Job Number

0108-28

Equipment type

Television - 14 inch

Maker + Model #

Seleco 16SB312

Made in

Comments

Chasis BS700

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

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.

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)