TDA2579 No Vertical O/P

[Shaun Davy]

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

 

[Franc Zabkar]

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

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

 

[Shaun Davy]

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

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