TDA2579 No Vertical O/P

Mar 31, 2005 2 Replies
S

Hi,



Im working on a NEC TV that has no vertical deflection (white horizontal line). There is no O/P on Pin 1 which is the vertical drive to the deflection amp (TDA3654). I have replaced the TDA2579 chip and still no drive on pin 1. Checked pin 3 with external cap and there is a proper sawtooth waveform there. Pin 2 has about 1.8V on it taking it just short of blanking the sandcastle O/P. Pin 4 (vert def current) appears dead which i assume would be due to no drive to vert amp and then no O/P. I have lifted off pin 1 of the TDA2579 and still nothing.



Any idea's to what could be causing this? there is horizontal defelection so chip has to be powered up. Could there being no feedback cause it to have no O/P, therefore the vertical amp possably being dead?



Cheers, Shaun


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On 31 Mar 2005 03:12:15 -0800, snipped-for-privacy@iinet.net.au (Shaun Davy) put finger to keyboard and composed:

I'd check the supply to the vertical amp. There could be an OC fusible resistor.

As for your horizontal white line, did you need to adjust the screen voltage in order to see it? I ask this because the TDA2579 datasheet has this to say:

"The IC also contains a vertical guard circuit. This circuit monitors the vertical feedback signal on pin 2. When the level on pin 2 is below 0.35 V or higher than 1.85 V the guard circuit inserts a continuous level of 2.5 V in the sandcastle output signal of pin 17. This results in the blanking of the picture displayed, thus preventing a burnt-in horizontal line. The guard levels specified refer to the zener diode reference voltage source level."

- Franc Zabkar

Please remove one 's' from my address when replying by email.
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S

Im pretty sure i checked the supply to the vertical amp.

Also the white horizontal, i didnt have to turn up the screen to see it. Even with the screen turned right down it was still visible.

Just puzzled as to why i have no O/P on the TDA2579. If the blanking circuit kicked in shouldn't it leave the defelection drive intact. My only thought was that there was no O/P due to there being the wrong levels at the feedback pins, but the chicken and the egg theory seems to prevail. Do i have feedback that is not letting the chip O/P, due to their being no O/P in the 1st place.

I hope this makes sense to some people :-) Shaun

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