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List Info Thread: File - Monitor and Tv
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ehv.ce.philips.com) for
his extensive contributions to this document.
Safety
------
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.
2.2) 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.).
This document addresses the basic principles of operation of
these types of
deflection systems. While most people with any familiarity
with TV or monitor
opration or repair have some vague idea of how these
circuits work (probably
just enough to be dangerous), many are incorrect or at least
very incomplete.
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.
Chapter 3) Horizontal Deflection System Fundametnals
3.1) 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.
Since the flyback is constructed with an air gap in the
core, it behaves more
like an inductor than a transformer as far as the primary
drive is concerned.
There are scan rectifiers and the coupling factor with the
primary is decent.
But they 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. The
flyback rectifiers
(especially the EHT) draw from the stored magnetic energy.
When the secondary
load increases, also the magnetisation current will
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.
_ _
\/
_/\_
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.
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 VB
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.
3.2) Basic deflection circuit operation
The following description is only the basics. For more
information, see
the article by David Sharples in "Electronics
World", June 1996.
A very simplified circuit is shown below - many components
needed to create
a practical design have been omitted for clarity. First
concentrate only the
portion of the schematic shown below to the left of the yoke
components:
B+
o
|
+
)|:|
Part of T2 )|:|
Flyback )|:|
Primary )|:|
)|:|
+
| <- Yoke
components ->
+-------+---------+----------+
| | | |
Horizontal | | | +
Drive | C | |
)|:|
T1 B |/ __|__ _|_
)|:| L2
--+ +-------| _/_\_ ---
)|:| Horizontal yoke
Driver )|:|( |\ | D1 | C1 +
Stage )|:|( Q1 | E | Damper | Snubber _|_
C2
(not )|:|( HOT | | | ---
S-correction
shown )|:|( | | | |
+--+ +---------+-------+---------+----------+
_|_ _|_
- -
Signal Ground B+ Return (may not be signal ground)
The current in the flyback primary and collector of the HOT
are *not* equal.
The horizontal deflection yoke, damper diode, HOT collector,
snubber HV
capacitor(s), and flyback primary all connect to the same
point. We begin our
adventure at the end of the scan - retrace - when the
flyback period begins:
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 that does not
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.
Thus, the current in the flyback (ignoring the yoke
components) is a nearly
perfect sawtooth. The ramp portion is quite linear due to
the essentially
constant B+ across the flyback primary inductance. The
current waveform can
be easily viewed on an oscilloscope with a high frequency
current probe. See
the section: "Probing TV and monitor yoke
signals".
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!
3.3) The deflection yoke connection
So, you ask: "Why can't the yoke just be placed in
series or parallel with the
flyback primary?"
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).
Note that some TVs and monitors cut off power to the
horizontal deflection
circuits if the yoke connector is removed. This is a
separate interlock
and not a result of the B+ flowing through the yoke. Its
purpose is to
protect the circuitry and the CRT. With no deflection, the
very bright spot
in the center of the screen would quickly turn into a very
dark permanent
unsightly blemish. With appropriate precautions to avoid
this costly
situation, it is possible to power a monitor or TV with the
yoke winding(s)
disconnected to determine if a defective yoke is messing up
the deflection
system operation. See the documents: "Notes on the
Troubleshooting and Repair of Computer and Video
Monitors" (or "...Television Sets") for
additional information.
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.
3.4) What are S and E/W or N/S correction?
These terms actually refer to the various corrections to
deal with what is
normally called scan linearity and pincushion distortion.
Most larger TVs
and nearly all high quality monitors will have various user
and internal
controls to optimize the corrections for each scan rate
(multiscan monitors).
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 sereous 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.
3.5) S-correction circuit operation
The first correction to apply, in both directions, is
S-correction. By simply
putting a capacitor in series with each coil, the sawtooth
waveform is modified
into a slightly sine-wave shape (the top and bottom are
somewhat squashed).
This reduces the scanning speed near the edges. Linearity
over the two main
axis should now be good.
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.
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,
800x600 at 56 Kz (35 KHz), and 800x600 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.
3.6) N/S and E/W correction circuit operation
Then remain the N/S and E/W errors, meaning that near the
corners the scanning
speed is still too large.
To a large extent the N/S errors can be corrected by a
suitable 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 pin-cushion.
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
->
+-------+---------+---------+----------+
| | | | | (Vb)
| | | | +
| | | | )|:|
| __|__ _|_ | )|:| L2
| _/_\_ --- | )|:|
Horizontal yoke
| | D1 | C1 | +
| | Damper | Snubber | _|_ C2
| | | | ---
S-correction
| | | | |
| +---------+---------)----------+ (Vm)
| | | |(cross) |
| | | | +
| C | | | L3 )|:| L4
B |/ __|__ _|_ _|_ Bridge )|:|
E/W coil
-----| _/_\_ --- --- coil )|:|
----
|\ | D2 | C3 | C4 +
---- (Vm) E
Q1 | E | Damper | Snubber | Snubber
+----+^^^^+---+-----+-----
| | | | |
| |
HOT | | | | C5 _|_
_|_ E \| B
| | | | Bridge ---
Elcap --- Q2 |---
| | | | cap |
cap | C /|
| | | | |
| |
+-------+---------+---------+----------+-------------+-----+
------
_|_
E/W amplifier (PNP)
-
B+ Return (may not be signal ground)
The deflection supply B+ gives a constant voltage Vb. At the
output of
the E/W amplifier there is a variable voltage Vm. Because
there can not
be an average voltage over any coil, the average voltage
over the
deflection circuit (L2 + C2) is Vb-Vm. The scanning width is
proportional
with this voltage Vb-Vm. By modulating Vm as a
field-frequency parabola
which is higher for the top and bottom lines it is achieved
that the
scanning width is reduced for the corners of the screen.
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 scre