Are these things I should consider bad

[jr_tech]

Originally Posted by Electronic M

The CRT shields on Zeniths of that era almost completely hide the dag.

Good enough! I have never seen one in person and thought that the HV filtering (and ringing suppression) would be really poor if it were missing from a sloppy re-build job or from somebody cleaning all that "dirty stuff" off of the CRT.

jr

[old_tv_nut]

Originally Posted by Electronic M

The CRT shields on Zeniths of that era almost completely hide the dag. I should know I have at least two Zeniths that use that CRT shield.

thanks for this comment - made me look again, and I now see that apparently the picture shows the internal dag (?). If the external only is underneath the shield, then it is probably OK because none of the external components, labels, etc. are disturbed.

I still think the dag is a wild goose chase for anything having to do with the jail bars (isn't that the one thing we are trying to fix here?).

Need to stop shot-gunning and try to follow clues - there is a jail bar pattern in at least some of the CRT leads, and there are questions as to why some of the waveforms are fat (high frequency or low frequency superimposed?). Should be chasing down if those scope waveforms are valid and going from there.

[TinCanAlley]

Originally Posted by old_tv_nut

thanks for this comment - made me look again, and I now see that apparently the picture shows the internal dag (?). If the external only is underneath the shield, then it is probably OK because none of the external components, labels, etc. are disturbed.

I still think the dag is a wild goose chase for anything having to do with the jail bars (isn't that the one thing we are trying to fix here?).

Need to stop shot-gunning and try to follow clues - there is a jail bar pattern in at least some of the CRT leads, and there are questions as to why some of the waveforms are fat (high frequency or low frequency superimposed?). Should be chasing down if those scope waveforms are valid and going from there.

I'm ready and willing to do any testing you guys come up with. I'm going to disconnect the lead for the setup side of the setup switch to see if there's some kind of issue in the switch. It seems to be a pretty basic open design switch, so I cleaned it with Deoxit. That didn't help as I still have the bars and I still can't get a setup line from the red or green and only a faint line from the blue.

[TinCanAlley]

Never mind. I missed a couple of points of connection, so I'll have to start over. I thought I was able to delete posts, but I don't see how.

[TinCanAlley]

Okay, spent the better part of today looking into why the setup doesn't produce a line. This is what I found out, but not why or how to fix.....

The G1 voltage controls brightness and the higher the voltage, the dimmer the output. The voltage should be around 142V for normal and actually measures out around 147 - 150V. That's not too bad. It's when the setup mode is entered that things change. The voltage of the G1s goes up to 190V and that must be why I can't see the lines when adjusting the G2. 190V is making it too dim to see.

Now why the voltage goes up that high is beyond me. I looked over the schematics and see an 18K and a 1.8K resistor on the setup side of the switch and am not sure if one of those (or a combination of both) could account for the voltage difference.

[old_tv_nut]

Originally Posted by TinCanAlley

Okay, spent the better part of today looking into why the setup doesn't produce a line. This is what I found out, but not why or how to fix.....

The G1 voltage controls brightness and the higher the voltage, the dimmer the output. The voltage should be around 142V for normal and actually measures out around 147 - 150V. That's not too bad. It's when the setup mode is entered that things change. The voltage of the G1s goes up to 190V and that must be why I can't see the lines when adjusting the G2. 190V is making it too dim to see.

Now why the voltage goes up that high is beyond me. I looked over the schematics and see an 18K and a 1.8K resistor on the setup side of the switch and am not sure if one of those (or a combination of both) could account for the voltage difference.

Something's wrong with your measurements - either you are not measuring G1, or something else is going on.

G1 voltage higher = more brightness. Cathode (K) voltage higher = less brightness. It's mainly the difference between cathode and G1 that controls beam current, so you have to measure both to figure out what's happening.

These voltages *should* change to a predetermined value when you go to the setup mode, so that you can set the G2s for proper cutoff; then when you go to normal, the signals on the cathodes and G1's will produce the right colors.

[TinCanAlley]

Originally Posted by old_tv_nut

Something's wrong with your measurements - either you are not measuring G1, or something else is going on.

G1 voltage higher = more brightness. Cathode (K) voltage higher = less brightness. It's mainly the difference between cathode and G1 that controls beam current, so you have to measure both to figure out what's happening.

These voltages *should* change to a predetermined value when you go to the setup mode, so that you can set the G2s for proper cutoff; then when you go to normal, the signals on the cathodes and G1's will produce the right colors.

This is what I got out of a book on CRTs.

"The control grid (G1) controls the brightness of the CRT. It is generally
placed directly on top of the cathode. If there is no voltage on G1, the
electrons can flow freely from the cathode. If there is some negative voltage
on G1, the electrons from the cathode are repelled somewhat, and the screen
appears darker. The greater the voltage on G1, the darker the screen becomes, because more electrons reaching the screen equals a brighter image, and less electrons hitting the screen equals less brightness."

I'm attaching the CRT section of the schematic. If I have the G1 information incorrect, I need to know. My understanding is pins 2, 11 and 6 are the G1s. I'm also attaching a pic of a chart from a CRT tester manual.

[TinCanAlley]

Just made a discovery. As most will recall, I can't get the setup line to show. That was until today. I managed to get the line to show, but under conditions outside of the normal setup procedure.

I set the switch into setup mode and there was no line. I then pulled the red tap off, completely and a bright red line showed up. I then did the same to the blue and green and got a bright whitish line in the center of the screen. Now outside of setup, the screen is way too dark to be useful. I then checked the voltage on the blue video output driver (yellow/blue wire on crt #11). It is 188 - 190V in setup with the tap in any of the HI, MED or Lo position. However, when I remove the tap connection it drops down to 132V and that when a very bright blue line shows up.

The Sams says to do this with the taps on "HI", so something must be off to keep the line from showing up with the haps connected.

So, does this new information give anyone any ideas as to what the problem might be? Is there something I should be measuring while the taps are disconnected? The collectors of the video output transistors?

[TinCanAlley]

Okay, another update and question.

Since finding out that disconnecting the screen tap wires from the taps allowed the setup line to appear, I decided to check voltages. I found something I don't understand, and therefore, don't know if it's normal.

I am attaching a pic from the schematics for the video outputs and screen taps. In it you'll notice that the screen taps get voltage from the collector and the 240V boost through a series of resistors. Those are the supply connections. Then there is the tap wire that you move from tap to tap. That wire receives the voltage and it is connected with a resistor to the CRT socket. That lead has no power indicated on the schematics unless it's connected to one of the taps (hi, med or lo). However, when I disconnect the lead and check it, it has 133V on it. If I disconnect the CRT socket and run the set, the lead has 0V. It's the same for all three colors. Is that normal? Should there be voltage on it from the CRT side? On all three colors?

Maybe I'm not fully understanding how this section is designed to work.

[jr_tech]

Originally Posted by TinCanAlley

This is what I got out of a book on CRTs.

"The control grid (G1) controls the brightness of the CRT. It is generally
placed directly on top of the cathode. If there is no voltage on G1, the
electrons can flow freely from the cathode. If there is some negative voltage
on G1, the electrons from the cathode are repelled somewhat, and the screen
appears darker. The greater the voltage on G1, the darker the screen becomes, because more electrons reaching the screen equals a brighter image, and less electrons hitting the screen equals less brightness."

I believe that the above quote is coinfusing, IMHO it should read: " The greater the NEGATIVE voltage on G1 the darker the screen becomes"

jr

[old_tv_nut]

Originally Posted by jr_tech

I believe that the above quote is coinfusing, IMHO it should read: " The greater the NEGATIVE voltage on G1 the darker the screen becomes"

jr

Yes. Hate this sloppy kind of writing, where it can only be understood by somebody who already knows the subject. Properly stated to avoid confusion, it would be "more negative," and not "greater" when referring to a negative quantity.

[boora2]

Ringing,assuming the damping resistor across the line linearity coil is ok,is often caused by the bypass cap in the video output stages being o/c,value usually 10-33uf 250v,make sure,as with everything you don't use chinese sxxt as replacements