NAP PTV6XX video problem
 

Hi;

This is another one I dunno if anyone can help on, I know it's hard by
correspondence. I'll do my best to describe this effectively.

I changed the board last week and got a worse problem, loss of sync to the
convergence, but that is not at issue here. This board was shutting down but on
a chance I found out it was the bummer aftermarket flyback. Now I'm back to a
video smear. This is a trailing negative and it gets worse to the right of the
screen.

Sources are normal, CRTs are a bit weak but we have some used ones to install.
I think it actually looks worse than it really is. We might get by without
doing that if this proves to be a video problem. With the other board the video
drive looked about the same but without the trailing negative, so I don't think
it's on the CRT socket boards.

This also occurs on the OSD, so that led me to after the chroma IC. I find the
video OK at Q604 and Q708, but a bit high in amplitude. I found the sub
contrast and brightness are cranked, so I returned them to normal levels.
Streaks are not gone.

I go to Jtest, right by J600 and find the proper DC, but 4V p-p video, but only
on the right side (it's supposed to be .7V) . It looks like a downgoing
sawtooth, but the top is flat. The sources are solid. This must have something
to do with it. Checking the circuit I find it hard to believe this waveform is
like that. These outputs go through a 10 ohm to the emitters of the final
outputs. To cause this effect in the waveform, a capacitance would be needed. I
can find no valid capacitor to account for this. There is a small cap between a
33 and a 47 at the bases of the outputs, but the total resistance is only 80
ohms, with the gain of the output, those outputs from the main board are seeing
a very low impedance, and still pretty low even if the outputs are in
saturation.

I thought the bases must be dropping, but the source to all the CRT boards is
solid, and the resistors etc. on each are seperate. The effect is actually
interactive, i.e. you can turn down the blue drive and see the effect abate in
the green waveform, even though it doesn't affect the green drive, in fact if
anything, lowering one drive seems to give a bit more to the other two. (on the
sceen but not in the waveform)

I can't believe weak CRTs are causing this, one because their load is
negligible to the 6K of collector load they're using, and two the smeared video
was not there with the other board.

For that "sawtooth" clipping of the whites at Jtest to disappear you need to
turn the drives very low, and now that the video levels are in spec up to this
point, it's gotta be right there somewhere. I might put the tubes in it if the
video can be fixed. Also the bias pots do nothing except change the DC level.
Everything else stays the same, and the effect on the sceen is less than I
think it should be.

Any ideas ?

Thanks in advance.

JURB