ELECTRONICS CORNER
My Repair Jobs
General information
Job Number |
|
0108-28 |
|
|
|
|
|
||
Equipment type |
|
Television - 14 inch |
||
|
|
|
||
Maker + Model # |
|
Seleco 16SB312 |
||
|
|
|
||
Made in |
|
|
|
|
|
|
|
||
Comments |
|
Chasis BS700 |
||
Symptoms observed
BAD |
1 |
Horizontal line at
centre of screen (loss of vertical deflection) |
|
|
|
|
2 |
Line about 66% of the
total screen width (loss of horizontal deflection amplitude) |
|
|
|
|
3 |
Horizontal line not very
bright |
|
|
|
|
|
|
GOOD |
1 |
Powers up, and operates
from standby mode (good logic channel display) |
|
|
|
|
2 |
EHT present (since of
white horizontal line) |
|
|
|
|
3 |
Sound OK |
|
|
|
Repair notes
Full repair description and observations |
|
|
|
1 |
The supply on the
vertical Deflection IC was measured and found to be 12V. It was assumed that
it is OK since the circuit diagram was not available. So it was decided to
replace the IC. On doing this, the fault remained the same with no
improvement. |
|
|
2 |
An oscilloscope was
required to trace the waveforms. It was noted that there was no vertical
deflection waveform on the scan coils as expected, but there was neither
vertical drive from the vert+ horiz sync/drive, IF IC that will be referred
to as the ‘jungle’ IC. |
|
|
3 |
The jungle IC was under
suspect, since was not producing any vertical drive pulses. Since a circuit
diagram was available, the voltages on the 28 pins were measured and
compared. The results are summarized in section F (Measurements). Only 2 pins
were not as expected, the vertical drive output (0v) and the VCR On/Off,
which remained always 0V on either a channel, or AV, but the latter problem
was not concerned with the main problem and so ignored for the moment. |
|
|
4 |
However, it was noted that the supply voltage on the deflection IC
should be 24V were actually it was 12V. This voltage was supplied by the line
output transformer (LOPTX). The other voltages given by the LOPTX was
measured and they were all found to be half of the expected voltage as shown
below: D304 - RGB Driver (collector
supply) = 98V ( expected = 200V ) D305 – Vertical deflection
Supply = 12V ( expected = 24V )
D306 – Tuning Voltage
supply = 22V ( expected
= 43V ) D307 - LOP transistor collector Supply = 60V ( expected = 120V ) |
|
|
5 |
The line output
transformer is either being loaded or it is receiving a low input operating
voltages. Each branch of the LOPTX was one by one disconnected to identify
any loading problems, but none of the 4 branches resulted in any improvement.
|
|
|
6 |
Hence the problem should
be either something in the input or the LOPTX itself. The waveform on the
Line Output transistor (BU508) was found to be as expected. However the
collector of the BU508 was receiving 60V instead of 120V. This explains why
all outputs are half as expected, and why the horizontal line on the screen
is shorter (about 66%) of the screen length. |
|
|
7 |
Some confusing results
were that on standby, the BU508 collector voltage raised to a normal value of
120V, but on activating the TV to turn on (by pressing remote control or Prog
+) it dropped to 60V. When disconnecting the 120V from the BU, the voltage
remains at the Switch mode supply rectifying diode. Both of these indicate
that the fault is not in the Switch Mode Power Supply section since it is
giving the 120V. |
|
|
8 |
However, the LOPTX was
found to be OK, and so the problem was around the 120V supply branch. This
branch had 2 decoupling (dc filtering) capacitors of same value. These were
disconnected and tested, and one of them was found to be dry (1uF instead of
47uF). |
|
|
9 |
On replacing a good
capacitor, all the LOPTX output voltages came well, but the LOPTX did a
strange sparking noise for few seconds and no raster was formed. On
inspection it was noted that the Focus voltage wire (from the LOPTX)
de-attached itself from the CRT base and came free. It was soldered back in
place. |
|
|
10 |
On turning on the TV, no
raster was formed again, but on turning the G2 pot near to max, a bluish /
violet raster, about 2/3 of the height (with flyback lines) was formed. |
|
|
11 |
On adjusting the height
and vertical shift controls, the picture height was adjusted accordingly,
while there was a dry joint at the collectors of 2 of the RGB driver
transistors. Once these were fixed, a grey white raster was formed with
flyback lines (since the G2 pot was turned near to max) but there was no
picture no sound, both with either an RF input, or a video signal from the
scart plug. |
|
|
12 |
A RF signal was tuned, (which was fairly weak) and the Composite
colour video signal (CCVS) was traced
with the oscilloscope to find any abnormalities. Initially, it was found that
there is no CCVS output from the Jungle IC. Again the fault suspect fell on
the IC, but when checking the Ic voltages and several waveforms, everything
was found to be good. Then a video signal was applied, and this time the CCVS
output was found. On applying again the RF signal, a stronger station was
found and the CCVS was present. Following its path along the circuit, there
was both good Luma and Chroma signal input to the RGB processor IC. Hence the jungle IC was ok. |
|
|
13 |
Now, the suspect fell on
the RGB processor IC, since there were good input signals, good supply, but
no RGB outputs. The video output stage (at the CRT plate) was examined and
tested and there were no signs of obvious faults. Then the voltages of the
40-pin RGB Proc Ic were measured and compared, and the results are given in
Table 2 in section F of this document. |
|
|
14 |
It was immediately noted
that the Contrast, Colour and Brightness had the minimal voltage from their
range allowed. There was no picture because these picture controls were all
present at their minimum. |
|
|
15 |
The sound, brightness,
colour and contrast were increased from the remote control and the picture
and sound came well (finally). However when switching off, the settings were
not memorized. The microprocessor was working properly since otherwise the TV
would not start and tune stations, the Digital to analogue converter was
working properly since it converted the digital input from the microprocessor
to continuous steps of dc voltages when increasing any of it controls that it
is responsible for, and so the suspect fell on the memory IC. After some pre
eliminary checks around the memory, everything seemed to be OK. |
|
|
16 |
Then, on studying the operation of the TV, it was discovered that to store the settings to memory, the user should press a special button on the front panel. On doing this, the picture and volume controls were maintained in memory even after switching off an on again the TV. The original fault (loss of voltage supply) made the memory to loose all its settings. Finally the stations were tuned accordingly one by one, and the TV worked well for hours under test. |
Repair summary |
DRIED CAPACITOR IN HT POWER SUPPLY OUTPUT BRANCH |
Precautions, fouls and advises |
|
|
|
1 |
When
there was no vertical, I assumed that the vertical lC is faulty without
checking how much supply it should take from the IC datasheet |
|
|
2 |
When
there was no Composite video, I assumed that the sync Ic was defective, were
in fact it was because there was a weak tuned station |
|
|
3 |
When no
RGB output was given, I assumed that the RGB processing IC was faulty, were
in fact it was that there was no Brightness, colour and contrast control
voltages (set to minimum). |
|
|
4 |
The
Vertical defl IC came in various model of the same part number, so to prevent
useless visits to electronic shops, it is much
better to remove the IC and take it at the shop to assure that it is
perfectly identical to the new one. |
Other measurements and technical comments
1 |
Voltage measurement on the JUNGLE IC: TDA 4502A
A = Vertical drive output -
found to be 0 V instead of 2.6V, no waveform was found either. B = VCR ON / OFF control voltage - should be 5V or 10V if AV / RF
Channels are selected Respectively. On both AV,
and RF channels selection, voltage remains constantly 0 V |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2 |
Voltage measurement on the RGB Processor IC: TDA 3301
* Colour, Brightness and Contrast control are set to their minimum
values, hence no picture is seen (R,G,B outputs low) |