Testing of Flyback (LOPT) Transformers

Version 1.58 (19-Nov-06)

Copyright ©; 1994-2013
Samuel M. Goldwasser
--- All Rights Reserved ---

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Table of Contents



  • 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 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.

    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 may result from the use of this material.



  • Back to Flyback Testing Table of Contents.

    Introduction

    Scope of This Document

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

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



  • Back to Flyback Testing Table of Contents.

    Safe Troubleshooting of Flyback Transformers

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

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

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



  • Back to Flyback Testing Table of Contents.

    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 the term IHVT - Integrated High Voltage Transformer):
    1. A special transformer which in conjunction with the horizontal output transistor/deflection circuits boosts the B+ (120 V typical for a TV) of the low voltage power supply to the 20 to 30 kV for the CRT as well as 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 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 in 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.

    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 (siva@bond.bocaraton.ibm.com).)

    1. The main difference between a flyback transformer and a regular transformer is that a flyback transformer is designed to store energy in its magnetic circuit, i.e., it functions like a pure inductor, whereas a regular transformer is designed to transfer energy from its primary to secondary and to minimize stored energy.

    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 times for 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 a regular transformer. This is because of a carefully calculated air-gap for storing energy (it's an inductor).

    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 decreasing linear sawtooths, whereas a regular transformer usually has sinusoidal currents.

    6. Finally, due to the properties of core materials, flyback transformers are most conveniently operated in the range from 10^3 to 10^6 Hz, whereas regular transformers have a much wider range, from a few Hz to 10^12 Hz.
    I may have succeeded in confusing you beyond redemption, so the best recourse for you would be to read any introductory textbook on switching power supplies for a more comprehensive picture.

    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 the world, it is either LOPT (Line OutPut Transformer) or simply LOT, or as noted IHVT - Integrated High Voltage 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 is generated by the collapse of the magnetic field in the core of the transformer during the short retrace period - when the electron beam in the CRT 'flies back' to the start of a new scan line. The flux in the core changes slowly during scan and is abruptly switched in polarity by the HOT turning off during the flyback or retrace period.

    Many off-line switchmode power supplies and DC-DC converters are also of the 'flyback' type with energy transferred to their output circuits mainly during the same time in the cycle - but there is no CRT involved. 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 in the output stage.

    I still think flyback is much more quaint! :-).

    Of course, others have their own definition:

    (From: Sam Riner (riner@inet2000.com).)

    When I was about 12 I touched the wire coming from the FBT on the picture tube, this was a BIG floor model TV, and I flew about five feet backwards. I know this isn't the real history for the name but for many years I believed it was.

    A Little History

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

    (From: Henry van Cleef (vancleef@netcom.com).)

    A flyback HV supply was a feature of the 1946 RCA 630 and GE 801 sets. They used either an 807 or 6BG6 horizontal output tube, 6W4 damper, 1B3 rectifier.

    The prewar TV's (yes, TV's were made and for sale before the NTSC standard was approved in 1941) generally used a 60 Hz. transformer and 2X2 similar to circuits used in RCA and Dumont oscilloscopes of the 1930's.

    Zworykin/Morton "Television" (Wiley, 1940) has schematics and a project home-brew TV set using an 81 tube for the HV off a standard power transformer. Of course, to follow your way around this book, you have to know vacuum tube theory and a lot of physics reasonably well, but it is an historical gold mine.

    (From: Brad Thompson (Brad_Thompson@pop.valley.net).)

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

    Why is the Deflection and High Voltage Combined?

    One of the main reasons that TVs and many monitors are designed with horizontal deflection driven flybacks is simply economics - it provides a cheap way to get the high voltage and many or most of the other voltages for the set with minimal hardware. (High quality computer monitors sometimes use a separate high voltage supply so that the horizontal deflection is then used just for deflection to reduce interactions between changing scan rates and the HV.) A side benefit is that if the horizontal deflection dies, the HV power supply voltage goe with it and prevents the CRT phosphors from burning do to undeflected high intensity beam.

    The use of the horizontal frequency rather than the AC line frequency of 50 or 60 Hz allows the power supply components to be small and light compared to a line operated power transformer and filter capacitors.

    Flyback Construction

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

    A typical flyback includes the following components:

    Most modern flybacks have all the windings on the same leg of the core. The drive winding and auxiliary windings will be wound and separately insulated under the high voltage winding. The high voltage winding will consist of many layers which have insulating material (i.e., mylar) between them.

    The other components will be mounted in a separate part of the assembly and the entire unit is then potted in an Epoxy type filler. Part of the core is generally accessible - often one entire leg.

    A flyback is not an ordinary transformer. The ferrite core contains a gap. Energy is stored in the magnetic field of the core during scan as the current is ramping up. Energy is also coupled to certain secondary outputs during scan. However, energy for the high voltage (HV) is coupled to the its secondary windings almost entirely when the primary current is shut off at the end of the scan (probably the source of the name flyback because it is during the retrace of the electron beam).

    Which type of coupling is in effect depends on the direction of the rectifiers on the secondary side of the flyback:

    
                      _  _
                       \/                                   _/\_
       B+ ------+    +----|>|-----+---o +V1  B+ ------+    +----|>|-----+---o +HV
               o )::( o  Scan     |                  o )::(   Flyback   |  
                 )::(  Rectifier _|_                   )::(  Rectifier _|_ 
                 )::(            ---                   )::(            ---
                 )::(             |                    )::(             |
           _/\_  )::(             |              _/\_  )::( o           |
      HOT ------+    +------------+         HOT ------+    +------------+
                                 _|_                                   _|_
                                  -                                     -
    
    
    Here, V1 is just a typical example of an auxiliary supply derived from a scan rectifier and HV is the best known example of the use of a flyback rectifier.

    Note that the ratio of the number of turns for each winding *cannot* be used to calculate expected output voltages since the rate of collapse of the magnetic field (determined by the design of the horizontal output circuit) affects this.

    The gap is critical to the proper operation and is usually determined by some plastic spacers. CAUTION: mark each one and replace them in exactly the same position if you disassemble the core for any reason.

    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 thousand turns of nearly invisible wire used in a typical flyback. To sustain the high voltages without arcing and to minimize the interwinding capacitance, the high voltage winding is constructed as many individual layers - perhaps 50 layers in all - of 50 turns each using super fine wire (#40 typical - thinner than a human hair). Each layer must be wound perfectly flat with all wires side-by-side and then individually insulated with mylar tape. Just breathing on such wire will practically break it let alone wrapping several thousand turns in perfect order!

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

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



  • Back to Flyback Testing Table of Contents.

    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's power supply or deflection circuits, in most cases, they simply expire on their own. Why?

    Flybacks are wound with many layers of really really fine wire with really really thin insulation. This entire assembly is potted with an Epoxy resin which is poured in and allowed to cure.

    In some ways, these are just short circuits waiting to happen.

    Flybacks get hot during use and this leads to deterioration of the insulation. Any imperfections, nicks, or scratches in the insulation or trapped air bubbles and impurities in the Epoxy fill material contribute to failure. Temperature cycles and manufacturing defects result in fine cracks in the Epoxy potting material reducing the insulation breakdown particularly in the area of the high voltage windings, rectifiers, and focus/screen divider network. They also physically vibrate to some extent. A whole bunch of other factors are also no doubt important.

    Once a breakdown - sparking or arcing - develops, it is usually terminal.

    It is amazing they last as long as they do with the stresses they are under.

    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 thin insulation so that shorted windings may already have developed. Even if the windings are currently in good condition, long term reliability of any such repairs is questionable.

      Nonetheless, it doesn't hurt to try cleaning and coating with multiple layers of high voltage sealer, corona dope, or even plastic electrical tape (preferably as a temporary repair though I have gotten away with leaving this in place permanently). If possible, moving the point to which the flyback is arcing further away (i.e., a piece of metal or another wire) would also help.

      (The following from: Tom Riggs (thriggs@mail.netusa1.net))

      For sealing flyback transformers, I have found that silicone sealer has worked very well. I used the clear variety, though others will probably work as well. I have heard of burn through with corona dope. (Author's note: make sure you allow ample time for the silicone sealer to setup completely - or else it will breakdown instantly - at least 24 hours. Also, some types (those that smell like vineger - acetic acid - as they cure may result in corroded wiring in the long term).

    2. Cracked or otherwise damaged core will effect the flyback characteristics to the point where it may not work correctly or even blow the horizontal output transistor and other expensive parts like the low 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 in place, it should be possible to perform testing without undue risk of circuit damage but consider a replacement flyback as a long term solution.

    3. Internal shorts in the FOCUS/SCREEN divider network, if present. One sign of this may be arcover of the FOCUS or SCREEN spark gaps on the PCB on the neck of the CRT.

    4. Internal short circuits in the windings.

    5. Open windings.
    More than one of these may apply in any given case. As noted, temporary repair, at least, is sometimes possible for failures (1) and (2). For failures (3) to (5) replacement is usually the only alternative.



  • Back to Flyback Testing Table of Contents.

    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, and discoloration. Look for bad solder connections at the pins of the flyback as well. If the TV or monitor can be powered safely, check for arcing or corona around the flyback and in its vicinity, or at the sparkgaps or gas tube protectors on the CRT neck board.

    Next, perform ohmmeter tests for obvious short circuits between windings, much reduced winding resistances, and open windings. Don't neglect to check between the CRT HV connector (suction cup) and the pins on the base. This should measure infinity.

    For the low voltage windings, service manuals may provide the expected DC resistance (Sams' Photofact, for example). Sometimes, this will change enough to be detected - if you have an ohmmeter with a low enough scale. These are usually a fraction of an ohm. It is difficult or impossible to measure the DC resistance of the HV winding since the rectifiers are usually built in. The value is not published either.

    WARNING: Make sure you have the TV or monitor unplugged and confirm that the main filter capacitor is discharged before touching anything as the flyback is usually connected to this point, perhaps directly! If you are going to remove or touch the CRT HV, focus, or screen wires, discharge the HV first using a well insulated high value resistor (e.g., several M ohms, 5 W) to the CRT ground strap (NOT signal ground).

    Measurements that are much less than the published values likely indicate a partially shorted winding. However, a difference of 10% may not be at all significant. Higher than normal readings might simply indicate that a design change was made. Yes, I know, hard to believe they would not have informed you of this! For example, various versions of the flyback used in the Apple MAC Plus - 157-0042A,B,C - are functionally similar but have minor variations in winding parameters. It is not known what effects this would have but they are interchangeable at least for testing.

    Of course, any continuity between separate windings is definitely a fault.

    Partially short circuited windings (perhaps, just a couple of turns) and sometimes shorts in the focus/screen divider will drastically lower the Q and increase the load the flyback puts on its driving source with no outputs connected. It is these types of failures, not detectable by simple ohmmeter tests or visual inspection, which the techniques described in the sections under "Advanced testing" address.

    While less common, I have seen shorts between the CRT HV connector and the low voltage windings on the base of the flyback. This implies a breakdown of the Epoxy potting material probably due to thermally induced microcracks or poor quality manufacturing. Once a small arc develops, it rapidly carbonizes the material around it further reducing the resistance. These rarely heal themselves and thus show up as obviously low resistance readings using an ohmmeter. It is an easy test and can be performed without removing the flyback. Discharge the CRT HV (though this will probably be dead) and just remove the connector from the CRT.

    It is also possible that various types of flyback faults can damage other circuitry (beyond taking out the horizontal output transistor and its associated parts). For example, a sudden short between the CRT HV connector and a low voltage winding or a short between two low voltage windings could conceivably blow solid state components powered from the flyback. This damage will generally not be apparent until the flyback is replaced. Therefore, if shorts are detected in the flyback, it is worth testing some of the components 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 that your flyback is the problem component. The following assumes that running the TV or monitor with the suspect flyback results in an excessive load on the low voltage (B+) power supply blowing a fuse (or attempting to blow a fuse - excessively bright series light bulb). The B+ likely drops from its normal 65 VDC to 140 VDC or more (depending on the actual TV or monitor and mode) to some low value like 25 VDC when measured on the low voltage power supply side of the flyback drive winding. (Measuring at the HOT can result in all sorts of weird readings due to the pulse nature of the waveform and is not recommended - especially when everything is working properly - 1,500 V pulses!).



  • Back to Flyback Testing Table of Contents.

    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 this purpose). 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, mains transformers, 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 well as shorts or opens that occur only when the flyback is driven near full voltage. Thus, do the basic tests FIRST and don't assume that the flyback is 100 percent good just because it passes the ring test (though the likelihood of this is very high).

    (Portions from: Gabe (ggabe@mcs.com).)

    This is called a 'ring test' and is the method often used by commercial flyback (or other coil/transformer) testers. The theory is that a faulty flyback (which cannot be found by simple resistance measurements) will have shorted turns in one of the coils. In such a case, the 'Q' of the transformer is greatly reduced. If excited by an impulse, a faulty transformer will resonate with a highly damped oscillation while a good one will decay gradually.

    1. Connect a high quality capacitor across one winding of the suspect device. Hope for a resonant frequency of a few kHz. You may need to select the capacitor value for best results. I have found that a capacitor in the .001 uF to 1 uF (non-polarized) will usually be satisfactory.

      Note that it doesn't matter whether the excitation is applied to the shorted winding or any other one. However, you should avoid trying to connect the generator to one of the very small windings like those for the CRT filament which may only have 2 or 3 turns.

    2. Apply a pulse waveform to the parallel resonant circuit. In 1960, most scopes had a 'sync out' on the timebase that provided a few 10s of volts at enough current for this. A circuit in "Television" magazine a couple of years ago used a BU508, a 12 V power supply, and a small oscillator built from a 4011 chip. A function generator or a 555 timer based circuit will also make a satisfactory stimulous. Also see the section: Flyback Testing Equipment.

    3. Look at the waveform across the resonant circuit with a 'scope. A good unit will give a nicely decaying oscillation, of at least a few cycles, possibly 10's of cycles. If there is a shorted turn *anywhere* in the device, the oscillations will be seriously damped, and you'd be lucky to see 2 complete cycles. Experience and/or comparison with a known good device will tell you what to expect.
                                             Scope
                             _                 o
                Pulse or   _| |_               |      Device under Test
                function o---------------------+-----------+    +---
               Generator                       |            )::(
                                High Quality  _|_           )::(     All other
                                Non-polarized ---           ):: +--- windings
                                    Capacitor  |            ):: +--- left open
                                               |            )::(
                  Ground o---------------------+-----------+    +---
      
      
    (From: James Elliott (jelliott@stlnet.com).)

    I tried the Q evaluation method using the 100 volt CAL voltage pulse from a Tektronix scope. It worked best when I used a series 200 pF capacitor. I got maybe 100 pulses before it decayed to zero. If I shorted two of the primary pins, the decaying pulse train went to zero almost immediately. So it works!

    I thought of another method. The Q of a resonant circuit is equal to the center frequency divided by the half power bandwidth. I applied an audio generator through a 22k resistor, found the peak frequency, then went off that frequency to .707 of that amplitude. Double this would be the bandwidth. I got Q's of 26 and 16 for two I tried. (Editor's note: This appears to be a valid approach.)

    Method 2

    The circuit below excites the flyback in much the same way as in normal operation. The only caution is that this tester probably does not put enough stress on the flyback to find an intermittent that fails only under full operating conditions. However, most flyback failures are solid - once a short develops, there is a meltdown of sorts and it is there to stay.

    You will require a 12 V power source of at least 2 or 3 amps capacity (regulation is not important - I just use a simple transformer, rectifier, filter capacitor type of power supply).

    The circuit is shown below. None of the component values are critical.

    
        +12     Q1   +----------------+
         o           |                 )::
         |       B |/ C                )::         <-- Flyback Under Test -->
         |  +------|    2N3055         )::
         |  |      |\ E            5T  )::            +------|>|----------o +HV
         |  |        |                 )::         ::(   HV Diode(s),
         |  |       -_-                )::         ::(   usually built in.
         |  |                          ):: +-----+ ::(      
         +--|-------------------------+ ::(       )::(
         |  |   Q2  _-_                )::(  10T  )::(
         |  |        |                 )::(  each )::(
         |  |    B |/ E            5T  )::(   _   )::(
         |  |  +---|    2N3055         )::(  _|_  )::(
         |  |  |   |\ C                ):: +-- --+ ::(
         |  |  |     |                 )::  Switch ::(
         |  |  |     +----------------+ ::         ::(
         |  |  |                        ::         ::(
         |  |  -----------------------+ ::            +------------------o -HV
         |  |                      2T  )::
         |  |             +-----------+ :: (Numerous other windings not shown.)
         |  |             |        2T  )::
         |  +-------------------------+     Note: :: denotes ferrite core.
         |                |
         |          R1    |    R2
         +--------/\/\/\--+--/\/\/\---+
                    110        27    _|_
                    2W         5W     -
    
    
    Note: if the circuit does not start oscillating at about 5 volts or less, interchange the two feedback connections to the transistor bases.

    The tester is just a chopper feeding the salvaged core from an old flyback (I removed the inductance control spacers for this core). The drive (5T+5T) and feedback (2T+2T) coils can be wound from hookup wire (#14-#20) and well insulated with plastic electrical tape. Connect the center taps directly to the coils - do not bring out a loop of wire. Make sure all the turns of each coil are wound in the same direction. Wind the feedback coil directly on top of the drive coil. The secondary of this core is a 10 turn well insulated coil similar to the other two wound on the opposite side of the ferrite core.

    You will need to remove the suspect flyback from the TV or monitor. Another 10 turn coil is wound on the suspect flyback core anywhere it will fit. Connect one end of this coil to one end of the 10 turn coil on your old flyback core. Use a wire nut or twist together securely. Provide an easy way of connecting the other ends momentarily - a 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:

    Method 2 Testing Procedure

    Once everything is wired and double checked, turn on the juice. If there are any shorted windings, then there will be no significant HV output and the load on the oscillator will increase dramatically.

    I have used this 'tester' on a dozen or so flybacks. It has never been wrong (though I have opted not to believe it and gotten screwed).

    Other Flyback Testing Procedures and Comments

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

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

    (From: Terry (terwes1@juno.com).)

    I first check for HOT shorts, secondary supply overloads, and everything else, disconnecting the flyback windings to any suspect circuits as I go. So, if I get to the following test, pretty much all connections to the flyback 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 a signal. Even the 60 Hz AC secondary from a 6 volt filament transformer will work, although you date yourself if you have one in your shop. (Note: I expect that the 6 VAC transformer works in this case because the core of the flyback saturates pretty quickly at this low frequency resulting in sharp edges to 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 damped pulse.

    3. Run a small piece of wire around one post of the ferrite core and short the two ends of the wire together (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 reduced by the existing short in the failed flyback, even one turn. If it does reduce dramatically, then you've just 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 always leaves physical evidence of its demise. If I don't see it, I check everything else before I try this loop test. I rarely 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 test horizontal circuits. When I tell 'em how I do it, they're pretty much speechless. Some are fascinated. Those are the ones who should switch from sales to tech.

    (From: Wild Bill (kwag98@tcis.net).)

    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 single instrument which will test all parameters.

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

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

    (From: Jurb6005 (jurb6005@aol.com).)

    I test flybacks by clipleading a beefy old TO3 horizontal output transistor into the circuit. This tests it at the actual 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 get hot, 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 a ballast resistor. If you simply leave the shorted regulator disconnected, it will run through the ballast and viola! You can non-destructively test the circuit.

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



  • Back to Flyback Testing Table of Contents.

    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 the professional, these are well worth the expense.

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

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

    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 for testing inductors and transformers (including flybacks) which displays characteristics on an oscilloscope.

    (Portions from: Tony Duell (ard@p850ug1.demon.co.uk).)

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

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

    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 transformer itself.

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

    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 (sabo@storm.ca).)

    Checking out flybacks can be frustrating and very time consuming without a good tester.

    Now, it just takes me a second to check for ringing on the HOT collector. No ringing? Check the HOT with a DVM for shorts. No shorts? Unsolder all flyback legs except the primary winding and check for rings again. No rings? Shorted turns in the flyback!

    Bob's estimate that 20% of faulty flybacks have internal leakage or arcing, or bad HV diodes, seems about right. And an LC102 (tester) won't catch these either :-). I've found that about half of these show up with a low resistance measurement between the EHT cap and ground.

    Sometimes scoping the output at the EHT cap shows unrectified ringing but stray capacitance probably accounts for that. Other times, it's clearly rectified, so go figure. As a last resort, I resort to Sam's chopper to wrestle the hold-outs to the ground, but it takes a bit of time to remove the flyback and put 10-15 turns around the core. The ringer has also helped me isolate a defective yoke, which explained why things wouldn't ring.

    Anyway, I think Bob's tester is a great little unit and am glad I have had the opportunity to test it--and keep the prototype! :-)

    (From: John Robertson (jrr@flippers.com).)

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

    Connect the generator to the leads that the horizontal output transistor and ground use (out of circuit, use HOT 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 the original 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 square wave as the source, but the theory is consistant.

    (From: Quick Fix (iradg@guru.nu).)

    If you don't don't repair that many TVs, the cheapest way to check a FBT is to connect its primary winding in series with the yoke (low side) of a working set. If the picture shrinks a few inches on both sides evenly and with no ringing or jail bars, your FBT is good. You can even measure the high voltage on your FBT with 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 larger flybacks) 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 or with typical flyback testing equipment. Actually, a simple ohmmeter test between the HV output and return might 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 and return 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 as the 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 ((netrekker@pdq.net).)

    Sencore Z-Meters are capable of applying sufficient bias to check those diodes for forward conduction and reverse leakage. Forward conduction should be confirmed first to rule out an open -- almost all multimeters will 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-in yoke/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 much due to the high voltage winding and/or HV multipler but rather related to its usual function as the source of multiple secondary power supply voltages used by various tuner, deflection, video, and audio subsystems. In addition, inductance, capacitance, pin configuration, and HV, focus, and screen outputs must be compatible.

    ECG and similar companies do have a line of generic FBTs and should have a catalog/cross reference for these similar to the one for semiconductors. See the section: Replacement Flyback Transformers.

    However, FBTs are where the designers of TVs and monitors can be really creative. After all, specifying the flyback windings gives them complete freedom to pick the number and types of secondary voltages! Your chances of picking up something off the street so-to-speak and expecting it to fit anything you have ever owned - or ever will own - isn't great.

    (From: an engineer at a TV manufacturer).

    We have one guy whose mission in life is doing exactly that... (and specifing HOT's too).

    Besides specifying auxiliary secondaries you can also specify an overturn on the primary (for deflection coils which would otherwise require a >1500 V HOT) and influence the tuning of the EHT secondary, to determine the EHT internal impedance. And finally you might specify a built-in EHT capacitor or bleeder resistor and various types of clicked-on potmeter modules (perhaps with a second focus voltage for DAF).

    Typical Flyback Schematic

    This diagram shows a typical flyback that might be found in a direct view color television or computer monitor. Resistances are included for illustrative purposes only and may be quite different on your flyback!

    The high voltage section on the right may actually be constructed as a voltage multiplier rather than a single winding with multiple HV diodes. The rectifiers or multiplier, and/or focus/screen divider may be external to the flyback transformer in some models.

    Flyback transformers used in black-and-white TVs and monochrome computer monitors do not have a focus and screen divider network. Older ones do not include a high voltage rectifier either - it is external.

    The ferrite core of a flyback transformer is constructed with a precision gap usually formed by some plastic spacers or pieces of tape. Don't lose them if you need to disassemble the core. The ferrite core is also relatively fragile, so take care.

    The focus and screen divider network uses potentiometers and resistors (not shown) with values in the 10s to 100s of M ohms so they may not register at all on your multimeter. The high voltage rectifiers (CR1 to CR3 on this diagram) are composed of many silicon diodes in series and will read open on a typical VOM or DMM.

    Note that there is no standardization to the color code. However, the fat wire to the CRT is most often red but could also be black. Of course, you cannot miss it with the suction cup-like insulator at the CRT anode end. The focus and/or screen connections may also be to pins rather than flying leads.

    
                                            +--|>|-----------o  HV to CRT
                  _        1             ::(   CR1             (25 to 30 kV,
                 |   B+  o-------------+ ::(                    suction cup on
        Drive    |                      )::(                    fat red wire)
        winding <                       ):: +-------+
                 |             1.32     )::         |
                 |         2            ):: +--|>|--+
                 |_  HOT o-------------+ ::(   CR2
                  _        3             ::( 
                 |   50  o-------------+ ::(
                 |                      ):: +-------+
                 |         4    .11     )::         |
                 |   35  o-------------+ :: +--|>|--+
      Various    |                      )::(   CR3  |
      auxiliary <               .28     )::(        /
      windings   |         5            )::(        \<-------o  Focus 
                 |   16  o-------------+ ::(        /          (3 to 10 kV,
                 |                      )::(        \           orange wire)
                 |         6    .12     )::(        |
                 |_   0  o----------+--+ ::(        |
                  _        7        |    :: +--+    /
                 |   H1  o----------|--+ ::    |    \<-------o  Screen
     CRT Heater <          8    .08 |   )::    |    /          (200 to 800 V,
                 |_  H2  o----------+--+       |    \           brown wire)
                                    |          |    |      9
                                    |          +----|--------o  To CRT DAG
                                    |               |            ground
                                    +---------------+
    
    

    Replacement Flyback Transformers

    Unfortunately, you cannot walk into Radio Shack and expect to locate a flyback for your TV or monitor. It is unlikely the carrots at the counter will even know what a flyback is or recognize one if it hit them over the head (wherever that would be on a carrot). They will probably attempt to sell you a 6.3 V power transformer :-). Fortunately, there are other options:

    (From: a-freak@freenet.de.)

    HRdiemen is a manufacturer of replacement line output transformers and have several thousand types available. But the nicest thing is their online database where they have pinout and internal schematics including typical voltages of every of these transformers. Just type your original letters/numbers into the search 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 (like me, to avoid ad banners and other doubtful background activities), you can also directly access http://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. I do not know how likely it is to have problems with 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.

    Here are several examples of incompatibility problems:

    (From: Petercoe (petercoe@aol.com).)

    There is some good and some bad to these flybacks. One thing I noticed is that the competition has caused the 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 get the set to work right after using a low priced replacement. I also had a Goldstar which would only work with the original flyback."

    (From: Michael Caplan (cy173@freenet.carleton.ca).)

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

    (From: Dave Moore (penguin@datastar.net).)

    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 of the screen and a bright area with retrace lines showing from top to bottom down the middle of the of the screen. At first I thought that this was the classic bad B+ filter to the crt board phenomena. But nope, filter good.

    So I figured that it had to be a bad filter in the brightness limiter or to the video circuits. A quick round about with 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 I had removed a small disk capacitor on the screen supply trying to unload the shorted screen supply. Well the problem turned out to be the picture tube and after I cleared the screen short I noticed a similar phenomena to the one that I was presently experiencing (the bright area in the middle of the screen from top to bottom with retrace lines).

    Well after I reconnected the little disk capacitor the problem went away. So!! I thinks to myself: The little disk cap on this Hitachi I'm working on must be bad. Well lo and behold - there is no disk cap on the hitachi. 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 and retrace lines in middle of screen. I can only assume that the cheapie flyback was the cause. This makes me wonder if the lack of horizontal width might be symptomatic of this "cheapie" flyback also. I'll probably just parallel some capacitance on the HOT since I don't have much width to make up. I already adjusted B+ to the 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 (gdanner@advancenet.net).)

    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 aligned quite right, and may have to be bent and adjusted slightly to fit the board. Screen and focus controls may not be in the same physical position on the new flyback. But, overall, I would say that most of the generic flybacks I've used have worked out OK electronically.



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