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Copyright ©; 1994-2007
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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.
In particular, before touching or probing the flyback or circuitry in its
vicinity:
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.
(From: Sivasankar Chander (siva@bond.bocaraton.ibm.com).)
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.
(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.
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.
A typical flyback includes the following components:
Some TV and monitor designs use a physically separate (external - not part
of the flyback transformer) voltage multiplier. In this case, the flyback
high voltage winding will generate 6 to 10 kVAC and the multiplier will
boost this typically 3X or 4X to 20 to 30 kVDC. The focus and screen (G2)
network will generally be part of the multiplier module in this case.
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:
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.
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!
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.
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).
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.
Power up the TV or monitor (preferably with a series light bulb or on a
Variac.
If the B+ now climbs to a more normal value, a problem with the HV (CRT
short) or one of the secondary loads is indicated. Connect each of these
up one a time (or test individual components) to localize the fault. The
flyback is likely good.
Power up the TV or monitor (preferably with a series light bulb or on a
Variac.
If the B+ now climbs to a more normal value, a problem with the original
flyback is indicated. However, more thorough testing may be desirable to
be absolutely certain.
If you do this regularly, keeping a selection of 'flyback simulators' - just
the drive windings and cores may be desirable.
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.
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.
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.
I tried the Q evaluation method using the 100 volt CAL voltage pulse from a
Tektronix scope. It worked best when I used a series 200 pF capacitor. I got
maybe 100 pulses before it decayed to zero. If I shorted two of the primary
pins, the decaying pulse train went to zero almost immediately. So it works!
I thought of another method. The Q of a resonant circuit is equal to the
center frequency divided by the half power bandwidth. I applied an audio
generator through a 22k resistor, found the peak frequency, then went off that
frequency to .707 of that amplitude. Double this would be the bandwidth. I
got Q's of 26 and 16 for two I tried. (Editor's note: This appears to be a
valid approach.)
The circuit below excites the flyback in much the same way as in normal
operation. The only caution is that this tester probably does not put
enough stress on the flyback to find an intermittent that fails only under
full operating conditions. However, most flyback failures are solid - once
a short develops, there is a meltdown of sorts and it is there to stay.
You will require a 12 V power source of at least 2 or 3 amps capacity
(regulation is not important - I just use a simple transformer, rectifier,
filter capacitor type of power supply).
The circuit is shown below. None of the component values are critical.
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.
There are several approaches that can be taken - possibly in combination:
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).
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":
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!
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:
(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.
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.
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).
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.
Here is one apperently just for flybacks:
and one that is mostly for flybacks:
some others:
And, here's one for your semi-antique (1950s) needs:
However, these may be of lower quality or not be quite compatible with your
original. In an effort to minimize the number of distinct flyback models,
some corners may be cut and one-size-fits-many may be the rule resulting in
all sorts of problems. Here are a couple of possibilities:
Thus, marginal or erratic behavior might result from generic replacements
greatly complicating your troubleshooting since without careful measurements
there is no way of knowing whether the problem is due to the new flyback or
a fault that still exists elsewhere. For some actual experiences, see the
section: Cheap Flybacks - Beware.
(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.
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.
-- end V1.58 --
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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.
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.
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.
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):
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:
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).
A Little History
So, how far back does the use of a flyback based high voltage go?
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.
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.
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.
_ _
\/ _/\_
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.
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.
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?
How Do Flyback Transformers Fail?
Flybacks fail in several ways:
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.
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.
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!).
Advanced Testing
When the Basic Tests Don't Reveal Anything
Also see the section: Flyback Testing Equipment.
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.
(From: James Elliott (jelliott@stlnet.com).)
Scope
_ o
Pulse or _| |_ | Device under Test
function o---------------------+-----------+ +---
Generator | )::(
High Quality _|_ )::( All other
Non-polarized --- ):: +--- windings
Capacitor | ):: +--- left open
| )::(
Ground o---------------------+-----------+ +---
Method 2
+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.
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!
100K
PS- o--------/\/\--------+--------o CRT (suction cup) connector on flyback
|
o
-
100 VDC Measure voltage here
+
o
|
PS+ o---------------------+--------o Probe to pins on base of flyback
Check each pin on the base of the flyback with the probe. Touching the
return pin will result in the voltage reading dropping to perhaps 50 or 60
volts. This is the forward voltage drop across the high voltage rectifier
stack inside the flyback. All other pins will result in it remaining at the
supply voltage (except for the ground connection to the F/G2 divider if it
is separate - then it may drop a fraction of a volt). Note that if you
cannot locate the HV return, your flyback may indeed be defective; it may
have an internal bad connection, open HV rectifier, or burnt out HV winding.
Or, if other pins drop the 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 there are any shorted windings, then there will be no significant HV output
and the load on the oscillator will increase dramatically.
Other Flyback Testing Procedures and Comments
Here is a Web site with some notes on flyback testing procesures:
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.
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.
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.
Why Do All Flyback Transformers Seem to be
Unique?
(Most of these comments also apply to SMPS high frequency transformers.)
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!
+--|>|-----------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:
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.
