Preface

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Acknowledgement

Introduction

Scope of This Document

TVs and most computer and video monitors depend on the use of similar (at
least in concept) circuit configurations to generate several outputs:

• Current waveform required in the deflection yoke coils of the CRT for linear sweep of the electron beam to create a high quality (geometry and
  linearity) picture. This is close to a sawtooth but not quite.
  

• CRT High voltage (20 to 30 kV or more) required to accelerate the electron beam and provide high brightness and sharp focus, as well as other related
  voltages - focus and screen (G2).
  

• Various auxiliary power and signals for other subsystems of the equipment (low voltage, CRT filament, feedback, etc.).
  

Most of this information applies to the horizontal deflection which operates
at the higher frequency in a raster scan display (except for peculiar rotated
portrait formats where the functions of the horizontal and vertical scan are
interchanged.

Equipment which utilizes this circuitry includes TV (direct view as well
3-CRT and light valve projection types), computer and video monitors, tube
based video cameras (e.g., vidicon), and other magnetically deflected CRT
devices.

Vertical deflection circuits are much less complex due to the lower scan rate
(e.g., 50 to 120 Hz V as compared to 15.734 kHz H for an NTSC TV or up to 120
kHz or more for a high resolution computer monitor). Most of the control and
output drive circuitry is contained in a special vertical chip in modern
equipment.

Deflection System Safety

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. See the specific documents:
Notes on the
Troubleshooting and Repair of Television Sets or Notes on the
Troubleshooting and Repair of Computer and Video Monitors as appropriate
for your equipment.

Horizontal Deflection System Fundamentals

How Does the Horizontal Deflection Circuit Work?

Although there are many variations, the basic operation of the horizontal
deflection/high voltage power supplies in most TVs, monitors, and other CRT
displays is very similar.

For understanding the working of the deflection circuit regard the flyback
transformer as an inductor. The airgap stores energy, some of which may be
tapped off during flyback by secondary rectifiers (e.g., vertical deflection,
signal circuits, and high voltage supplies) and non-rectified loads (e.g.,
filament supply) but these have hardly any influence on the basic working
principles.

The scenario described below is only true in the steady state - the first few
scans are different because the picture tube capacitance is still discharged.
This represents a short-circuit at the secondary side of the flyback. It
prevents proper demagnetizing, hence the core will go into saturation (unless
special soft-start measures have been taken, like a B+ supply that comes up
slowly). Generally, a hard start of the line deflection circuit represents a
very heavy load on the HOT. This will happen after a picture tube flashover
or if the B+ is connected suddenly (due to intermittent contact) and can mean
instant death to the HOT due to secondary breakdown. See the section:
More on HOT Failure.

Basic Deflection Circuit Operation

A very simplified circuit is shown below - many components needed to create
a practical design have been omitted for clarity. First concentrate only on
the portion of the schematic shown below to the left of the yoke components:

                         B+
                          o
                          |
                          +
                           )::
                Part of T2 )::
                   Flyback )::
                   Primary )::
                           )::
                          +
                          |
                          +-------+---------+----------+
                          |       |         |          |
         Horizontal       |       |         |          +
           Drive          | C     |         |           )::    (Horizontal
             T1       B |/      __|__      _|_          ):: L2  Deflection
         --+    +-------|       _/_\_      ---          )::     Yoke - HDY)
    Driver  )::(        |\        |   D1    | C1       +
    Stage   )::(     Q1   | E     | Damper  | Snubber _|_ C2
    (not    )::(     HOT  |       |         |         --- S-correction
     shown  )::(          |       |         |          |
         --+    +---------+---+---+---------+----------+
                             _|_
                              -
               B+ Return (may not be signal ground)

1. At the end of scan, current is flowing through the flyback primary to the HOT, Q1. At the start of the flyback period, Q1 turns off. (This must be
   done in a controlled manner - not just a hard shutoff to minimize stresses
   on the HOT - but that is another story). Since current in an inductor
   (the primary of the flyback has inductance) cannot change instantaneously,
   the current is diverted into the snubber capacitor, C1. The inductance of
   the flyback primary (T2) and C1 forms a resonant circuit so that the voltage
   climbs on C1 as the current goes down. At its peak, this voltage will be
   1000 V to 1500 V.
   

2. C1 now begins to discharge in reverse through the primary of T2 (back into the B+ supply - the filter capacitor will stabilize the B+ output) until
   its voltage (also C-E of the HOT) reaches 0.
   

3. If there were no damper diode (D1), this voltage would go negative and continue to oscillate as a damped sinusoid due to the resonant circuit
   formed by T2 and C1 (and the other components). However, D1 turns on as
   the voltage goes negative and diverts the current through it clamping the
   voltage near 0 (-Vf for the diode).
   

   Note that the damper diode D1 may have been built into the HOT T2 in the
   case of an inexpensive or small screen TV or even some monitors that don’t
   have any circuitry for E/W correction.
   

   Steps (1) to (3) have accomplished the flyback function of quickly and cleanly
   reversing the current in T2 (and, as we will see, the deflection yoke as well).
   The full flyback (and yoke current) are now flowing through the forward biased
   damper diode, D1.
   

4. At the beginning of scan, the damper diode (forward biased) carries the bulk of the current from the yoke and flyback. The nearly constant
   voltage of the B+ across T2 results in a linear ramp of current now
   through the damper diode since it is still negative and decreasing
   in magnitude.
   

5. At approximately mid-scan, the current passes through zero and changes polarity from minus to plus. As it does so, the damper diode cuts off
   and the HOT picks up the current (with a voltage drop of +VCEsat). Current
   is now flowing out of the B+ supply.
   

   The base-drive to the HOT must have been switched on before this point!
   Timing is not very critical as long as it happens between the end of the
   flyback and the zero crossing of the summed current. The location of the
   zero crossing depends on the secondary load, notably the beam current.
   Larger beam current requires that the HOT be switched on earlier. The
   designer has to do some optimizing here…
   

6. During the second half of the scan, the HOT current ramps up approximately linearly. This is again due to the nearly constant voltage of B+ across
   the inductance of the flyback primary.
   

7. Near the end of scan, the HOT turns off and the cycle repeats.

   The HOT has a storage time between 3 us and 7 us, thus the base-drive is
   switched off earlier, in a controlled way to properly remove the charge
   carriers from the collector region in the HOT. The peak amplitude of the
   base current and the way it is decreased determine the ultimate dissipation
   in the HOT and are thus subject of heavy optimization. This is hampered
   by the fact that there is much spread in HOT parameters.
   

The voltage across the C-E of the HOT is a half sinusoid pulse during the
flyback (scan retrace) period and close to zero at all other times (-Vf of
the damper diode during the first half of scan; +VCEsat for the HOT during
second half of scan).

Caution: Without a proper high frequency high voltage probe, it is not possible
or safe to observe this point on an oscilloscope with full B+. However, where
the equipment can be run on a Variac, this clean pulse waveform can be observed
at very reduced B+. Excessive ringing or other corruption would indicate a
problem in the flyback, yoke, or elsewhere.

Normally you would use a 100:1 probe suitable for 2 kV peak. You could always
make your own voltage divider out of a couple of suitable resistors and use a
regular 10:1 or 1:1 probe. Beware that also the capacitive division ratio
must be correct because the line frequency is high enough to make it relevant.

The current through Q1+D1 is several amperes peak-peak. There’s a lot of
power circulating here, making this a dangerous circuit in every way!

The Deflection Yoke Connection

There are several reasons including:

• The desired yoke current is not quite a sawtooth but includes two major corrections: S and E/W (described below). These cannot be applied easily
  with such a configuration.
  

• The flyback also generates the HV and secondary output voltages and the primary current might then be affected by these and change as a function
  of beam current (picture brightness) or audio level (although feeding the
  audio amplifiers from LOT windings is not common anymore).
  

The yoke is placed across the C-E of the HOT in series with a capacitor
(S-correction) and other components which in effect form a variable power
supply (analogous to the constant B+) which is used to compensate for the
various problems of scanning a nearly flat screen.

What are S and E/W or N/S Correction?

Because the screen of most CRTs is relatively flat (even those not advertized
as flat) and the electron gun is relatively close, any picture tube will
naturally have serious linearity problems and pincushion distortion if there
were no corrections applied. Near the edges and corners of the screen the
spot will move faster because the same angular speed translates to a larger
linear speed. This is simple trigonometry.

There are 3 ways of correcting N/S:

1. The design of the deflection coil (distribution of the windings). This is cheap and robust but it requires a huge design effort.
   Also, if for some reason the glue that holds the wires together
   gets soft then the geometry and convergence may be affected.
   

2. With permanent magnets. After the tube has been built it is switched on and some magnets are stuck on the back to improve
   geometry, convergence and or landing. This is very difficult
   but relatively cheap. I would expect it in better monitors.

3. With active N/S correction. A line-frequency complex waveform is injected as current into the raster deflection coil. This allows
   arbitrary correction of N/S waveforms, within the limitations of
   the waveform generator and the bandwidth of the system. This can
   be easy to adjust, or not, depending on the design, and it is
   most expensive of all. I would expect it in workstation monitors.
   

As for problems with N/S geometry, if there is an active circuit at all, then
it would correct only 4-way symmetrical errors, so you would expect an error
in all 4 corners. I don’t know of any circuits that would affect only 1
corner. A magnet might affect only 1 corner, it might have dropped.
A coil might have softened in 1 corner, due to heat. That would be beyond
repair, I guess. But it is not very likely either. My guess is that the error
has always been there, from the start and it was just noticed!

S-Correction Circuit Operation

When we add in the yoke components (only the horizontal deflection coil and
S-correction capacitor or S-cap are actually shown above) conditions are only
slightly more complex:

First, consider what would happen if instead of the S-cap, the yoke were
connected to B+ like the flyback. In this case, the total current would
divide between the flyback primary and the yoke. It would still be a sawtooth
as described above. Of course, component values would need to be changed
to provide the proper resonant circuit behavior.

That’s called ‘tuning of the flyback capacitor’, to achieve the proper
duration of the flyback pulse, matching the blanking time of the video
signal, and to achieve the proper peak flyback voltage, matching the
Vces specification of the HOT with a reserve of about 20%.
That’s two conditions, requiring two degrees of design freedom.
There are 3 freedoms: supply voltage, flyback capacitor and yoke
inductance.

With the S-cap and yoke wired as shown above, the inductance of the yoke and
S-cap form a low pass filter such that voltage on the S-cap will be a smoothed
version of the pulses on the HOT collector (similar in effect to the B+ feeding
the flyback but not a constant value). The average value of the S-cap voltage
will be positive.

The S-capacitor together with the yoke inductance forms a resonant circuit
whose frequency is tuned lower than the line frequency. It has the effect
of modifying the sawtooth current into a sine-wave shape. This is called
‘S-correction’. It reduces the scanning speed at the left and right edges
of the screen.

The value of the S-cap can be selected so that the voltage varies in such a
way as to squash the current sawtooth by the appropriate amount to largely
compensate for the fact that the electron beam scans a greater distance with
respect to deflection angle near the edges of the screen.

Think of it this way: When the scan begins, the yoke current is at the maximum
value in the direction to charge the S-cap. The voltage across the S-cap
is causing the current to decrease but the S-cap is also gaining charge so
the rate of decrease is increasing. At the time the current passes through
0, the S-cap is charged to its maximum. The current now reverses direction
retracing its steps. Got that! :-). This is another example of a portion
of a resonant circuit. The voltage on the S-cap is varying by just the right
amount to compensate for the geometry error.

Many CRTs, especially the flatter ones, have a need for geometry correction
that goes beyond simple S-correction. Most tubes need inner pin-cushion
correction, which is also called “dynamic S-correction”. It is an adaptation
of the diode modulator circuit, which is only found with tubes that need
E/W (pin-cushion) correction in the first place. See the section:
N/S and E/W Correction Circuit Operation.

Some tubes need more S-correction only at the extreme edges, this is called
“higher-order S-correction”. This requires another resonant circuit, tuned to
a higher frequency, which is more difficult to implement. Most sets will not
have such circuit.

Many tubes in the USA (with only 100 degrees deflection angle) are
“raster-correction-free” so they will not have much correction circuitry other
than basic S-correction. There will be no diode-modulator for E/W-modulation.
Such sets will have larger remaining errors that tubes (with 110 degrees
deflection angle) that need active correction to begin with.

For multiscan monitors, S-caps must be selected for each scan range since the
timing varies with scan rate. These are only approximate corrections but
good enough for most purposes. MOSFET or relay circuits take care that for
each range of scanning frequencies the correct combination of S-correction
capacitors is selected.

As an example, consider a multiscan monitor which supports VGA (31.4 kHz,
800×600 at 56 Kz (35 kHz), and 800×600 at 60 Hz (38 kHz):

For good geometry between 31, 35 and 38 kHz, two discrete values for the S-cap
is barely enough. Actually a 3rd value optimized for 35 kHz would be better. If
there is only one S-cap and it is optimized for 38 kHz, then at 31 kHz you will
be using a too large angle of the sine (of the resonant frequency between the
S-cap C and the deflection coil inductance). Possibly even > 180 degrees,
making the current fold back. Apart from an obvious geometric distortion,
there is also increased risk of HOT failure because you’re operating too close
to the resonant frequency.

Applications of CRTs in TVs are a lot less critical than computer monitors, so
don’t expect too much of them. It is simply not worth the money until more
critical applications like WebTV come along. The market is not willing to
pay for corrections to problems that most people would never notice (with
typical TV pictures).

N/S and E/W Correction Circuit Operation

To a large extent the N/S errors can be corrected by a suitable yoke coil
design. For smaller tubes (90 to 100 degrees types) this is also possible for
E/W errors. For larger tubes (110 degrees) or high quality tubes, electronic
E/W correction is required. This is the well-known pincushion.

E/W correction is modulation (which implies multiplication) as a function
of vertical beam position. The amplitude of the horizontal deflection current
is modulated with a parabola waveform which is derived from the vertical
deflection circuit. This squeezes the top and bottom lines back into the left
and right screen borders.

N/S correction (if any) is a method of injection - addition of a high
frequency waveform (harmonics of the line frequency) to the low-frequency
field-deflection waveform.

This requires costly nasty circuitry which is better avoided.

This is how the diode modulator for E/W correction works:

           B+
            o
            | (Vb)
            +
             )::
  Part of T2 )::
     Flyback )::
     Primary )::
             )::
            +
            |                    < - Yoke components ->
            +-------+---------+---------+----------+
            |       |         |         |          |
            |       |         |         |          +
            |       |         |         |           )::    (Horizontal
            |     __|__      _|_        |           ):: L2  Deflection
            |     _/_\_      ---        |           )::     Yoke - HDY)
            |       |   D1    | C1      |          +
            |       | Damper  | Snubber |         _|_ C2
            |       |         |         |         --- S-correction
            |       |         |         |          |
            |       +---------+---------)----------+
            |       |         |         |(NC)      |
            |       |         |         |          +
            | C     |         |         |        L3 )::
        B |/      __|__      _|_       _|_   Bridge )::  L4
     -----|       _/_\_      ---       ---     coil )::  E/W Coil    
          |\        |   D2    | C3      | C4       +     ::::         (Vm)
       Q1   | E     | Damper  | Snubber | Snubber  +----+^^^^+---+-----+-----
       HOT  |       |         |         |          |             |     |
            |       |         |         |      C5 _|_          +_|_   E \|B
            |       |         |         |  Bridge ---        C6 ---  Q2  |---
            |       |         |         |     cap  |             |    C /|
            |       |         |         |          |             |     |
            +-------+---------+---------+----------+-------------+-----+------
           _|_                                            E/W Amplifier (PNP)
            -
       B+ Return (may not be signal ground)

The required field-parabola waveform is derived from the field deflection
circuit. Amplitude and DC-level are adjustable, for correcting pin-cushion
distortion and setting the screen width. EHT is not influenced !

The E/W amplifier usually has a PNP emitter follower only, because it
must only sink current and dissipate a bit of power. The coils L3 and L4
take care that the E/W amplifier sees no line-frequency.
The “bridge” components L3 and C5 resemble the deflection coil with its
S-correction capacitor and carry the large amplitude alternating current.
L3+C5 are tuned to approximately the same frequency as L2+C2.

C1, C3 and C4 must be tuned so that EHT is independent of Vm and peak
voltage over D2+C2 is high enough but not too high when Q2 is off.
C4 is usually a small ceramic capacitor of approx. 1 nF mounted close
to the HOT Q1, it also suppresses EMI. Flyback capacitors are critical
components. Wrong types may overheat and burn. Bad contacts here or
elsewhere in the deflection circuit may arc and also cause fire.

There are many variants to this circuit, e.g. for dynamic S-correction.
Multi-sync monitors need added circuitry to make the EHT independent of
the line frequency (if there is not a separate EHT supply, that is).

If the E/W modulator fails then you will see that the top and bottom lines
will be much too wide. There are several parts that could have failed. It’s
usually not too difficult to find why there’s no parabola. If you have
partial loss of E/W modulation, notably in the extreme corners, then you
should suspect the tuning of the 3 flyback capacitors that belong to the
diode modulator circuit. That’s a specialist job…

S-Correction Problems

• An open S-cap will result in no horizontal deflection - a vertical line.

• A shorted S-cap will likely load down the B+ possibly resulting in a blown fuse or other power supply components.
  

• An S-cap that changed value (or in the case of a multiscan monitor, selected to be the wrong value) will result in distortion at the left and right sides
  of the screen:
  
  • Too low: picture will be squashed towards edges.

  • Too high: picture will be stretched towards edges.

  Note that this is not the same as what is commonly called linearity which
  would likely affect only one side or gradually change across the screen.
  

Horizontal Linearity Correction

The waveform becomes a damped sinewave, which will be ‘undamped’ by restoring
energy during the flyback.

One way to deal with this is to add a magnetically biased saturable inductor in
series with the horizontal deflection yoke. This is called the linearity coil.

Its core is magnetically biased near the point of saturation such that the
inductance decreeases with increasing current and this helps to stretch the
right hand side of the scan. In other words, during the scan the coil
saturates so that the inductance decreases. At the end of scan there is
practically no voltage left over the linearity coil so that the deflection
coil gets maximum voltage.

E/W Correction Problems

Pincushion Amp adjusts the amplitude of the correction signal.

Pincushion Phase adjusts where the correction is applied on the vertical scan.

• Failure of the E/W correction circuit will result in very noticeable pincushioning distortion of the vertical edges.
  

• Excessive E/W correction will result in barrel distortion of the vertical edges.
  

• A bad power supply derived from the flyback could also result in similar symptoms due to ripple or lack of power to the pincushion circuitry.
  

Differences Between N/S and E/W Correction Implementation

E/W correction is easy: the lower frequency modulates the higher frequency.
This reduces to simple amplitude modulation. Well, simple in principle. The
line circuit is a high-energy circuit. For this purpose the diode modulator
circuit has been invented. It allows an energy exchange between the line
deflection circuit and a pseudo deflection circuit.

N/S correction is difficult: the higher frequency modulates the lower
frequency. It can be done with sort-of amplitude modulation by using a
‘transductor’. This is not a transformer but a component with 2 coils and a
saturable core where the (line frequency) current through 1 coil modulates
the inductance of the other coil. If there are tuned parts in the circuit
then the correction will be highly sensitive to line frequency variations.

It can also be done with a regular transformer, by injecting a strong signal
(from an amplifier) with line frequency components into the field deflection
circuit.

Either way, it’s an expensive solution which should be
avoided by designing the deflection coils in such a way
that the picture tube needs no active N/S correction.

• Scan power is obtained during the forward stroke as with a ‘normal’ transformer. Energy is transferred while the HOT/damper diode is conducting.
  The output rectifier is oriented so that current flows during scan time.
  (Dots on the transformer winding match.)
  

  The scan rectifiers make no use of the stored magnetic energy, they load the
  primary directly during the scan part. They do not cause an increase of the
  stored magnetic energy so a heavy load is not a problem.

• Flyback power is obtained from the stored energy in the flyback transformer’s inductance when the HOT shuts off. The output rectifier is
  oriented so that current flows at flyback time. (Dots on the transformer
  windings oppose.)
  

  The flyback rectifiers on the other hand (especially the EHT) draw from the
  stored magnetic energy. When the secondary load increases, the magnetization
  current will also increase. Ultimately this will cause saturation of the
  ferrite core. Excess beam current is a common cause for this and should be
  avoided by the beam current limiter. The advantage of a flyback rectifier
  is that it provides 7 times more volts per winding than a scan rectifier.
  

• AC power (usually only for the filament or a feedback signal) flows during both scan and 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.
Since the deflection system runs at 15 kHz or higher, fast recovery diodes
must be used as rectifiers. However, at these frequencies, the uF ratings of
the filter capacitors can be quite small compared to power line (50/60 Hz)
based systems.

EHT (High Voltage) Generation

The EHT (Extra High Tension or HV to the CRT ) is generated from a secondary
winding on the flyback transformer having several thousand turns of very fine
wire. Being a flyback supply, the actual output voltage is many times what
would be calculated based on turns ratios alone. The HV rectifier consists of
a stack of silicon diodes with a total PIV rating of 50 kV or more.

Because the flyback pulse is so narrow, the rectifier diode will
conduct only a short time. Thus the peak current in the winding will
be quite high, resulting in a significant voltage drop when loaded.
The internal impedance of the EHT source is in the order of 1 MOhm,
so with a load of e.g. 1 mA the EHT will drop 1000 V = -3%.
Usually the EHT voltage is far from stable, 10% drop is quite normal.

If the EHT voltage drops, then the electrons will be accelerated less
and will move through the deflection field at a lower velocity. As a
result they will be easier to deflect by the magnetic field, and the
picture size will grow. Without special measures, brighter pictures
will be larger. The measure is to feed some EHT information or beam
current information to the deflection circuits, reducing the deflection
current amplitude a bit for bright pictures. For horizontal deflection
this is done by the E/W modulator. This is called anti-breathing.

Sets with raster correction free picture tubes don’t have an E/W
modulator. There the correction may be done by means of a power
resistor in series with the B+ supply. A large beam current causes
more power consumption, this lowers the B+ supply voltage and thus
reduces the line deflection current. That also reduces the EHT even
further, but the deflection current has a stronger effect on the
picture width than the EHT. Better methods exist too.

The EHT information is also used to protect the flyback transformer
from overload. As the load increases, the average primary current
rises. Ultimately it may reach a level where the transformer core
may go into saturation. This causes large peak currents in the HOT
which might lead to destruction. To prevent this, some EHT information
is fed to the contrast controller, to automatically reduce the
picture brightness whenever the white content is too much. This is
called the average beam current limiter.

A failure in the video path, like a video output amplifier stuck at
0 V, causes a high beam current that will not react to the contrast
controller. In that case the beam current limiter will not work and
the set should switch off automatically, usually within a few seconds
after applying power. When the cathodes heat up, you’ll see an even
picture with diagonal retrace lines and then it will switch off.

The Difference Between the Ideal and the Real

You may see all sorts of additional passive components as well as transformers
for generating additional voltages not provided by the flyback. There may be
diodes in places you would think would be impossible. Therefore, to really
understand even approximately how each design works may require some head
scratching but the basic operation of them all seems to be very similar.

Horizontal Output Transistor (HOT) Information

Why are There So Many Different HOTs?

Actually there is still some progress going on :-).

TV’s for Europe DO get higher scan rates, you know. Our entire high-end
range runs on 31250 Hz. Sets with VGA capability often run even higher.
100 Hz HDTV was supposed to run on 62500 Hz but that is a big technological
problem as you might imagine.

Larger screen sizes (32″ 16:9, 33″ 4:3) do tend to have less sensitive
deflection coils, so the peak-peak amps go up. Combined with the
higher scan rate this often means that some new transistor must be
found, even if it is only a higher rated selection from an existing
type. There are also 1700 V Vce types next to the regular 1500 V.

And in USA it is known that setmakers (like ourselves) have
standard transistors marked with a different type number, to prevent
repairmen from putting in just any would-be replacement type.
For a fact, it IS risky to put in the wrong replacement, the faulty
drive conditions may destroy it early. So try and avoid this.

To comfort you, the BU508 is still widely used!

HOT Specs and Substitution

Every line transistor has its own requirements for:

• Amount of base drive current, especially the Ib at end-of-scan.
• Waveform of base drive current (rising, steady, falling)
• Speed of reduction base drive current at switch-off.

• Overdriving causes a slow switch-off behavior, some collector current keeps flowing during the beginning of the flyback and will cause dissipation.
  

• Underdriving causes bad saturation, the collector voltage will start to rise before the flyback should start. This too causes dissipation.
  

The dissipation as a function of the base drive current is a more-or-less
parabolic function with a global minimum. The minimum will be different for
high-gain and low-gain types. By measuring the curves for both extreme types
and combining them, an optimum drive for the random type will be found, with
a figure for the worst-case dissipation.

All this will only be true if you insert a device which is a member of the
population spread for which you optimized the base drive. If you just insert
a random other device (different type, same type but different brand, same
type and brand but much older/newer batch) then all bets are off. Dissipation
may be way too high, with early failure as a result (and possibly a distorted
picture geometry due to excess damping of the waveform).

It is certainly not possible to substitute a standard HOT (BU508) in place of
a more advanced type (in >> 15 kHz applications like a monitor). It is also a
very bad idea to substitute a BU508 in place of a much lighter type like a
BUT11 (used in < = 17″ sets). It will fail!

With horizontal output transistors, it is not true that ‘bigger is better’.
If you substitute a heavier transistor (more amps, more volts, more watts,
faster switching, whatever) for a lighter one, then there is a very big chance
that it will fail earlier, not later. The reason is that the drive conditions
will now be wrong (most likely underdrive) and the transistor will overheat
from too high conduction losses (Ic * Vce,sat). So do yourselves a favour and
get a correct replacement type.

If cost weren’t an issue, transistors and other parts could be hand selected
(and some are in any case). But, you wouldn’t be able to buy a monitor for
$200 if that were required!

Is there a Universal HOT Replacement for TVs?

(From: Chris Jardine (cjardine@wctc.net).)

I shouldn’t say this, but, the TV repair shop I worked for a number of
years ago stocked 1 universal Horizontal Output Transistor 2SC1308K or
NTE238. This worked in almost every set out there that used a
transistor and not SCR’s (RCA, etc.). This may not be the best way of
substituting, but, these 2 part numbers seem to have fairly high gain,
power capability, voltage ratings, current ratings, etc. These
characteristics made it a good substitute and when you buy 100 at a
time you would get a really good price.

This may not work in your case, but, my $.02

Is There a Universal HOT Replacement for Monitors?

“Would anyone like to comment on BU508′s, they should be to the same spec.
and born equal. My experience has been that different manufacturers BU508′s
behave differently. One make will fry and last about 3 weeks, put in a
different make, no circuit change, and it runs cool, and is still running 3
years later. Price doesn’t seem to be a guide, a $1 one may run cool and
have no mfr. code, while a branded one might cost a lot more and run hot.”

There is such a thing as component spread. Base drive must be optimized for
the whole range of gain within a type. That range can be so large that at the
limits of the spread the dissipation can still be too large. The reason is
that the device with the largest gain will be overdriven, causing a tail in
the current at switch-off, whereas the device with the smallest gain will be
underdriven, causing it not too saturate enough. Each condition can be easily
viewed on the ’scope. By varying the base drive you can minimize dissipation.

Normally one basedrive is set for the entire population, accepting the
variation in dissipation and its upper limit. Sometimes the variation is so
large that this will not be acceptable. But this is unlikely for a BU508 in a
16 kHz application. Substituting it with a similar type from a different brand
with different parameter spread may indeed cause it to dissipate too much and
thus fail early. This has nothing to do with price or quality, just with a
different optimum base drive. If base drive has been optimized for a brand
with low parameter spread then it can be that the heatsink may have correctly
been selected smaller…

Since you live in the UK, you should definitely read David Sharples’ article
in Electronics World (was it June 1996 ?). He literally optimizes base drive
for a living.

Typical Types of HOTs Used in Monitors

14", SVGA (38 kHz)         A-types: BU2508AF, 2SC4830,  2SC5148.

                           D-types: BU2508DF, 2SC4762,  2SC4916,  2SC5149,
                                    2SC4291,  2SC5250.

15", XVGA (64 kHz)         A-types: BU2520AF, BU2522AF, 2SC3885A, 2SC3886A,
                                    2SC4757,  2SC4758,  2SC5129,  2SC4438,
                                    2SC4770,  2SC4743,  2SC5067,  2SC5207,
                                    2SC5251,  2SC5002.

                           D-types: BU2520DF, 2SC3892A, 2SC3893A, 2SC4531,
                                    2SC4763,  2SC4124,  2SC4769,  2SC5296,
                                    2SC4742,  2SC4744,  2SC4927,  2SC5003.

                          C                            
                          o                      C o--+--+
                          |                           |  |
                        |/                          |/  _|_
                   B o--|             B o--+--------|   /_\ Dd
                        |\                 |        |\   |
                          |                |   Rd     |  |
                          o                +--/\/\----+--+--o E
                          E                               
                                                          
                A-Type without damper      D-Type with Damper

In nearly all the above cases, the devices will plug-in substitute for each
other within a category.

For designs with larger screen sizes (and higher frequencies) the device
selection is not so straightforward as some designs which split the horizontal
deflection and high voltage generation circuitry.

Continuous dissipation is hardly ever the cause of failure. Failure is usually
due to some infrequent transient condition. For multi-frequency monitor designs
of 1991-1994 mode-change was/is a big killer. When repairs are made it is wise
to cycle through a mode-change sequence. Delays of about 1 min. between changes
should be used, shorter delays can cook the device.

• As a rule, once an engineer has a bad experience with mode change he takes greater care in the small-signal circuitry of his next design.
  This has led to mode change, in general, becoming more benign in the
  last couple of years. However, in Taiwan & Korea there is a high turn
  round of engineering staff and some, shall we say, less than perfect
  designs do still reach production.
  

• To “cook” a device by mode changing would take at least 30 mins. of continuous changing with a delay of about 20 seconds between each change.
  

• If a device fails during such a sequence the old spit test is good indicator of why a device fails. That is, a drop of spit on the HOT
  immediately after failure can tell us a lot: if it sizzles, then the device
  has probably cooked, if it doesn’t then the device failed instantly after
  one stressful cycle. Frontiers of technology it is isn’t but it is a useful
  technique.
  

• If you do get failures which haven’t been caused by the HOT “cooking” I have no easy solution, sorry!
  

• Winfield Hill’s comments reflect the experiences of many. School physics teaches that bipolar’s are current driven and MOSFET’s are voltage
  driven. In practice, of course, this is a gross over-simplification.
  A MOSFET in deflection would, as we say in the north of England, have
  to be a “bloody big bugger”. For example, a 1500V MOSFET that behaved
  as a BU2508 would have to be nearly twice the size and at least twice
  the price. Getting 10V on all the gate cells of such a big chip requires
  a lot of charge to be injected (i.e., current) and then removed (reverse
  current); much like a bipolar drive.
  

Varieties of BU508 HOTs

BU508 series seem to come in a number of variants, I haven’t sorted out the
specification for each listed variant, but have found that it’s worth trying
replacements by different manufacturers. Also if the device is on a grounded
piece of metal as heatsink try adding another radiator (twisted vane is my
preference). Some TV manufacturers introduce post production mods to change
resistance values to provide more drive into the base, this may be in the base
or emitter. Scope the waveform for parasitic high frequency oscillation and
check that the waveform looks clean. Check voltage rails, supply and derived,
and that the set is not over scanning. Check all the components around the
horizontal output stage, it may be the manufacturer. had a duff batch of some
component (often a capacitor that goes OC) that keeps failing
which then stresses the BU508.

Why Do Apparently Similar or Better HOTs Sometimes Run Hot and Blow?

There is more to characterizing a transistor than just maximum voltage,
current, and power dissipation.

One important parameter is current gain: Too low a gain for a particular
operating point may result in incomplete turn-on during scan resulting in high
dissipation. You want the transistor to be in the fully saturated state. A
larger HOT is more likely to have a lower current gain.

If you read the app notes put out by the manufacturers like Motorola you
will also find that fast turn off based drive (negative step) is actually
not what you want since this traps excess carriers in the high resistivity
collector region which leads to continued conduction and heating. The ideal
waveform also provide adequate drive during scan but not excessive overdrive
and is thus an increasing ramp to account for the increasing collector current
during scan.

Characteristics like this are not dealt with by the basic specs but can
differ substantially among otherwise similar transistors.

Also see the section: HOT Specs and Substitution.

Storage Time of HOTs

Variations in storage time would translate into horizontal position
errors of the picture. That’s why the base-drive to the HOT is
generated by a PLL that measures the phase of the output of the HOT,
which is the flyback pulse at the collector. This PLL is called the
PHI-2. It keeps a constant phase relation between the sync at the input
and the flyback pulse. As a result, you will see the base-drive pulse
shifting in time as a function of HOT load, this is normal.

Most deflection processors generate a base-drive pulse with a constant
duty-cycle. This means that also the switch-on moment of the HOT will
vary with the load. This makes it extra difficult to optimize the
base-drive because there is only a limited time interval where the HOT
may be switched on and that interval is shorter with high beam current
load. On-time is typically between 50% and 55%, depending on the IC.

The feedback of the flyback pulse to the PHI-2 PLL is not perfect
because the shape of the pulse distorts as a function of beam current.
This will give a dynamic geometry error. It is compensated by feeding
a certain amount of EHT information to the PLL.

Typical HOT Dissipation

Just measuring the actual power dissipation in a HOT is not trivial due to
the nasty shapes of the voltage and current waveforms. You can’t do this with
your DMM! A couple of ways of doing this are:

• Monitor the voltage and current waveforms for the HOT. Integrate the instantaneous measured V*I over the duration of one scan line and multiply
  by the horizontal scan rate.
  

• Mount the HOT and a fixed power resistor on identical heat sinks so that their thermal enviroments are the same. Then, adjust the current through the
  resistor (and thus its power dissipation) so that the temperature rise of
  the heat sinks for the two are equal.
  

(From: David, a Philips application engineer).

14-21", 16 kHz: About 1 W      (some have the HOT running in free-air)
21-36", 16 kHz: Less than 2 W  (some new large CRT's only need 9 A p-p)
25-36", 32 kHz: Less than 4 W  (dissipation really is prop. to frequency)

Why MOSFETs are Not Generally Used for HOTs

True, a MOSFET is much much much easier to drive. In modern switched-mode power
supplies they reign, rightfully so. They are effective and rugged.

But in line deflection things are not so easy. I’ll give you 4 reasons:

1. The flyback pulse is regularly 1200 V peak, allowing for margins you need a 1500 V device. There are few or no FETs available for that. In a small
   set, a 1000 or 1200 V device may be usable. With effort, the deflection
   circuit impedance may be re-scaled for greater current and lower voltage.
   

2. The conduction losses (Vds = Id * Rds) of a FET are quite high, this not only is wasteful dissipation but it also affects the linearity
   of the picture (it’s a damped sine-wave, going slower and slower).
   

3. For the same power losses a FET needs a much bigger silicon area, also FETs are made in a more advanced process, with IC-like features,
   this translates directly into greater cost! They are several times more
   expensive.
   

4. A FET has no storage time, hence it has no storage-time modulation, which is a disadvantage because that would help to improve the natural
   stability of the control of the phase of the line deflection.
   

Optimizing Base Drive for HOTs

The recipe itself isn’t all to difficult: mostly we change the value of the
power resistor that feeds the line drive circuit until the temperature of the
HOT heatsink is at minimum. Too much basedrive current increases the switching
losses, too little basedrive current increases the conduction losses. The
optimum is somewhere in the middle. All you need is a handful of power
resistors and a thermometer on the heatsink.

If you need to optimize for a general HOT type (as opposed to one single
sample) then you kindly ask the manufacturer to provide some limit case
samples (lowest and highest hFE found) and find a basedrive that satisfies
both extremes. Of course you must select a slightly larger heatsink. So far so
good. The difficult bit is when you find that you need to change other
components in the drive circuit as well, like the spread inductance of the
driver transformer, a damping resistor, a duty cycle of a drive signal etc.
And of course you may find that the HOT you planned to use is entirely
unsuitable for this application …

Anyway, it’s never a case of just ‘drive it hard’.

What is This Diode Across My HOT?

The damper is a special high voltage fast recovery type of diode - a 1N400x
type will not work in its place.

BTW, many of these HOTs have a D after the part number to indicate that
they have the internal damper and include a B-E resistor (which may confuse
transistor testing) of about 50 ohms. However, the D is not a sure indication
of an internal damper - nor is its absence an indication of a lack thereof.
The entire part number must be checked to be sure.

What is This Funny Capacitor (or Capacitors) Across My HOT?

If this capacitor is open or missing, excessive flyback voltage will result
probably killing the HOT. If the HOT does not fail, the result will likely be
greatly increased high voltage. Should the X-ray protection circuitry not
shut down the deflection, there could be internal or external arcing and/or
destruction of components like the flyback or tripler.

For proper operation and continued safety, only proper exact replacements
should be used for these parts.

Horizontal Output Transistor Failure and Testing

HOTs Keep Blowing (or Running Excessively Hot)

You have just replaced an obviously blown (shorted) horizontal output
transistor (HOT) and an hour (or a minute) later the same symptoms
appear. Or, you notice that the new HOT is hotter than expected:

Would the next logical step be a new flyback (LOPT)? Not necessarily.

If the set performed normally until it died, there are other possible
causes. However, it could be the flyback failing under load or when it
warms up. I would expect some warning though - like the picture shrinks
for a few seconds before the poof.

Other possible causes:

1. Improper drive to horizontal output transistor (HOT).
   • A too weak drive (or a HOT with a too low Hfe) causes Vce(sat) to be too large, giving conduction losses.

   • A too strong drive (or a HOT with a too high Hfe) causes it to switch off too slowly, giving switching losses.
     

   Base drive should be optimized to balance between these 2 losses. Check
   driver and HOT base circuit components. Dried up capacitors, open
   resistors or chokes, bad connections, or a driver transformer with shorted
   windings can all affect drive waveforms.
   

2. Excessive voltage (B+) on HOT collector - check low voltage regulator (and line voltage if this is a field repair), if any.
   

3. Defective safety/flyback capacitors or damper diode around HOT. (Though this usually results in instant destruction with little heating).
   

4. New transistor not mounted properly to heat sink - probably needs mica washer and heat sink compound.
   

5. Replacement transistor not correct or inferior cross reference. Sometimes, the horizontal deflection is designed based on the quirks
   of a particular transistor. Substitutes may not work reliably.

   Well, you can always try to optimize the base drive by changing the value
   of the power resistor that feeds the drive circuit at the primary of the
   drive transformer. But you’re on your own here! Clearly label what you
   have done or else your name will be mud if the unit ever needs to be
   repaired by someone else in the future.
   

6. And, of course, bad connections in the drive or output circuitry can be the cause of almost any sort of failure! The HOT should not run hot if properly mounted to the heat sink (using
   heatsink compound). It should not be too hot to touch (CAREFUL - don’t
   touch with power on - it is at over a hundred volts with nasty thousand
   volt spikes and may be line connected - discharge power supply filter caps
   first after unplugging). If it is scorching hot after a few minutes, then you
   need to check the other possibilities.
   

   However, it is possible that the deflection circuit is just poorly designed
   in the first place and it has always run hot (though it is unlikely to have
   always been scorching hot). There is no way to know for sure without a
   complete analysis of the circuit - not something that is a realistic
   possibility. In this case, the addition of a small fan may make a big
   difference in HOT survival.
   

   It is also possible that a defective flyback - perhaps one shorted turn - would
   not cause an immediate failure and only affect the picture slightly. This
   would be unusual, however. See the document:
   Testing of Flyback
   (LOPT) Transformers.
   

   Note that running the set with a series light bulb may allow the HOT
   to survive long enough for you to gather some of the information needed
   to identify the bad component.

   Base Drive and Hot HOTs

   One common cause of a HOT running excessively hot is wrongly dimensioned base
   drive. This may be due to problems in the drive circuit due to bad components
   (often a capacitor) or as a result of poor design. There is usually easy
   evidence if you look at the rising edge of the collector voltage. If it rises
   too early, before the end of the scan, then the transistor is underdriven and
   there will be excess conduction losses. If it rises too late, or rather when
   there is still a lot of collector current during the flyback, then the
   transistor is overdriven and there will be excess switching losses. The
   condition can be varied by playing with the power resistor that supplies the
   line drive circuit. It would be appropriate to vary this as a function of
   parameter spread within one type of transistor. This is not necessary for 16
   kHz deflection because there is sufficient margin for error.
   

   Other deviations may be due to an inappropriate waveform of the base
   current. It should not fall too quickly because the HOT needs time for
   recombination of electron-hole pairs. This is typical of high voltage
   transistors which have a very wide and high-impedance collector-base region,
   difficult to control. Setting this right is the one-time responsibility of
   the TV designer. It can be disturbed by a wrong transistor substitution of
   course, so check whether a correct type has been mounted. That’s what I mean
   when I say that a bigger HOT is not better. A BU508 requires a totally
   different base drive from a BUT12.
   

   Also see the section: Optimizing Base Drive for HOTs.

   HOTs Blowing at Random Intervals

   The HOT may last a few months or years but then blow again.
   

   However, a combination of mode switching, loss of sync during bootup, running
   on the edge of acceptable scan rates, and frequent power cycles, could test
   a monitor in ways never dreamed of by the designers. A TV may suffer from
   similar failures due to repeated power cycling, video input selection, or
   channel changing. It may take only one scan line that is too long to blow
   the HOT. Newer horizontal processor chips are quite smart about preventing
   HOT killing signals from reaching the horizontal driver but they may not be
   perfect.
   

   On the other hand, the cause may be along the lines of those listed in the
   section: HOTs Keep Blowing (or Running Excessively Hot) and just not as obvious - blowing in a few days or weeks instead of a few
   seconds but in this case, the HOT will likely be running very hot even after
   only a few minutes.
   

   Another possible cause for random failures of the HOT are bad solder
   connections in the vicinity of the flyback and HOT (very common due to the
   large hot high power components) as well as the horizontal driver and even
   possibly the sync and horizontal oscillator circuits, power supply, or
   elsewhere.

   Preventing Random HOT Failures

   As noted above, a bigger HOT is not necessarily the answer. A selection of the
   same HOT for 1700 V breakdown voltage may help but is not an option outside
   the design lab. Sometimes very exotic HOT type numbers occur, which are really
   a selection from a standard type, used for statistical failure analysis.
   

   A separate EHT supply (only in the most expensive monitors) would also help to
   save the deflection transistor, but might kill its EHT twin. Of course, not
   an option in the field.
   

   Soft-start circuits make a biiig difference, but are an inherent part of the
   design, not an afterthought.
   

   The chance of failure may also be a function of an unspecified transistor
   parameter, so sometimes the mere swapping of a HOT may solve it permanently.
   

   And, if in the unlikely event it is an EMI problem (like a cellular phone lying
   on top or the set) then obviously the cause must be eliminated. A layout change
   is a better remedy but out of reach of a repair shop.

   More on HOT Failure

   “I’m sure this has been discussed before, but I have worked on several
   monitors lately with shorted HOT’s, replacing that one part has fixed all of
   them, and I have not had a reoccurrence yet. Does something cause HOT’s to
   blow, or do they just short occasionally for no reason?”
   

   Actually, we know fairly well why HOTs blow, at least in television. They
   almost always fail from secondary breakdown, not from average dissipation.
   Secondary breakdown occurs instantly if the combination of voltage and current
   is too high, even for only 1 line period.
   

   Usually this happens due to a hard-start condition of a flyback converter. In
   case of a CRT display this usually applies to the high voltage output which is
   loaded by the built-in capacitance of the tube. When the FBT starts to charge
   that capacitance it sees a short-circuit. This causes the core of the FBT to
   go into saturation and the primary current rises to a multiple of what it
   should be.
   

   When the HOT switches off such a large current it breaks down. That is, some
   HOTs break down. Most are better than spec and survive. If it survives the
   first hard start it will likely survive many.
   

   Such situation occurs most likely after a picture tube flashover, when the EHT
   capacitance has been shorted. After about 3 line periods the current through
   the HOT becomes too high. Only a soft-start method can really prevent it but
   it is a bit hard to design a circuit.
   

   Other causes for HOT failure may be false drive pulses, e.g. due to a bad sync
   input (not likely for a monitor with a slow line PLL) or due to EMI. It should
   not happen, but it will. Some manufacturers have better designers than
   others. Ahem…

   Saga on Swapping of HOTs

   (From: Malik (M.Dad@mmu.ac.uk).)

   The prologue:
   

   I have in for repair a Tatung monitor model CM14UAE. Originally it was
   dead this fault was quickly remedied by replacing the line output
   transistor which was ‘dead short’, causing the power supply to shut down.
   

   Now the transistor I replaced seems to get a hell of a lot hotter than it
   should, when it gets to a certain temperature after about half an hour the
   width begins to collapse at which point I quickly switched off. If I switched
   off for a few seconds and back on again I would get the width back but again
   it quickly began to collapse.
   

   Things you need to know:
   

   The LOP stage is independent from the EHT transformer. i.e. there is a
   separate transformer for the EHT (and A1 supplies etc) with its own
   transistor and a separate line output only transformer with its own
   transistor.
   

   I have also tried substituting the efficiency diode, still no joy.
   I have also replaced the EW transistor and its driver.
   

   To me it seems that the transistor is not being switched properly,
   although the waveforms do look OK.
   

   Original line output device 2SC3893A, which I replaced with a BU508DF
   (European equivalent according to the books. I also tried a 2SC3883 which
   used for the same job in other SVGA monitors. Both give the same problem,
   

   I will be trying to get the original transistor in case this is the
   problem but I don’t think it will make a difference.
   

   And the conclusion:
   

   Anyway after all that headache I finally received through the post the
   original 2SC3893A. I fitted this and left the monitor on test. Sigh of
   relief, the problem has gone away.
   

   It seems the timing on this device differs from all the others I
   tried. The transistor runs at a reasonable temperature even after a few
   hours use.
   

   The moral of this story is, use original line output transistors.
   Unless you are 100% sure the replacement works in its place. Don’t rely
   too much on equivalent books, these should be used as a guide. The
   book I looked at specified the BU508DF as a direct replacement for a
   2SC3893A, but as I know now it doesn’t work properly.
   

   The HOT should not run too hot to touch. If the replacement sizzles, it
   won’t last long and is probably deficient in some specification. If the
   monitor or TV appears to work normally otherwise, try an exact replacement
   HOT if available before swapping other expensive parts like the flyback
   (LOPT). For testing, however, a substitute can usually be used - with a
   series light bulb and Variac.
   
   

   Brief Comments on Testing the HOT

   For a TV or monitor with no blown fuses that will not start, here are two
   quick checks to see if the HOT is good and has power and drive:
   
   • HOT tests - check across each pair of pins for shorts (preferably removed from the circuit board). No junction should measure less than 50 ohms
     or so. Lower readings almost certainly indicate a bad HOT. If in-circuit,
     however, the reading between base and emitter will be near zero due to the
     secondary of the driver transformer. See the document:
     Basic Testing of
     Semiconductors. Don’t be confused by internal damper diodes and B-E
     resistors.

   • Power - measure across the collector to emitter with a multimeter (with the HOT removed or if there is no deflection, this is safe with it in
     place). There should be solid B+, typically about 100 to 160 V (for TVs, 115
     VAC sets - possibly higher for 220 VAC sets), 60 to 170 V or higher for
     auto-scan monitors. If this is missing, either there is a problem with the
     power supply or the emitter fusable resistor has blown (probably in addition
     to the HOT) and there is no return for your voltmeter. If it is pulsing,
     the power supply may be cycling on overvoltage - the HOT may be good in this
     case but there is no base drive.
     

   • Drive: put an oscilloscope on the base - there should be pulses around .7 V for most of the scan and probably going negative a couple volts at
     least for retrace. Specific timing will depend on the actual scan rate. If
     drive is weak or missing, If drive is weak or missing, determine how it
     is derived as there may be a problem in the startup power supply or
     deflection IC.
     

   WARNING: Use an isolation transformer for the oscilloscope tests (and whenever
   you are probing a TV in general)!!! This part of the circuit, in particular,
   is usually line connected. See the document:
   Safety Guidelines
   for High Voltage and/or Line Powered Equipment.

   Testing of Replacement HOTs

   The following is useful both to confirm that a substitute replacement HOT is
   suitable and that no other circuit problems are still present. However,
   single scan line anomalies (particularly when changing channels and/or where
   reception is poor with a TV or when switching scan rates and/or when no or
   incorrect sync is present with a monitor) resulting in excessive voltage
   across the HOT and instant failure are still possible and will not result
   in an HOT running excessively hot.
   
   • Function - Confirm that the monitor or TV behaves EXACTLY as you expect. Look for any sign of changes in picture width and other aspects of geometry
     that might indicate a less than happy horizontal deflection system.
     

   • Temperature - After letting the unit run for a while, unplug the unit and confirm that the voltage on the HOT collector is near zero (discharge the
     power supply filter capacitors if it is not) and see how hot the HOT is.
     Note: Unplugging without switching of off may result in the capacitors
     discharging faster if the unit has a soft (logic controlled) on/off switch.
     Careful, the HOT may be really hot - start at the far end of the heat sink
     and work your way towards the transistor case. Obviously, a temperature
     probe (insulated!) would be better as it would be able to make measurements
     while the HOT is powered. You can also use a cheap thermometer for this
     purpose - attach its sensor to the heat sink near the HOT.
     

   (From: Raymond Carlsen (rrcc@u.washington.edu).)
   

   After installing a replacement HOT in a TV set or monitor, I like to check the
   temperature for awhile to make sure the substitute is a good match and that
   there are no other problems such as a weak H drive signal. The input current
   is just not a good enough indicator. I have been using a WCF (well calibrated
   finger) for years. For me, the rule of thumb, quite literally, is: if you can
   not hold your finger on it, it’s running too hot, and will probably fail
   prematurely. Touching the case of the transistor or heat sink is tricky….
   

   Metal case transistors will be connected to the collector and have a healthy
   pulse (>1,200 V peak!) and even with plastic case tab transistors, the tab will
   be at this potential. It is best to do this only after the power is off and
   the B+ has discharged. In addition, the HOT may be hot enough to burn you.
   

   A better method is the use of an indoor/outdoor thermometer. I bought one
   recently from Radio Shack for about $15 (63-1009). It has a plastic ‘probe’ on
   the end of a 10′ cable as the outdoor sensor. With a large alligator clip, I
   just clamp the sensor to the heat sink near the transistor and set up the
   digital display near the TV set to monitor the temperature. The last TV I used
   it on was a 27″ Sanyo that had a shorted H. output and an open B+ resistor.
   Replacement parts brought the set back to life and the flyback pulse looked
   OK, but the transistor was getting hot within 5 minutes… up to 130 degrees
   before I shut it down and started looking for the cause. I found a 1 uF 160
   volt cap in the driver circuit that was open. After replacing the cap, I
   fired up the set again and monitored the heat sink as before. This time, the
   temperature slowly rose to about 115 degrees and stayed there. I ran the set
   all day and noticed little variation in the measurement. Test equipment doesn’t
   have to cost a fortune.

   Oscillation or Ringing at HOT Base?

   “Could this be happening because I’m using the wrong HOT?”
   

   At these relatively low frequencies your scope probe is probably not suspect,
   although you should keep loops small and not underestimate the effect of high
   dV/dt inside the line transformer.
   

   First, how do we know if the oscillations were already there with the
   original HOT ? I suppose you have never measured that …
   

   Second, it is NEVER a good idea to replace a HOT with just any other
   type because there can be significant differences which are not at all
   visible in the spec. Even for specialists it’s quite difficult to
   optimize the drive conditions for a new HOT type. You’re on your own here.

   Third, some oscillations are normal because the inductance of the base
   drive transformer does form part of a damped resonant circuit. Usually
   these oscillations show after switching OFF the HOT and they can be a
   problem if the negative Vbe is insufficient during the flyback pulse
   and the transistor might be turned back on, which would kill it.
   As a rule of thumb, Vbe should be -2 V during flyback, but there are
   exceptions. Depending on the HOT type, some damping resistor may have
   to be applied. I’ve never heard of oscillations during HOT ON.
   

   If the collector waveforms (V and I) seem OK and the HOT does not
   overheat then maybe you shouldn’t worry too much. But do check at high
   beam current too (max. brightness on a white picture) because then the
   HOT must be switched on earlier to provide the magnetizing current to
   the line transformer too.
   

   (From: Alan McKinnon (a.mckinnon@pixie.co.za).)
   

   I’ve run into this kind of thing several times recently on different
   sets, this is what I’ve found:
   

   1. Do you have a damper resistor (about 30 or so ohms) across B-E of the HOT. It can go open circuit, causing weird stuff.
      

   2. Check the supply feed resistor into the transformer of the line drive circuit. These can go high, especially if it’s a high value resistor
      dropping a supply of 120 odd volts down to 30 or so.
      

   3. Check all small resistors and caps in the line drive circuit, take then out and measure them, in circuit reading are funny in this area.
      

   4. Try a new drive transformer.

   
   

   ---

   
   
   
   
7. Back to Deflection Systems Table of Contents.

   Additional Deflection System Information

   Web Resources on Deflection Systems

   Here is an article originally from Sencore:
   Understanding
   the TV Horizontal Output Stage. I have archived it at
   Sam’s Copy of
   Sencore’s Understanding the TV Horizontal Output Stage in case the
   link dies. While specifically written for television sets, most of it
   also applies to monitors.

   Why are Nearly All Horizontal Driver Circuits Transformer Coupled?

   Almost every TV and monitor in the universe uses a small high frequency
   transformer to couple the drive signal from the horizontal oscillator to the
   horizontal output transistor base. There are several reasons why this is so
   popular:
   
   • One (probably secondary) reason is that this provides one of the isolation barriers between a line-connected HOT and flyback primary
     and the signal circuits of the TV.
     

   • A more important rational is that a transformer is nice easy way of impedance matching the horizontal driver circuit (100s to 1000s of ohms)
     to the few ohm input impedance of the horizontal output transistor
     base which requires upwards of several amps for proper drive. A typical
     driver transformer may be in the 5-10:1 turns ratio representing 25-100:1
     impedance ratio.
     

   • A byproduct of all this is that it is unlikely for a faulty driver stage to kill the HOT. Unlikely but not impossible.
     

   However, there are apparently some smaller TVs that use a direct coupled
   drive scheme but these are definitely the exception.

   S-Correction Circuits on Multi-Scan Monitors

   (Quoted text from a curious but frustrated tinkerer/technician.)
   

   “Hi there. Recently, I’ve encountered this circuit (or ones similar to it)
   several times in VGA monitors and I’d like to know what it’s purpose is.

   
                  +-------------------+-----------------+
                  |D                  |                 |
                 _+--+                |                 |
              G||  __|__             _|_              __|__
        S0 ----||  _/_\_ IRF620      ___ .64 uF       _\_/_
               ||_   |                |                 |
                  +--+                |                 |
                  |S        .64 uF    |B                |
        Ground o--+-----------||------+---o * See Text  +------o To +76 V
                  |S                A |C                |
                 _+--+                |                 |
              G||  __|__             _|_              __|__
        S1 ----||  _\_/_ IRF620      ___ .64 uF       _/_\_
               ||_   |                |                 |
                  +--+                |                 |
                  |D                  |                 |
                  +-------------------+-----------------+
   

   The IRF620′s are N channel Enhancement Mode MOSFETs.
   

   The connection at this point is to the HOT collector via the horizontal deflection yoke (HDY), width, and linearity coils.”
   

   You will probably find that 0, 1, or 2 of the MOSFETs will be turned on
   (S0, S1 set to ground or +15 V (typical) depending on the scan rate. This
   would introduce varying amounts of S-correction (E/W) depending on the
   horizontal scan rate.
   

   The diodes are probably for clamping and protection. The MOSFETs are used
   as switches. You, in effect, get 1, 2, or 3 x .64 uF between the yoke return
   (the point marked ‘*’) and ground.
   

   This circuit is from an auto-scan computer monitor where it switches the
   effective S-correction capacitor to one of 3 possible values depending on
   scan rate.
   

   Note that this approach is used both in multi-scan and auto-scan monitors.
   The multi-scan (largely obsolete technology though still used in some specific
   applications) has two or more discrete line frequencies and the auto-scan has
   a whole range of available frequencies. You would not guess this from the
   discrete switching of the S-correction capacitors. Obviously, the S-correction
   is optimized only for 3 frequencies and approximated for all others. This is
   generally good enough (even) for auto-scan.
   

   The internal diodes anti-parallel to the MOSFET (A to S, K to D) are important
   to set the DC voltage over the extra S-capacitors to a reasonable value before
   they are first switched in. At lower line frequencies the extra capacitors are
   permanently switched on.
   

   For each given scan frequency there is only one correct value for the
   S-correction capacitor. If you limit your circuit to only 3 discrete values
   (like 0.64, 1.28 and 1.92 uF) then the horizontal geometry can be optimized
   for only 3 scan frequencies.
   

   Somewhere between these 3 frequencies there will be 2 switch-over points where
   more S-correction capacitance is added by the switches. Generally, around
   these border frequencies the geometry will be worst (because whatever the
   position of the switch, there will be an error one way or the other) and it
   would be wise to switch over near some little-used frequencies and thus
   optimize for the much-used frequencies like 31, 38, 48 and 64 kHz. For the
   other scan frequencies, the geometry will then be only approximately correct.
   Luckily, S-correction is not very critical.
   

   For any given scan frequency, the position of the MOSFET switches is constant:
   either on or off. These are not used in a switched mode.
   

   There will be one or two other MOSFET switches that switch at a line scan
   rate in order to take care of the correct dependency of the (average) supply
   voltage and E/W voltage in function of the scan frequency.
   

   There is a difference in the horizontal deflection circuit of a TV versus
   an auto-scan monitor. In the diode bridge of a TV, the deflection circuit
   is in the upper half of the bridge and the “bridge circuit” is in the
   lower half of the circuit. The E/W amplifier sinks current from the
   centre node (and dissipates). The VB supply voltage is constant.
   

   In the diode bridge of a auto-scan monitor, the deflection circuit is in
   the lower half of the bridge. This makes the S-correction capacitor grounded
   and thus easier variable (without need for level shifters, opto-couplers,
   etc). The E/W amplifier sources current into the centre node. The VB supply
   voltage is made variable, linearly proportional to the line frequency, by
   means of a switched-mode down-converter topology. The same topology is also
   used for the E/W amplifier, which now actually delivers power to the circuit.
   With this topology you can have a constant (even regulated) EHT and a
   constant deflection current amplitude, independent of the line frequency.
   

   “In another similar monitor, this circuit had completely self-destructed
   and taken out a few other things too (fire). The monitor shuts down
   immediately unless the HOT is disabled or connections A, B, and C are all
   broken (no other combination works). It almost seems like it’s used to
   accommodate or change the horizontal frequency for different video modes.
   For some reason, the monitor is now powering up so seems like it might be
   an intermittent short somewhere.”

   By disconnecting A,B,C you effectively take the coils of the horizontal
   deflection yoke (HDY) out of the circuit. By disabling the line transistor
   (HOT) there remains only DC voltage over the bridge- and S-correction
   capacitors. Either way there is no voltage over the HDY. I would suspect a
   short or arcing in the deflection coil. Just a wild guess, though.
   

   There is enough energy present in the deflection circuit (also in the HDY)
   to create a very nice fire, let there be no doubt about that. A loose contact
   in series with this circuit is potentially fatal.

   More on Horizontal Driver Circuits

   Usually, the primary voltage is constant when the driver transistor is ON
   and thus the HOT is OFF. Then when the driver switches OFF, the stored magnetic
   energy switches the HOT to ON.
   

   This is called non-simultaneous base drive, which is most common. The primary
   voltage that you see then is mostly a transformed version of the secondary
   voltage, over the series base impedance. The voltage at HOT=ON is not forced
   from the primary side.
   

   Usually the “cold” side of the primary of the driver transformer is connected
   via a power resistor and filtering electrolytic capacitor to the B+. The R
   determines the average voltage and thus the base drive current. Higher R means
   less base drive and vice-versa. Varying the value of this resistor is the
   first choice for minimizing the power dissipated in the HOT.
   

   The filtered DC voltage at the cold side is (typically) a little over
   1/2 * V(B+). Then when the driver switches off the voltage at the hot side
   (of the primary) will be higher, but typically not higher than V(B+). But how
   high exactly is a coincidence.

   Why do Some Monitors Fail if Driven at the Wrong Horizontal Frequency?

   I think it should not have failed. It is the purpose of the deflection
   processor to guard that the line deflection never runs at a forbidden
   frequency. It is possible that such protection is not good enough,
   that when the PLL is not in lock it might generate a very irregular
   line drive. That can prove to be immediately fatal to a line transistor.
   All it takes is one line length that is too long, followed by a flyback
   pulse that is too high, and it’s all over. Second breakdown, terminal.
   

   Trying a too high line frequency is usually not harmful, generally your
   EHT will be too low and your deflection current amplitude too.
   The HOT might eventually die from overheating due to bad drive conditions,
   but otherwise any decent monitor should be able to withstand it.
   In the better cases it simply refuses to sync on an illegal frequency.
   

   1. While increasing the frequency of the horizontal drive *all other factors being equal* should not result in HOT death, all other factors are not
      always equal. The sync circuits may select an improper set of voltages,
      the drive waveform (as mentioned) may be insufficient, etc.
      

   2. The sync may lock to a submultiple so the drive may be at 1/2 H resulting in a too long on-time and poof.
      

   As noted, this can be a one shot affair - absolutely no warning. My
   recommendation remains to attempt if at all possible to obtain the
   specs for any monitor you intend to use.

   Tweaking the Deflection Rates in a Fixed Frequency Monitor

   Pulling a fixed frequency monitor by more than a few percent will likely
   be a problem. I know this is not the answer you were looking for but
   getting a new inexpensive video card may be a better solution.
   

   If not, you are looking for an adjustment called horizontal oscillator,
   horizontal frequency, or horizontal hold. If you do tweak, mark everything
   beforehand just in case you need to get back to the original settings.
   There is some risk - changing it too far may result in damage either
   immediate or down the road - I have no idea.
   

   Here is a discussion about modifying a monitor to run at other scan rates than
   for which it was originally designed (in this case, a lower one):

   “I would like to convert IBM XGA2 (39.4 kHz JH) monitors (9515, 9518) to work
   at VGA (31.4 kHz H) rates.
   

   I have the schematics, and with datasheets/application notes from chipmakers’
   websites altering the scan-frequencies and the mode-switching logic has been
   relatively straightforward.
   

   That’s as regards the ’small-signal’ bits (and frame output), to leave a
   39.4 kHz scan for 75 fps graphics and provide 31.5 kHz for the 400-line DOS
   text (VGA startup) mode, and 350-line graphics.
   

   But the HOTs blow up at 31.5 kHz: after a while in the first of a number
   of 9515s, almost instantly in 9518. The corpses and heatsinks are very hot,
   as in ‘high temperature’ as well as function :-((”
   

   Blowing up the HOT after minutes is due to bad base drive, or (much more rare)
   due to LOT core saturation. Blowing up a HOT immediately is due to too high
   collector pulse.
   

   “The base drive looks fine on a ’scope as regards holding the HOTs (or now an
   experimentally-substituted forward diode) firmly saturated for the longer ‘on’
   time, and then sharply/cleanly turning them off.”
   

   There’s more to a good base drive than just ‘looking fine’ on a scope. The aim
   is to minimize the dissipation in the HOT. This requires just the right amount
   of base current and reducing it in a particular manner at the end of each line.
   (A process that we call ‘de-holing’, removing all the holes from the
   high-impedance collector region, this must not go too fast.)
   • Too little base drive current will cause the collector voltage to start rising too early, before the beam has reached the right end of the screen.
     The HOT goes out of saturation too early, causing dissipation.
     

   • Too much base drive current will cause the collector current to reduce too slowly during the beginning of flyback. The HOT goes out of saturation too
     late, also causing dissipation.
     

   Usually for lower line frequency you need a bit more base drive.
   

   “I have only a rather vague mental picture of how a LOPT works. I think that,
   as a load on the HOT, it is almost purely inductive from turn-on (though not
   from the EHT-generating turn-off).”
   

   If there are secondary scan-voltage rectifiers then they add to the load during
   (beam) scan. Otherwise it is always inductive. During flyback, the flyback
   capacitor and the secondary flyback rectifiers cause the characteristic flyback
   pulse shape.
   

   “Hence I suppose that the longer ‘on’ period allows the current to
   increase to a higher (and destructive) level.”
   

   Not the higher current in itself is destructive, but the increased magnetic
   energy is. During flyback, 1/2*L*I^{2 is converted to 1/2*C*V}2 in the flyback
   capacitor. More energy means more voltage over the HOT. Over 1500 V peak it
   will be destroyed immediately.
   

   “If magnetic saturation were to be reached, it would even look more like a dead
   short than an inductance.”
   

   That’s true, but it occurs mainly due to excess beam current. Secondary
   flyback load causes the primary scan current to rise.
   

   “My questions are twofold: (1) how correct is my understanding?”
   

   Fair enough. You can get a fair idea by running simulations with ideal
   circuit models. But to understand what happens inside a HOT you need very
   advanced models. Which don’t even exist (yet)!
   

   “(2) More to the point, what solution is there?”
   

   You’re doing a good job guessing. I’ll let you continue.
   

   “One that springs to mind is to reduce the voltage of the HOT’s supply,
   something like proportionally to the increase in ‘on’ time, so that the peak
   current is back to what it is at the higher frequency.”
   

   Exactly. Peak current and more importantly peak voltage, because then
   also the EHT will be kept constant.
   

   “This would be a non-trivial task, both to provide a second voltage (at quite
   high current) and to switch between them.”
   

   Maybe not trivial but a lot easier than you think.

   “Though some multi-frequency monitors have such multiple (switched) supplies,
   I don’t think all do: what alternatives can be adopted? How does one
   calculate in this area, or is it (unfortunately) a matter of having LOPTs
   intended for the purpose?”
   

   The LOPT must be optimized for the higher line frequency and the corresponding
   short flyback duration. For lower line frequencies, you want to reduce the B+
   voltage so that it is proportional to the line time. This is in fact very easy
   if you use a switched-mode down-conv