> A
single thin horizontal line across the centre of the screen. The brightness of it can be altered by
adjusting the brightness control of the set.
Audio OK.
> A
thick horizontal line of about 1 to 2 inches in height across the centre of the
screen. Audio OK.
> The
upper part of the display is OK; but there is no screen at the lower part. Audio OK.
> The
lower part of the display is OK; but there is no screen at the upper
portion. Audio OK.
> Displayed
picture has linearity trouble. Either
the upper or the lower part of the picture is elongated or compressed. Audio
OK.
> No
screen, no raster, now audio to some brand sets like Sony, Hitachi etc:(Beware: With some sets, the power supply section will be directly connected [Live chassis]. In this case, an isolation transformer should be used before start any tests described here.
A
SINGLE THIN HORIZONTAL LINE ACROSS THE CENTRE OF THE SCREEN
The
symptom indicates that there is absolutely no vertical scan signal comes to the
deflection yoke [vertical deflection winding].
The
first test to carry out is; to check the continuity of the winding [vertical
deflection] of the deflection yoke. Make
sure that there is continuity and there is no open circuit. You can check this
with the help of a multi-meter set to Ohms range.
If its seems OK;
If its seems OK;
Switch ON the set and check whether the supply voltage comes to the vertical scan
output IC. Check for it at the main input
voltage terminal of this IC [refer the circuit diagram]. If not, check the current limiter resistor used (if any);
in series with this vertical scan output supply voltage. If it found “open”, make sure that there is
something wrong with the IC used. Before
replacing this resistor, check the DC resistance of the supply voltage terminal
to this IC to Ground. If it measures
very low resistance, check for its reason, as it indicates a short circuit
within the IC or with any external components. It will be best to de-solder out the IC from circuit, and to check.
If
the above mentioned test are OK, check the connections of the vertical
deflection yoke from the main board.
Make sure that the there is no break in this connection, due to bad soldering . One end of the vertical deflection yoke
connection will be connected to the (-ve) terminal of an electrolytic
capacitor, and the other end to Ground, via a small value resistor, usually of
(0.05 to 0,1 Ohm). This resistor is
called yoke return resistor. If this resistor
get opened, there will be no signal path to ground from the deflection yoke,
resulting a horizontal line display.
Check this resistor. With some
sets, there will be more than one resistor fitted in parallel. Check both of them. If anyone among it get opened, it will affect
the height of the picture. Most modern circuit
has more than one resistor in place. In
most cases it might be due to a short circuit within the IC itself. If so, the IC is damaged, and should be
replaced with same number.
If
all the above tests are OK;
Locate
the vertical scan signal output terminal of the IC. (Refer the circuit diagram).
Switch
ON the set; measure the voltage at this terminal. It should be around half the voltage of the
main supply voltage to the IC. If not,
the IC is defective. But, before make
sure about this, check all the external components connected to this IC; mainly
all the resistors. If there is proper
voltage measured at this pin; locate the vertical signal input terminal to this
IC; (refer the circuit diagram).
Touch
that terminal with the blade of a screw driver [beware; not to make a short
circuit in between adjacent solder terminals of the IC; while you do this
test]. After make a good contact to that
terminal with the screw driver blade, just touch the blade of the screw driver
with your bare finger; while watching at the TV screen. This test is very like a “Hum” test that we
do with audio amplifiers.
If
the IC is functioning, the horizontal line display will expand a little (at
about 1 inch at least). If not, the IC is faulty;
non-functional.
If
the above mentioned test is OK, it is clear that the IC do not get vertical
scan output drive signal to its input pin, and you should find the reason for
that. A faulty VIF, chroma IC can be the cause.
Here
one main point I've to specify is; the vertical scan output IC will heat up
while it works. This is true to any
brand CRT TV. Frequent heating and
cooling may make the solder terminal/s of it to get loose contact by dry
soldering. I suggest to re-solder all
the suspected solder terminals; and test the function, before you suspect and replace this
IC. 50% of the fault will be cleared by
this procedure of re-soldering. When
replacing the IC; make sure to tight the seating screw to its heat sink ; and to
use silicon grease (heat-sink compound).
A loose fitting to the heat sink is the most probable reason for its
failure.
A
THICK HORIZONTAL LINE ACROSS THE CENTRE OF THE SCREEN
Indicates
the vertical scan output section circuit fabricated inside the IC is working;
but the drive signal amplitude it get might be very low. First check the input voltage to IC, and make
sure that it is OK. All the other terminal
voltages might be OK; otherwise the horizontal line displayed wont expand. As I have stated above, re-solder all the IC
terminals, and make sure that there is no loose soldering. Try to adjust the height of the display, if a
vertical height adjustment pre-set control is present at the main board. This type of adjustment pre-set resistor can
be seen in older television circuit, but with newer, there will be no such
pre-set resistor. You have to enter the
service mode of the set to adjust all parameters like these.
If
all the tests are OK, and you can’t adjust the height of the displayed line
check and replace the feedback signal bypass capacitor. This capacitor [an electrolytic type; very
low capacitance value] will be connected in between the yoke return signal
point to the linearity – height control pin of the vertical scan output
IC. Be sure, to replace it with the same
capacitance value used, and not to reverse it polarity while soldering to the
PWB. (be sure not to reverse its (+ve)
& (- ve) terminals.). Check the
supply voltage to the IC, and make sure that it is OK. If found lower than the voltage specified in
the circuit diagram; check out for the reason.
If the voltage to very low than specified, this kind of fault can occur.
If
found very low, check and replace the rectifier diode and the supply smoothing
capacitor used. Usually this supply
voltage is derived from one among the windings of the LOT [in most sets]; and
rectified by a single diode and smoothed by an electrolytic capacitor. Either a leaky rectifier diode or a smoothing
capacitor can make this voltage to go low.
It is best to replace these two components along with a new vertical
scan output IC. With some TVs, supply
voltage to vertical scan output is derived from the SMPS. If so, check and rectify the fault there.
All
the above tests are OK; and still you won’t get any improvement; replace the IC with a new one, and check.
THE
UPPER PART or LOWER PART OF THE PICTURE IS ABSENT
IC
fault. As in an amplifier circuit, the
vertical scan output section circuit fabrication inside the IC will have two transistor used. One of it is a PNP type
and the other is an NPN type. If the PNP
transistor inside is short circuited, the upper part of the picture will be OK,
but the lower part will be absent; and vice versa. There is nothing more to check for this
fault. Replace the IC. It is faulty.
DISPLAYED
PICTURE HAS NO LINEARITY
As
I stated above, check all the solder terminals of the vertical scan output IC
for loose contact. Re-solder all the
terminals by applying a little more fresh solder to each terminal; without
making any solder bridge short in between the adjacent terminals. Replace all the electrolytic capacitors used
in the vertical scan output section circuit at one time. Enter the service mode, and adjust the
vertical linearity parameter to make the linearity of the picture.
Service
mode entry, adjust and exit procedure to many popular brand TVs and
other devices are given by this site itself.
Search this site by type in the brand name to your set, in the search
box; at the top right of the Home page; to get gathered the related pages together.
NO
SCREEN – SET APPEARS DEAD – BUT THE POWER LIGHT IS ON
To
some brand sets, there will be no screen, or the set won’t power up, if the
vertical scan output section have any fault or short circuit. Sony and Hitachi are the examples. There is a short circuit detection and
protection circuit to these sets. It
will detect any short circuit, or any abnormality to any of the circuit section,
the main system control IC will detect it, as it is linked with all other
circuit parts of the set. If an abnormality is detected with any part of the circuit, a stop signal will be
provided by the system control to the main SMPS section circuit, to kill
it. To such sets, we have to isolate the
supply voltage to suspected circuit part to proceed further. That is why; repair to these sets seems to be
somewhat difficult.
You
should use your own techniques too to troubleshoot. It might be different to that of mine. Good Luck.
