Picture
rolling
There
are two types of rolling can be noted to TV picture. Vertical Rolling & Horizontal Rolling. To some faulty sets, both Horizontal and
Vertical rolling will be present at the same time. There are many reasons for this picture
rolling fault. If the local oscillator frequency of the vertical scan output IC drifts more than the allowed limits, vertical
rolling will start. If the vertical
synchronizing signal from the Video Processor section is absent at the
synchronization input terminal of the vertical scan output IC is absent; or its
amplitude is very low than specified; picture rolling will start. If the amplitude of the vertical
synchronization signal is excess (more than normal); rolling will start. Before going deep into the matter, we should
understand, what the video signal is and what is this synchronization signal.
RF (Radio Frequency) signal transmitted from a TV transmitter will have;
* Carrier
wave.
* Luminance
signal.
* Chroma
signal.
* Audio
signal.
* Synchronization
signals.
* Transmitter
identification signal.
Carrier
wave
For
any RF signal transmission, whether it is for TV or Mobile Phone signal; a carrier
wave is necessary to carry signals. All the other signals
mentioned above are modulated to this carrier wave, and is being transmitted as
electro-magnetic waves through air and come in contact with the terrestrial
antenna; and then directed to the tuner section circuit of the TV set; via
feeder cable. The function of the tuner is to convert the RF signal it receives
to IF signal. This IF signal is then
passed to VIF (Video IF) signal processor section for further processing and
amplification. Let us take the case of
video IF.
The
carrier wave is modulated with;
Luminance
signal (the black and white part of the picture)
Chroma
signal (Red & Blue colour information of the picture.)
Here one thing we have to understand is, only Red and Blue colour information signals are being transmitted by the TV transmitter. The Green colour information signal is generated within the TV set, at its chorma matrix section circuit, of chroma processor. The Blue color information of the picture is modulated into the carrier wave as it is, and the Red color information is Phase Inverted. This inversion is done at the transmitter to recognize these two signals by the color matrix section circuit within the TV receiver. The matrix section compares these signals with the Luminance signal, and the difference between these signals are converted to Green color information (all these information are in terms of very small voltages); and then supplied to RGB output section to drive the respective cathodes of the color CRT of a CRT based colour TV or to drive the timing control board (T’Con) in an LCD TV. It is very simple to say this process, but it is much more complicated to explain.
Here one thing we have to understand is, only Red and Blue colour information signals are being transmitted by the TV transmitter. The Green colour information signal is generated within the TV set, at its chorma matrix section circuit, of chroma processor. The Blue color information of the picture is modulated into the carrier wave as it is, and the Red color information is Phase Inverted. This inversion is done at the transmitter to recognize these two signals by the color matrix section circuit within the TV receiver. The matrix section compares these signals with the Luminance signal, and the difference between these signals are converted to Green color information (all these information are in terms of very small voltages); and then supplied to RGB output section to drive the respective cathodes of the color CRT of a CRT based colour TV or to drive the timing control board (T’Con) in an LCD TV. It is very simple to say this process, but it is much more complicated to explain.
Then
come the synchronization signals. These
signals, both horizontal and vertical; are transmitted along with the CCVS (Color Composite
Video Signal). These signals are
responsible to lock the picture to be Rock Rigid at the TV screen. For a PAL (Phase Alternating Line) system;
the frequency of the vertical synchronization signal will be 50Hz. Frequency of the Horizontal synchronization signal
will be 15.625KHz [15625Hz]. These
signals comes from the transmitter will be extremely accurate in frequency and in amplitude. So if we see a vertical rolling
problem to the TV set, can make sure that the components in the TV set has the
fault, and not with the signal it receives from the transmitter.
Vertical rolling only
Both vertical and horizontal rolling
Let’s
take the case of vertical rolling. The
picture of the faulty TV will roll either upward or downward. The speed of the rolling might be very fast,
or very slow. If the rolling speed is
very fast, we will see multiple frames of picture rolls vertically, or seem to
be stationary. If the rolling speed is
low, frames of picture will be seen either goes upward or downward.
What
might be the reason for this fault? I here
try to describe this fault; according to my service experiences so far.
The
amplitude of the synchronization signal might be excess than normal or lower
than normal or does not reach to the synchronization input terminal of the
vertical scan output IC, or the local oscillator section of the vertical scan
output IC is oscillating abnormally, or there is fault to the synchronization
signal path from the synchronization separator output pin of the video processor IC to the synchronization signal input pin of the vertical scan output
IC. These are the main causes.
The
RF signal from the transmitter is processed by the tuner of the TV set,
converted to VIF signal, and then passed to VIF processor section circuit. Usually an IC is used in the VIF processor
section. The function of this IC is to
process the VIF signal; and to separate the vertical and horizontal
synchronization signals out from the CCVS. The horizontal
synchronization signal is then used to process the other functions within the
IC itself, and the vertical synchronization signal is send out from it though
one among it terminal to feed the vertical synchronization signal input
pin of the vertical scan output IC.
The
first check to be done is; is there vertical synchronization signal is present
at the pin of VIF processor IC. [Refer
the circuit diagram of the set to locate this pin]. Measure the voltage at this pin. Check the measured voltage and make sure it coincides with the
voltage marked at the circuit diagram.
If this signal is present at this pin, a slight variation to voltage can
be noted; when the set receive a video signal.
Mesure this voltage; with and without connecting the antenna to the TV
set, while the set have been tuned to a strong local station. If this voltage varies, make sure that the
synchronization separator section circuit inside the IC is functioning properly. You can check the signal wave-form test at
this point, if you have an oscilloscope.
If
the result of this test is OK; we can proceed.
If not, the VIF processor IC might be defective, and further checks to that section circuit should be done to rectify the fault.
If not, the VIF processor IC might be defective, and further checks to that section circuit should be done to rectify the fault.
Let’s
assume, this test is OK.
As
I described earlier, there is a local oscillator section is fabricated inside
the vertical scan output IC. There are two types of oscillators. To PAL-50 system, the local oscillator
frequency of oscillator inside the IC will have a frequency of 50Hz; and with PAL-60
system, this frequency will be 60Hz (50 frames per second and 60 frames per
second). This oscillator frequency is controlled
by an external capacitor; usually 0.15MFD/Polyester. To some sets, VCO [Voltage
Controlled Oscillator] will be used. To
such ones, this external capacitors will be absent. [Refer the circuit diagram of the set to know
what type of vertical scan output IC is used.]
The
most common fault I’ve faced is with loose solder terminals to vertical scan
output section circuit. Check all the
solder terminals; especially, the solder terminals of the vertical scan output
IC. If suspected, de-solder out all the
solder from these terminals, using a solder sucker, and re-solder all these
terminals by applying fresh solder; without making any solder-bridge short in
between adjacent solder terminals. Check all other solder terminals of the
components at this vertical scan output section circuit. Be sure that all of it are OK.
If
vertical rolling still persists, trace the PCB track between vertical
synchronization separators signal out pin of VIF IC to vertical synchronization
input pin of the vertical scan output IC for any break or solder defects.
Check all the other components too.
Check the foil track has any crack, or there is any loose soldered
component/s at this track. Beware; even a slight leak to any of the ceramic
capacious connected to this track can make problems.
Still
rolling is there? If yes,
Connect
the TV to a VCR or CD player; select the input to AV, and play a video tape or
Disc.
A perfect picture now denotes the VIF section circuit has the fault and video processor section is OK.
If rolling is still there, vertical scan output IC is faulty.
Modern circuit uses, a jungle IC. [System control Chroma Jungle]; which have all the main functions. If so, replacement of this IC is the only remedy.
A perfect picture now denotes the VIF section circuit has the fault and video processor section is OK.
If rolling is still there, vertical scan output IC is faulty.
Modern circuit uses, a jungle IC. [System control Chroma Jungle]; which have all the main functions. If so, replacement of this IC is the only remedy.
You
can adopt your own techniques too to troubleshoot. It might be different to that of mine.
Good Luck.
