On 31 Mar 2005 03:12:15 -0800, (Shaun Davy)
put finger to keyboard and composed:

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