[vol.2]

Originally Posted by Username1

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Hi!

Hello! Happy Thursday morning!

An Observation - At the edges of the screen on both the right & left it seems
that where the scan lines end it looks a bit brighter - Is it just the picture
artifact, or is it real?

The "bright" part is not real, but there is a real dark line in that spot on the edges of the screen on both sides. The Sharpness is creating the light artifact on the edge of those dark lines.

In your scope picture it looks like your scan is 60hz, frame rate, If you set it
to sync to horiz sync and scoped the video amp or any of the color drivers
you are looking at, I think you would see the vert lines in each line scan.
Can you count the ripple on what should be a flat line?

Not really. I tried to do that, but it looks like the ripple left over from the flyback derived power supply occurs out of frame, and I don't particularly see a distinct waveform that reflects the lines I see on the raster. If I display a dark grey (that shows the lines on the raster) and look at the cathodes (any of the three), the scope looks pretty flat (during scan time) with just some random noise. If it's in the video signal, it's very subtle because I can't see it on a scope. It may be that it's masked by my probe. Not sure.

That's why I initially thought this was not in the video. Am I correct in thinking that it has to be in the video signal if I can effect it with the Sharpness control?

Sharpness knob is a single knob attached to the NTSC IC:



And here in the schematic, it's pin 51 (in the lower lefthand corner)

You may want to find similar bright or dark lines on the tv screen across the entire
face of the screen, count them up, and multiply times 15,734 to see what the
frequency might be, and then use some math and see what inductor & cap.

So, I think I did approximately this by using the cursors on my scope to measure the ripples present on the power supply in various places, and it seems like it's somewhere between 250kHz-600kHz. The lines are more obvious on the left side of the screen, and basically disappear (on the raster) by the time they become about 350kHz.

I tried to dampen them by putting a 0.22uf ceramic cap near the 1ohm resistor that feeds the 12V supply on the video amplifier circuit. I succeeded in damping the flyback ripples from about 300mVpkpk to about 230mVpkpk, but it made zero difference to the raster that I could perceive.

pair makes a filter close to that frequency and those components may
be the trouble spot..... You know that tv has a LOT of coils in it.

This TV does NOT have a lot of coils in it, and almost all of the coils are in the RF section, which I am not at all using in any of these tests. There is a relay switch that isolates the RF side of the signal and switches the set to composite mode.

The non-RF coils are the choke in the main AC rectifier and three small transformers used in the power supplies, the audio and the deflection.

There are zero coils on the video input board and the RGB board which handles the OSD mux and color outputs to the neck board.

There is exactly one inductor on the neck board, L521, and it is between the 178VDC supply and the three collectors of the output transistors heading to the cathodes.

And that voltage source that you have growing on the neck board is another
possible noise injection point. There is probably a focus voltage showing
up on that board to do it's business, and is it possible it's drooling a
few stray electrons around....?

What does the fact that the Sharpness effects the signal say about that hypothesis? The Sharpness happens in the NTSC IC all the way back in the signal chain. Could it effect the lines on the screen if they were sneaking into the signal at the cathodes?

You may also want to do a little unorthodox troubleshooting for this problem....
Take a 1/2" size nut (steel not a squirrel kind of nut) mount it tightly on a
wooden dowel and carefully move it around near all the goodies that can
be a problem area, like the coils, horiz. everything, output, flyback, etc.
Keep a good eye on the tv screen while doing it..... You know that
white screen above with the sharpness turned up. You never know
you may find a spot that changes something.... You might also
try a small magnet on a stick...... (Weak magnet)

Sure I can try that. I have a couple of those small convergence strips with tiny weak magnets in the end. I can tape one to a wooden dowel and wave it around inside. They are already plastic covered anyway, and this is a small set so easy to do by myself.

Another thought I has was that maybe this is due to the nature of the set being a hot chassis that has an optically isolated composite video input. I had a thought that maybe this has to do with either the opto IC getting weak, or possibly a side-effect of the floating ground used for the opto-input section. Maybe this is all weird grounding issues caused by that arrangement?

You can see here that is has a separate little ground loop for the input: