More screen anomalies

[TinCanAlley]

Originally Posted by old_tv_nut

The moire' patterns occur where the focus is a little too sharp and the scan line structure beats with the CRT screen structure. You can show this easily by adjusting the focus in and out or even by adjusting the vertical linearity or height slightly to change the line spacing. Nothing can be done to fix this.

Regarding the jail bars: I lost track of what set this is and how it's constructed. If it has more than one printed circuit board, make sure all the boards are well grounded to the chassis at their ground points. The ringing frequency will be well within the capability of almost any scope - it looks to be 10 or 12 times horizontal, so that's 200 kHz at most. The problem is, if it's getting into the video and caused by ground currents, the scope would need to be hooked to the same ground point and hot point as the video circuit to see whatever the video circuit is seeing. So, you have a huge number of possibilities of how to hook up your scope. Also, it may be difficult to see if it's getting in at a low-level point in the video circuit and your scope does not have much gain.

As others have said, it might not be in the video, but on one of the CRT electrodes. Does the strength increase and decrease as you turn the contrast (picture) control? If so, it's in the video. If not, it may be getting to the CRT through some other path, or getting into the video after the contrast control.

If there are multiple printed circuit boards (or just multiple ground points on one board), it may be worthwhile to connect some of them to each other or to the chassis with ground wires and observe what happens to the jail bars when you do so. You may find some hot spots this way.

Good luck!

There are five boards press fitted on the chassis. I have removed each one, cleaned them, their contacts and reseated them. No effect.

If I crank up the contrast, the dark bars get lighter until the match the existing lighter bars. So if I crank it up, the entire screens becomes over saturated and it's pretty hard to see the bars (pretty sure they're still there).

Should I be looking for ground points only in the horizontal circuit? It's pretty crazy trying to find out all the ground points on all the terminal strips and such. Only the ground points of the modules are noted. All others have to be located manually. Not an easy process.

I'm going to replace the voltage divider as the current only is leaking. On humid days it buzzes and has a blue corona. If you put your finger near it, it tries to reach out and touch you. Could this cause the moire?

I believe the scope I am using goes up to 500 kHz. I also got a new x10 probe. I just have to figure out what ringing would look like when comparing the wave I get to the one in the SM.

[old_tv_nut]

Originally Posted by TinCanAlley

There are five boards press fitted on the chassis. I have removed each one, cleaned them, their contacts and reseated them. No effect.

If I crank up the contrast, the dark bars get lighter until the match the existing lighter bars. So if I crank it up, the entire screens becomes over saturated and it's pretty hard to see the bars (pretty sure they're still there).

Should I be looking for ground points only in the horizontal circuit? It's pretty crazy trying to find out all the ground points on all the terminal strips and such. Only the ground points of the modules are noted. All others have to be located manually. Not an easy process.

I'm going to replace the voltage divider as the current only is leaking. On humid days it buzzes and has a blue corona. If you put your finger near it, it tries to reach out and touch you. Could this cause the moire?

I believe the scope I am using goes up to 500 kHz. I also got a new x10 probe. I just have to figure out what ringing would look like when comparing the wave I get to the one in the SM.

Not sure what you mean by "saturating?" Do you mean the whole picture gets brighter? Does the bar pattern stay visible if you crank up the contrast AND turn down the brightness (to prevent "saturating")?

My suggestion is to try connecting known ground points directly to where they are supposed to go in case the connectors on the PC boards are still not making good contact even though you cleaned them. Also, you can try connecting different ground points on the PC board connectors to each other to see if that helps.

What is the maximum gain (volts/division) that your scope has? Depending on where in the circuit, you may need more gain than your scope provides to see this waveform, especially if you use a 10x probe.

Your scope needs to be synced to the horizontal sweep. If you cannot sync it independent of the probe input, it will be next to impossible to trace the waveform. You definitely then need a better scope. If the scope is locked to horizontal sweep and showing at least one complete sweep cycle, the jail bars will appear as a sine wave of decreasing amplitude from left to right, with about 10 or 12 cycles within one horizontal sweep (as you can count by looking at the TV screen). This decaying sine wave will be added on top of whatever normal waveform is at the place you are probing.

[TinCanAlley]

Originally Posted by old_tv_nut

Not sure what you mean by "saturating?" Do you mean the whole picture gets brighter? Does the bar pattern stay visible if you crank up the contrast AND turn down the brightness (to prevent "saturating")?

My suggestion is to try connecting known ground points directly to where they are supposed to go in case the connectors on the PC boards are still not making good contact even though you cleaned them. Also, you can try connecting different ground points on the PC board connectors to each other to see if that helps.

What is the maximum gain (volts/division) that your scope has? Depending on where in the circuit, you may need more gain than your scope provides to see this waveform, especially if you use a 10x probe.

Your scope needs to be synced to the horizontal sweep. If you cannot sync it independent of the probe input, it will be next to impossible to trace the waveform. You definitely then need a better scope. If the scope is locked to horizontal sweep and showing at least one complete sweep cycle, the jail bars will appear as a sine wave of decreasing amplitude from left to right, with about 10 or 12 cycles within one horizontal sweep (as you can count by looking at the TV screen). This decaying sine wave will be added on top of whatever normal waveform is at the place you are probing.

I have jumpers with alligator clips, would those be good for grounding? I can clip from ground to ground on the modules and then to a good chassis ground.

The scope has a X1, X10 and X100 setting and the probe has X1 and X10. I'm not sure what the maximum is, but I understand this one can handle all the way to the yoke and high voltage.

The scope can either sink with the probe, it's internal (using the AC line, I believe) and an external source (of which I don't have).

Where in the horizontal should I start? Should I start at the HOT and work my way back?

[old_tv_nut]

Originally Posted by TinCanAlley

I have jumpers with alligator clips, would those be good for grounding? I can clip from ground to ground on the modules and then to a good chassis ground...

Yes, these are exactly what you can do to experiment. Be cautious and make sure you don't connect to something besides ground. Turning the set off while setting up the connection is a good idea.

Regarding all the other questions about your scope, I see you are getting a good one to work with. When you have that, you will be able to use one channel/probe to sync the scope to horizontal and the other to look at various points for your problem.

If it is a ground problem, however, you may see the bars change when you hook up the probe ground leads to the chassis or circuit cards, the same as experimenting with clip leads. This could be either exasperating or fun, depending on how you want to think about it!

[TinCanAlley]

Originally Posted by old_tv_nut

Yes, these are exactly what you can do to experiment. Be cautious and make sure you don't connect to something besides ground. Turning the set off while setting up the connection is a good idea.

Regarding all the other questions about your scope, I see you are getting a good one to work with. When you have that, you will be able to use one channel/probe to sync the scope to horizontal and the other to look at various points for your problem.

If it is a ground problem, however, you may see the bars change when you hook up the probe ground leads to the chassis or circuit cards, the same as experimenting with clip leads. This could be either exasperating or fun, depending on how you want to think about it!

About now I'm wishing I had a test jig. This poor set has been on it's side and top so I can get to the underside for testing and repair.

[TinCanAlley]

Originally Posted by old_tv_nut

Not sure what you mean by "saturating?" Do you mean the whole picture gets brighter? Does the bar pattern stay visible if you crank up the contrast AND turn down the brightness (to prevent "saturating")?

If I turn down the brightness, the screen goes black no matter how high the contrast is set. Because of this, I can't see if the bars are still there. Now if I turn the contrast all the way down and the brightness all the way up, I can see faint bars.

[old_tv_nut]

Originally Posted by TinCanAlley

If I turn down the brightness, the screen goes black no matter how high the contrast is set. Because of this, I can't see if the bars are still there. Now if I turn the contrast all the way down and the brightness all the way up, I can see faint bars.

Ok - so basically the bars are stronger when contrast is turned up, which says that at least a good part of it is in the video signal.

[TinCanAlley]

Originally Posted by old_tv_nut

Ok - so basically the bars are stronger when contrast is turned up, which says that at least a good part of it is in the video signal.

I guess I should have asked this a long time ago, but which part of the jail bars is not supposed to be there? The light or the dark bars?

[old_tv_nut]

Originally Posted by TinCanAlley

I guess I should have asked this a long time ago, but which part of the jail bars is not supposed to be there? The light or the dark bars?

Neither. The jail bars are a plus and minus (darker and lighter) (sinusoid) added to the nominal signal. Ideally you would see only the average brightness all across a blank picture.