Job Number

0210-60

Equipment type

Television – 14inch

Maker + Model #

Seleco 14SM112

Made in

Malta !

Client Info

Donald Parsons

Date in

11-Oct

2002

Date of repair

16-Oct

2002

Date out

18-Oct

2002

Comments

BAD

1

 Turns to standby mode after few seconds

2

 During the few seconds on, there was no raster

GOOD

1

 Good sound and so tuning

Full repair description and observations

1

It was noted the the line output transistor and an associated resistor were getting very very hot within few seconds.

2

 The resistor was found to be 5,6k (7W) but the transistor – S2055, was found to have low voltage drop between base emitter. However when I bought a new transister it gave the same readings between its pins. Replacing it did not solved the problem as expected

3

 When the high tension power supply branch which supplied the Line was disconnected from the power supply, the tv did not turned to standby. This clearly showed that the fault was in the Line section.

4

 During measuring the voltages of the power supply it was noted that on standby, the voltages where normal (130V, 18V, 13V) but when the tv was turned to operation mode, the voltage gradually and quickly fell until a threshold was reached and the TV turn to standby mode. Probably the voltage on the  microprocessor fell below 5V and so the Tv turned to standby mode.

5

 I decided to check the line output transformer before starting analysing in depth the line circuit, and I was lucky because it was found that the primary of the LOPT was short circuit and hence explained why the Line output transistor got very hot.

6

The transformer replacement was an HR7520 and it was replaced, after the tube was discharged accordingly

7

 On switching on, I had a very bad experience. There was no good picture, although the sound could be heard. There were strange patterns on the white picture. On increasing the Screen/G2 from the LOPTX, caused strange shrinking and pattering of the raster, while adjusting the focuse casued the raster to just go white, and sometimes an arc inside the tube could be heard.

8

I was thinking that some IC, like the chroma processor TDA3301b was broken since it was not outputing picture info, but I had no oscilloscope to check the signals. I could only compare the DC voltages, and the results are shown here:

TDA 3301b (Chroma Processor)

It is clearly shown that there is no colour output from pins 14, 17 , 20. But it is not because the IC is not working properly, but because of the beem limiting / contrast adjustment voltage, which is set too high (max 3.3V). The problem is that the beam limiting is trigered and is shuting the processor to work for protection!

9

On analysing the line output transformer, a critical mistake was found. The original transformer focus cable colour was red and the screen/G2 cable colour was black. The new HR equivalent colours were inverted (red = screen, black = focus). Hence I had connected the focus voltage to the screen, and the Screen voltage to the focus!

10

I exchanged the wires to the correct connection, and the Picture came perfect (thanks God that I did not made any damage to the tube!)

11

The focus and brightness of the picture where adjusted from the transformer until a sharp picture was obtained.

Repair summary

 Short circuit in the Primary of the Line Output Transformer

Precautions, fouls and advises

1

Never assume that the colours of the wires of replacement transformers re the same, check for the wire thickness (Focus having the strongest voltage, has a thicker wire),and if available the cable names on the transformer pots.

1

When connecting the screen cable  to the focus anode and the Focus to the Screen anode, the following effects were observed.

1)       On power up, sparking/arcing noise was heard at the crt base for 5 seconds and no raster was formed. Then the sparking stopped and the raster was formed

2)       When the screen pot was lowest value, a greyish vertical bar (20% of picture width) was shown

3)       On increasing this the bar widened, to a grey strange shaped raster (RBC shape!) and then a full raster. On increasing more, the greyish raster turned to white in the form of forming vertical blinds (about 8) which started thin, and then widened and merged into each other forming a complete white raster

4)       On increasing further, an internal arcking in the tune would be heard and the tube will go green and turn back to very bright white. Adding further was risky after hearing this spark, and so did not increased further

5)       On inreasing the focus pot (connected to G2) caused the rster to go white gradually

6)       When there was a white raster, it had strange very pale (nearly unnoticed) colour patterns in a symetrical fashion.