Plus - 157-0042A,B,C - arefunctionally similar but have minor variations 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) and sometimes shorts in the focus/screendivider will drastically lower the Q and increase the load the flyback puts on its driving source with nooutputs 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 onthe base of the flyback. This implies a breakdown of the Epoxy potting material probably due to thermallyinduced microcracks or poor quality manufacturing. Once a small arc develops, it rapidly carbonizes thematerial around it further reducing the resistance. These rarely heal themselves and thus show up asobviously low resistance readings using an ohmmeter. It is an easy test and can be performed withoutremoving the flyback. Discharge the CRT HV (though this will probably be dead) and just remove theconnector from the CRT.

It is also possible that various types of flyback faults can damage other circuitry (beyond taking out thehorizontal output transistor and its associated parts). For example, a sudden short between the CRT HVconnector and a low voltage winding or a short between two low voltage windings could conceivably blowsolid state components powered from the flyback. This damage will generally not be apparent until theflyback is replaced. Therefore, if shorts are detected in the flyback, it is worth testing some of thecomponents in the vicinity and vice-versa.

The Process of Elimination

Before attempting the more advanced tests suggested below, there may be ways of being more certain thatyour flyback is the problem component. The following assumes that running the TV or monitor with thesuspect flyback results in an excessive load on the low voltage (B+) power supply blowing a fuse (orattempting to blow a fuse - excessively bright series light bulb). The B+ likely drops from its normal 65VDC to 140 VDC or more (depending on the actual TV or monitor and mode) to some low value like 25VDC 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 thedrive (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 thesecondary loads is indicated. Connect each of these up one a time (or test individual components) tolocalize 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 goodflyback for a similar size TV or similar type of monitor (as appropriate). It may be close enough tokeep 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 coresmay be desirable.


Advanced Testing

When the Basic Tests Don't Reveal Anything

Also see the section: Flyback Testing Equipment.

There are several ways of testing flybacks (assuming you do not actually have special test equipment for thispurpose). Here are two possibilities. The first is easier if you have a scope but the second is more fun.

Method 1

The following technique, called a 'ring test', works for flybacks, chopper transformers, motors, mainstransformers, deflection yoke windings, VCR video and other magnetic heads, and other transformers, coils,or inductors.

However, note that it can miss certain problems like open windings (if they are not used for the test) as wellas shorts or opens that occur only when the flyback is driven near full voltage. Thus, do the basic testsFIRST and don't assume that the flyback is 100 percent good just because it passes the ring test (though thelikelihood of this is very high).

(Portions from: Gabe ([email protected]).)

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) willhave shorted turns in one of the coils. In such a case, the 'Q' of the transformer is greatly reduced. If excitedby an impulse, a faulty transformer will resonate with a highly damped oscillation while a good one willdecay gradually.

1. Connect a high quality capacitor across one winding of the suspect device. Hope for a resonantfrequency of a few kHz. You may need to select the capacitor value for best results. I have foundthat 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 likethose 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 thetimebase that provided a few 10s of volts at enough current for this. A circuit in "Television" magazinea couple of years ago used a BU508, a 12 V power supply, and a small oscillator built from a 4011chip. A function generator or a 555 timer based circuit will also make a satisfactory stimulous. Alsosee the section: Flyback Testing Equipment.

3. Look at the waveform across the resonant circuit with a 'scope. A good unit will give a nicelydecaying 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 will tell you what toexpect.

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 a Tektronix scope. It workedbest when I used a series 200 pF capacitor. I got maybe 100 pulses before it decayed to zero. If I shortedtwo 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 halfpower bandwidth. I applied an audio generator through a 22k resistor, found the peak frequency, then wentoff that frequency to .707 of that amplitude. Double this would be the bandwidth. I got Q's of 26 and 16 fortwo I tried. (Editor's note: This appears to be a valid approach.)

Method 2

The circuit below excites the flyback in much the same way as in normal operation. The only caution is thatthis tester probably does not put enough stress on the flyback to find an intermittent that fails only under fulloperating conditions. However, most flyback failures are solid - once a short develops, there is a meltdownof 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 justuse 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 ):: | +------| 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 feedbackconnections to the transistor bases.

The tester is just a chopper feeding the salvaged core from an old flyback (I removed the inductance controlspacers 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 - donot bring out a loop of wire. Make sure all the turns of each coil are wound in the same direction. Wind thefeedback coil directly on top of the drive coil. The secondary of this core is a 10 turn well insulated coilsimilar 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 thesuspect flyback core anywhere it will fit. Connect one end of this coil to one end of the 10 turn coil on yourold flyback core. Use a wire nut or twist together securely. Provide an easy way of connecting the otherends momentarily - a pushbutton 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.

Identifying the High Voltage Return on a Flyback

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 toanything else. Therefore, if you test between all combinations of pins on the flyback (removed fromthe circuit board) and find a pin that appears open to all other pins but is connected to a pad on thecircuit 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 flybackpins 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 powersupply 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 thevoltage reading dropping to perhaps 50 or 60 volts. This is the forward voltage drop across the highvoltage rectifier stack inside the flyback. All other pins will result in it remaining at the supply voltage(except for the ground connection to the F/G2 divider if it is separate - then it may drop a fraction ofa volt). Note that if you cannot locate the HV return, your flyback may indeed be defective; it mayhave an internal bad connection, open HV rectifier, or burnt out HV winding. Or, if other pins dropthe voltage, you may have already found shorts in the flyback!

Method 2 Testing Procedure

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 tocreate 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 oscillatorwill increase dramatically.

If you get arcing or corona from *under* the flyback - at the pins - either did not locate the correctHV 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 tobelieve it and gotten screwed).

Other Flyback Testing Procedures and Comments

Here is a Web site with some notes on flyback testing procesures:

Noahtech Testing Flyback Transformers - How To Test And When To Replace it

They have other useful information related to monitor repair as well as well as many links back here!

(From: Terry ([email protected]).)

I first check for HOT shorts, secondary supply overloads, and everything else, disconnecting the flybackwindings to any suspect circuits as I go. So, if I get to the following test, pretty much all connections to theflyback are now open anyway. Next, I perform "The Loop Test":

1. Ring the transformer at the HOT primary winding. You don't need fancy signal generators to inject asignal. Even the 60 Hz AC secondary from a 6 volt filament transformer will work, although you dateyourself if you have one in your shop. (Note: I expect that the 6 VAC transformer works in this casebecause the core of the flyback saturates pretty quickly at this low frequency resulting in sharp edgesto produce the ring pulse - I would rather use a 555 timer or pulse generator --- Sam.)

2. Scope any flyback secondary for the little ringing pulse. Observe the number of cycles in the dampedpulse.

3. Run a small piece of wire around one post of the ferrite core and short the two ends of the wiretogether (loop).

4. If the number of damped cycles doesn't reduce dramatically then the transformer already has a*single turn short* or *worse*. It's bad. There is little change because Q is already greatly reducedby the existing short in the failed flyback, even one turn. If it does reduce dramatically, then you'vejust strongly affected Q by introducing your own single turn short, so the flyback is OK. What is"dramatically"? Try out a few known good and bad ones and find out for yourself. :)

This *should* work in-circuit but any defective (heavy load, etc) circuit on any flyback lead will reduce Q,so you have to eliminate these other possibles anyway. It is my experience that the flyback almost alwaysleaves physical evidence of its demise. If I don't see it, I check everything else before I try this loop test. Irarely have to use it.

I just love it when the $encore guys call to tell me I need $2000 worth of test equipment to reliably testhorizontal circuits. When I tell 'em how I do it, they're pretty much speechless. Some are fascinated. Thoseare the ones who should switch from sales to tech.

(From: Wild Bill ([email protected]).)

There are numerous instruments which will check certain flyback/IHVT parameters, and not others.Thorough testing can only be accomplished with several instruments. As far as I know, there is no singleinstrument which will test all parameters.

Testing for internal faults includes continuity, shorts, shorted turns, winding-to-winding and winding to coreleakage, the HV rectifier (multiplier) stack, focus-screen divider (and internal spark gap), and a drive pulseinput - relative proportional output test. And after all of the above tests, the device might still break down atthe actual circuit working voltages/temperatures.

The minimum tests should include ohms, leakage, and ringing. An open in the HV winding can't be detectedwith an ohmmeter if the xfmr contains a HV rectifier stack. as the ohmmeter won't provide the necessaryvoltage to bias the rectifiers. A well designed (fairly inexpensive) leakage tester can provide the necessaryvoltage to check this.

(From: Jurb6005 ([email protected]).)

I test flybacks by clipleading a beefy old TO3 horizontal output transistor into the circuit. This tests it at theactual operating voltage and will show all faults. Believe it or not, this also works on sets that use a GCS(Gate Control Switch, GTO SCR?) like the 2SG264 and 613. If you use it on one of these sets it may gethot, but it will run long enough to test things. (Even a 'beefy old HOT' may not survive certain faults. ---Sam.)

Also, on sets that use a linear regulator (not a switchmode power supply or regulator) there is usually aballast resistor. If you simply leave the shorted regulator disconnected, it will run through the ballast andviola! You can non-destructively test the circuit.

These methods are especially good if you are writing the estimate, you need not solder anything in!


Additional Flyback Testing and Service Information

Flyback Testing Equipment

Sencore and others sell test equipment that includes the 'ring test' or similar capabilities built in. For theprofessional, these are well worth the expense.

However, the hobbyist could probably purchase lifetime TV replacements for the cost of once of thesefancy gadgets.

Bob Parker (of ESR Meter fame) has now designed an inexpensive, easy to use LOPT/Flyback Testeravaiilable through Dick Smith Electronics. Information is available at:

Bob Parker's FBT Page

Various electronics magazines have published construction articles for various types of simplified versions of

these devices. Here is a pointer to one such article:

The "Think Tank" column of Popular Electronics, December, 1998, provides information on a unit fortesting inductors and transformers (including flybacks) which displays characteristics on an oscilloscope.

(Portions from: Tony Duell ([email protected]).)

The February 1998 issue of 'Television' magazine, has a simple circuit for an LOPT (Line OutputTransformer - flyback transformer) tester.

It uses a TBA920 chip as an oscillator, driving a BUT11AF which supplies the primary of the LOPT. Thevoltage developed across this winding (the back EMF when the transistor is turned off) is shown on aDMM. There's also a 'scope point to look at the waveform produced.

Another chip or an oscillator constructed from discrete transistors can be substituted for the TBA920.Some possibilities: 555 timer or MC1391, or a multivibrator can be built from 2N3904s.

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 transformeritself.

3. There is a capacitor shown from pin 13 (decoupling) which almost certainly should be a bypass toground, not to the collector of the drive transistor.

Quickie In-Circuit Flyback Tests

Note: Larry has 'beta tested' Bob Parker's (of ESR meter fame) flyback tester described at:http://www.ozemail.com.au/~bobpar/fbt.htm.

(From: Larry Sabo ([email protected]).)

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 HOTwith a DVM for shorts. No shorts? Unsolder all flyback legs except the primary winding and check for ringsagain. 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 aboutright. And an LC102 (tester) won't catch these either :-). I've found that about half of these show up with alow resistance measurement between the EHT cap and ground.

Sometimes scoping the output at the EHT cap shows unrectified ringing but stray capacitance probablyaccounts for that. Other times, it's clearly rectified, so go figure. As a last resort, I resort to Sam's chopperto wrestle the hold-outs to the ground, but it takes a bit of time to remove the flyback and put 10-15 turnsaround the core. The ringer has also helped me isolate a defective yoke, which explained why thingswouldn'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 keepthe prototype! :-)

(From: John Robertson ([email protected]).)

I use an audio signal generator set it to about 15 kHz and a scope or AC voltmeter on one of the outputwindings.

Connect the generator to the leads that the horizontal output transistor and ground use (out of circuit, useHOT and B+ leads --- sam). You should see on the scope a reasonably nice waveform similar to the input.If you are using a voltmeter, you should get approximatly the correct ratio output voltage relative to theoriginal voltages. So if your generator puts out 10 VAC and the original HV input levels were 100 VDC,then the voltage levels should be about 1/10th of the original. I do this in-circuit, and try to get a squarewave as the source, but the theory is consistant.

(From: Quick Fix ([email protected]).)

If you don't don't repair that many TVs, the cheapest way to check a FBT is to connect its primary windingin series with the yoke (low side) of a working set. If the picture shrinks a few inches on both sides evenlyand with no ringing or jail bars, your FBT is good. You can even measure the high voltage on your FBTwith this method.

Testing for Bad High Voltage Diodes

A single diode failure would be tough to find if it is in series with other diodes (as is typical on largerflybacks) as it would only be a problem when run near full output. However, this sort of failure is unlikely.

General diode failure (shorts) would probably not be detected with the sorts of tests described above orwith typical flyback testing equipment. Actually, a simple ohmmeter test between the HV output and returnmight suffice! If this doesn't reveal anything, I suggest the following:

One possible way to test for this would be to attach a high voltage capacitor between the HV output andreturn of the flyback. If the diodes are good, the tester's excitation should then charge this cap up (watch out- the voltage might get to be quite high!). While charging, this load will make the flyback fail any ring test.Once charged, it should pass. However, if the diodes are shorted, I would expect the flyback to test bad asthe cap will continue to present an AC load on the output and never charge properly.

I haven't tried this, however, so no guarantees.

(From: dB King (([email protected]).)

Sencore Z-Meters are capable of applying sufficient bias to check those diodes for forward conduction andreverse leakage. Forward conduction should be confirmed first to rule out an open -- almost all multimeterswill always show open HV diodes due to their limited voltage output.

Indispensable for capacitor tests as well. I dunno how I got by w/o mine! They also have built-inyoke/flyback ringer. :)

Quite expensive. You might wanna try to find a used one.

Why Do All Flyback Transformers Seem to be Unique?

(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 muchdue to the high voltage winding and/or HV multipler but rather related to its usual function as the source ofmultiple 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 becompatible.

ECG and similar companies do have a line of generic FBTs and should have a catalog/cross reference forthese 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 theflyback windings gives them complete freedom to pick the number and types of secondary voltages! Yourchances of picking up something off the street so-to-speak and expecting it to fit anything you have everowned - 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 coilswhich would otherwise require a >1500 V HOT) and influence the tuning of the EHT secondary, todetermine the EHT internal impedance. And finally you might specify a built-in EHT capacitor or bleederresistor and various types of clicked-on potmeter modules (perhaps with a second focus voltage for DAF).

Typical Flyback Schematic

This diagram shows a typical flyback that might be found in a direct view color television or computermonitor. 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 singlewinding with multiple HV diodes. The rectifiers or multiplier, and/or focus/screen divider may be external tothe flyback transformer in some models.

Flyback transformers used in black-and-white TVs and monochrome computer monitors do not have afocus 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 plasticspacers or pieces of tape. Don't lose them 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 10sto 100s of M ohms so they may not register at all on your multimeter. The high voltage rectifiers (CR1 toCR3 on this diagram) are composed of many silicon diodes in series and will read open on a typical VOMor 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-------------+ ::( | ):: +-------+ | 4 .11 ):: | | 35 o-------------+ :: +--|>|--+ Various | )::( CR3 | auxiliary < .28 )::( / windings | 5 )::( \

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.

And, here's one for your semi-antique (1950s) needs:

Linear Electronics, Walthan, MA. Phone: 1-617-894-7300, Fax: 1-617-894-8890. (Theyalso have vacuum tubes.)

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 theirreplacement based on TV or monitor model and/or the part or house number on the flyback:

NTE (NTE Electronics, Inc)ECG (Philips) (now same as NTE)HR (HR Diemen)ASTI (ASTI Magnetics)Flybacktransformer

However, these may be of lower quality or not be quite compatible with your original. In an effort tominimize the number of distinct flyback models, some corners may be cut and one-size-fits-many maybe 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 therewill 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 isessential 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 someother points in the circuit. If this is not done, operation may not be correct or other parts mayblow 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, ormismarking. Internal circuitry such as the focus and screen(G2) divider could be improperlywired, configured for a different model, or omitted entirely. Such defects can be very tough toidentify.

Thus, marginal or erratic behavior might result from generic replacements greatly complicating yourtroubleshooting since without careful measurements there is no way of knowing whether the problemis due to the new flyback or a fault that still exists elsewhere. For some actual experiences, see thesection: Cheap Flybacks - Beware.

(From: [email protected].)

HRdiemen is a manufacturer of replacement line output transformers and have several thousand typesavailable. But the nicest thing is their online database where they have pinout and internal schematicsincluding typical voltages of every of these transformers. Just type your original letters/numbers into thesearch box, then you get the replacement transformer type and a link to its internal construction.

Very helpful if you want to "recycle" an old transformer for a new circuit.

Javascript must be enabled for this to all happen automagically. If you prefer to work without javascript (likeme, to avoid ad banners and other doubtful background activities), you can also directly accesshttp://www.hrdiemen.es/data/esquemas/HR7491.gif or whatever number your transformer has.

Cheap flybacks - Beware

There have been reports of inexpensive replacement flybacks resulting in a variety of strange symptoms. Ido not know how likely it is to have problems with these. In most cases, I would expect the replacement todrop 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 checkingbefore buying if possible.

Here are several examples of incompatibility problems:

(From: Petercoe ([email protected]).)

There is some good and some bad to these flybacks. One thing I noticed is that the competition has causedthe price of the name brands to drop.

However, these flybacks may not work right in all cases. I know I had to modify a circuit in a Sony to getthe set to work right after using a low priced replacement. I also had a Goldstar which would only workwith the original flyback."

(From: Michael Caplan ([email protected]).)

The FBTs that I tried (three samples in two generic brands available here in Canada) all seem to be missingthe required internal voltage divider. This was confirmed by comparison with a new oem Sony part. TheOEM part exhibits the proper resistance measurement. It is through this resistance that the Hold Downvoltage is derived. "No resistance = no Hold Down voltage", as far as I can see."

(From: Dave Moore ([email protected]).)

I recently put a cheapo sub flyback Hitachi P/N 243384 in a Hitachi model CT2647 26" tv.

Apart from inadequate horizontal deflection, the TV exhibited ringing like jail bar shadows on the left side ofthe screen and a bright area with retrace lines showing from top to bottom down the middle of the of thescreen. At first I thought that this was the classic bad B+ filter to the crt board phenomena. But nope, filtergood.

So I figured that it had to be a bad filter in the brightness limiter or to the video circuits. A quick round aboutwith my trusty DS ESR meter did sniff out a weak cap in what seemed to be the brightness limiter circuit.

Did this cure the problem? As John Belushi would say: Nooooooooooooo. Well I recalled a similar

problem that I had encountered while experimenting with the screen voltage on a Zenith TV. At one point Ihad removed a small disk capacitor on the screen supply trying to unload the shorted screen supply. Wellthe problem turned out to be the picture tube and after I cleared the screen short I noticed a similarphenomena to the one that I was presently experiencing (the bright area in the middle of the screen from topto bottom with retrace lines).

Well after I reconnected the little disk capacitor the problem went away. So!! I thinks to myself: The littledisk cap on this Hitachi I'm working on must be bad. Well lo and behold - there is no disk cap on thehitachi. No place for one either. It was designed to not need one apparently.

So I put a .01 uf 1400 volt disk cap (cause it was handy) on the screen and voila! End of jail bars andretrace lines in middle of screen. I can only assume that the cheapie flyback was the cause. This makes mewonder if the lack of horizontal width might be symptomatic of this "cheapie" flyback also. I'll probably justparallel some capacitance on the HOT since I don't have much width to make up. I already adjusted B+ tothe High Voltage section and at full clockwise position of the control the picture doesn't open up all the way.I left the control at mid position and played the set for a couple of hours. Everything's running cool.

(From: Gregory Danner ([email protected]).)

As far as "generic" flybacks, be prepared to do some slight adapting as far as mechanical installation.Sometimes new ones aren't the same diameter and height, and don't fit with existing metal support brackets,which may have to be cut away or bent. Sometimes the pins that go through the circuit board aren't alignedquite right, and may have to be bent and adjusted slightly to fit the board. Screen and focus controls maynot be in the same physical position on the new flyback. But, overall, I would say that most of the genericflybacks I've used have worked out OK electronically.


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Documents

Testing of Flyback (LOPT) Transformers