...

Those voltages test out as 248VDC and 761VDC. So their not on the low side.

I was hoping someone could look over the schematics I posted earlier and tell me what happens when the setup switch is activated. To me, a beginner, it looks like it does something and collapses the vertical. That's my best guess based on what I get looking over the schematics. If that's what's happening, then the lack of setup line for all but a faint blue is puzzling.

If it comes down to replacing the FB or yoke, I'll live with the bars. I don't want to get that involved. I think for the heck of it I'll look over all the caps and resistors in the 750V boost area. Could be a decoupling cap.

Service switch kills vertical also kills video & gives the 3 video outs a
constant voltage. On this chassis it also adds a crude blanking pulse.
If all the DC voltages are correct you should get very bright service
lines. At that point its 99% the CRT. Sometimes the CRT can look
& test good but be a little weak.
Good news ! Since I was a bench man & only did about 25 of these
sets I ran it by our old road man over a beer. He did a repair on
one once & it had the exact same jailbars. Bad news is the set was
very old & they didnt want to pay to pull the chassis. At least you
know this wasnt self induced & a real failure, possably common.
Just for kicks remove the wire from service switch that go to
jcn of R295 & R297. See if they go away.

73 Zeno:smoke:

you thinking a leaky switch Zeno?

A long shot but yes. When in service instead of video they tie in
a pulse from the FBT. Zeniths had problems with the tiny
remote/man SW & on/off buttons on this vintage getting leaky
& causing random on/off. Rare but they can get leaky & its
gonna be something strange if ever found.

I wonder if anyone here is on the old Electronix pay site ? I bet
the answers there..........

73 Zeno:smoke:

Another *very* long shot, but I gotta ask... Is the external dag coating on the CRT ok and properly grounded? Any shielding around the CRT or Yoke that is not properly grounded?

jr

Okay, I don't know about the coating other than there is a small scratch from where the metal shroud around it must have touched it at one point. The metal shroud has a ground from chassis to it and two from the CRT socket connected to it. How do I go about checking for proper CRT grounding?

Okay, Sunday is the day to disconnect the wire from the junction noted to the switch. So I am to leave it in normal mode without that lead in order to see if the switch has a bad connection and might be allowing current from that lead from the setup side to the normal side, right?

Just thought I'd post a pic of the switch. I don't know if this could cause any problems, but it looks like terminal two of the setup side of the switch is wired to ground, but also the T2 of the AGC pot is also connected to the same terminal on the setup switch. I've circled the ground side in yellow and the power side in red (the lead to be disconnected on Sunday).

So is there any connection between the shroud and the dag coating? It sounds as if the shroud and I'm guessing perhaps spark-gaps at the CRT socket are grounded to the chassis. Might want to ohm check the ground connection to make sure.

jr

Its OK they are using it as a tiepoint for ground.
Yes run it in normal for the test.
The CRT dag usually grounds through a few paths. The DGS shield
has fingers on it touching the CRT, usually a spring mounted wire
across the bell under the shield, sometimes the ground straps
touch the dag. Then usually straps run from the shield to the
chassis, tuner assy, & convergence assy. If it were floating you
should hear hissing, used to happen to B&W's sometimes.

Lead dress keeps me worried here also. Be sure the CRT harness
is well away from any HV or large hoz pulses. Focus, yoke,
and the 2 HV leads. IIRC its pretty hard for that to happen on
this set.
Also bad caps may have caused it. Every S cap that opens
brings up everything from the HOT on up 10%. In the olden days
it usually stressed the divider then the F terminal of CRT
socket arcs. If a bunch of them go it starts tossing lightning
bolts like you read about ! Quite a show........
So over time things may have been under stress.

73 Zeno:smoke:

Here are a couple of pics of the CRT and the shroud w/ground leads. You can see the two from the CRT socket and the one that goes from the shroud to the chassis. If you look closely at the left side of the neck you'll see the scratches, but I don't think they went through the coating. They appear more as surface scratches.

As for the setup switch, I'm gong to use some Deoxit on it first to see if it needs a little cleaning. I figure that being in the same position for so many years and the fact it's an open design, it might be dirty/oxidized.

You should not need to spend much time on the dag connections. The dag appears intact, as do all the wires. Just look for obvious mechanical problems, and wiggle the connections a bit to see if the problem you are trying to fix shows intermittent effects.

So is the horizontal still there when in setup? Reason I ask is that the bars go away.

Also, don't remember if I checked the voltages on the G2s and G1s when in setup mode. Maybe that is a next move.

the bars go away cause the vert is collapsed.

I must be missing something... I don't see any external dag on the tube... is there just a little area of dag on the lower part of the tube where the grounding strap wraps under?

jr

Here's a pic from when I first got her and hadn't been cleaned yet. This metal shield goes all around the back side of the CRT. It had ground straps from the CRT socket and chassis on it.

So what area of the tube does the external dag cover... I am still not sure that I see it. Note the highly visible pattern here where the external dag can be clearly seen *almost* to the yoke: http://upload.wikimedia.org/wikipedi...17px-Crt14.jpg
jr

In the picture you'll see the metal (bronze colored) shield connected with springs and other screw type adjusters/fasteners (you can see a long screw that is used to pull the sections together for a tighter fit). That's the part that in contact with the painted on DAG of the CRT and then connected to the chassis via a yellow ground connector.

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.

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

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.

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.

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.

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.

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?

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.

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.

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