Sylvania 1-076 arrived!

[jeyurkon]

I installed the 12LP4 tonight by moving the bridge to the next set of mounting holes back. By the way, there are another set of holes that would accommodate a 16LP4. That's really stretching a design.

I will have to make new side straps because of moving the bridge back. They're 3/4" too short now. To my surprise, the cross brace still fits.

The raster was shifted to the left. I was able to center it with the horizontal center control, but the raster is out of focus on the right. I had to greatly reduce the focusing field to get it to focus, compared to the 10MP4.

I've included a photo of the dot pattern to show the focus, though it's obvious in the movie photo.

I just noticed that the focusing magnet is pretty far off center vertically. It's centered horizontally. This doesn't seem to match the focus issue.

The yoke is very slightly off axis. It's such a small amount that I can't believe it's the cause. I'll try moving it to see what happens.

The GE data-sheet for a 12LP4 says the center of the focus magnet should be 4-1/2" from the reference line. The Sylvania data-sheet for a 12LP4 says it should be 3-1/4" from the reference line. Mine is 3-1/2" from the reference line. I wonder if I should move it back a bit.

Any suggestions as to what might be causing the left shift and the bad focus on the right would be welcome. It seems to be unaffected by the ion-trap position.

John

[jr_tech]

I think that I also see some keystone from left to right (yoke alignment?) or perhaps some voltage shifts? Are power supply voltages, boost, HV constant ? or do they change with the horizontal position of the beam?

jr

[jeyurkon]

Originally Posted by jr_tech

I think that I also see some keystone from left to right (yoke alignment?) or perhaps some voltage shifts? Are power supply voltages, boost, HV constant ? or do they change with the horizontal position of the beam?

jr

The key-stoning was there with the 10MP4 also, but it didn't have the focus issue and it was centered horizontally. I'll have to check the supply and the boost. The HV will be constant if the power supply is since it's an RF oscillator type. At least it wouldn't be coupled to the horizontal.

It could be the power supply. Since I had to make a large horizontal center adjustment, and the pot that does that also drops the 180V to supply 170V to other sections, it could now be affecting the 170V that I boost to 250V. The 170V is also supplied to the vertical output stage.

Until I'm able to secure the CRT I won't be able to do any measurements under the chassis.

It's probably not related, but right where the screen gets blury is where the right edge of the scan stopped until I recentered it.

John

[jeyurkon]

I scoped the G2 voltage and there is minimal ripple. Perhaps a volt. The G2 is +254V. This shouldn't account for the focus problem. Also, since this is derived from the +180V supply, it shouldn't be causing the keystoning either. I can't get to the -125 supply yet to check it, but it could have ripple causing the keystoning.

The gun does not appear to be a slashed field type. It doesn't seem bent either, but I can't see enough of it to be sure either. This is a rebuilt 12LP4 and the gun is different than the original type. If the end is offset like I see in some diagrams, that might be causing the left shift that I'm having with the raster. I've tried centering the focus magnet better and playing with the yoke with minimal improvement.

Since the gun is different I started to wonder if I needed to use a single field trap instead. The instructions that came with a Miller adjustable single field trap says that CRTs up to 10" or 12" having a straight gun, generally use a double ion trap of 38 gauss. I think by straight gun they mean one with a slashed field.

Then they go on to say that CRTs of 14" to 17" having a bent gun, use a single ion trap of 35 to 45 Gauss. Larger CRTs use a single ion trap of 45 to 55 Gauss.

Since it doesn't appear to be a slashed field type I decided to try a single field type. I had to adjust the field down to 35 Gauss for best results. This is the same strength as the double field trap that I have. With it adjusted to the best position I wasn't able to do any better than I did with the double field trap. With both of them I have to butt it up against the base. I get the impression that it might be better to even move it over the base. Perhaps the field is too strong, but I can't adjust the adjustable trap any lower than 35 Gauss.

The HV is at 8.8KV. The manual says it should be between 9.5KV to 7.5KV depending on the brightness setting. I heard some corona in the coffee can today so I've got to find that. I've suspected it in the past, but I've never been able to see it. I was also afraid of getting my nose too close.

I have another 12LP4 that is rebuilt that I can try. I noticed a strange charging effect when I used the single field trap. I'd adjust it, it would be o.k. for a few seconds and then drift. This didn't happen at the best position though.

If it's a matter of not having the slashed field type gun, I have one unrestored set that has a good original 12LP4 in it. I hate to take it out but I may have to eventually try that.

John

[jr_tech]

Perhaps it is indeed a straight gun...Do you get any beam to the screen with NO magnet?

"I have one unrestored set that has a good original 12LP4 in it. I hate to take it out but I may have to eventually try that."

It seems that many times the most difficult experiment yields the most useful data!

Since you mentioned charging effects, I had another terrible (perhaps far fetched) thought about the key-stoning/focus... since the tube is not aluminized, is the landing energy of the beam constant as it travels across the screen ? Is it possible that the phosphor surface discharges somewhat during retrace?

jr

[jeyurkon]

Originally Posted by jr_tech

Perhaps it is indeed a straight gun...Do you get any beam to the screen with NO magnet?

"I have one unrestored set that has a good original 12LP4 in it. I hate to take it out but I may have to eventually try that."

It seems that many times the most difficult experiment yields the most useful data!

Since you mentioned charging effects, I had another terrible (perhaps far fetched) thought about the key-stoning/focus... since the tube is not aluminized, is the landing energy of the beam constant as it travels across the screen ? Is it possible that the phosphor surface discharges somewhat during retrace?

jr

O.K. I do get a raster with no magnet. I tried the other rebuilt CRT and it behaves the same way. The raster is best without the magnet. However it is still badly focused on the right side.

The keystone is much reduced without the ion trap. I did have a keystone with the 10MP4 also. I suspect it is the -125V supply not being rigid enough, but I haven't tested it yet.

In defense of your odd theory when I had the keystone effect pretty bad with the 10MP4 I thought about charging since the raster would move if I got my hand near the front of the CRT. I blew on the CRT expecting moisture to drain any external charging. The raster blew up and disappeared for a second then returned with only a tiny keystone. I think the small keystone is a supply issue.

I did some research and the only way I can see that I could get a raster without an ion trap is if G2 and G3 are at the same potential. I think this would be extremely unlikely. Especially for both CRTs.

I will have to try the old 12LP4 that is in a Sonora that is further down the restoration queue.

BTW, I did look up the specs for a 12LPC. Although it is aluminized, from the factory it still requires a dual field ion trap.

I also found this page where they converted a set from a Triode to Tetrode CRT. They had to reduce the focusing field, which I also observed.

Something else that is happening now is that I get a bright spot for a couple of seconds after turning the TV off. With a gun that requires an ion trap, the electrons have to have the right velocity to make it out of the gun. As soon as the HV drops they get lost. But the straight gun without a trap doesn't filter by velocity

John

[John Marinello]

I would try moving the focus magnet. Specs for a 12LP4 call for a double-field ion trap, but once they're rebuilt, it seems anything goes. All of the rebuilt CTRs that I've seen used a single-field ion trap. For that reason, I would ignore the focus magnet orientation specs and adjust it for best pic.

[jeyurkon]

Originally Posted by John Marinello

I would try moving the focus magnet. Specs for a 12LP4 call for a double-field ion trap, but once they're rebuilt, it seems anything goes. All of the rebuilt CTRs that I've seen used a single-field ion trap. For that reason, I would ignore the focus magnet orientation specs and adjust it for best pic.

Thanks for the suggestion. I tried moving it further back and that helped with the centering, it improved the focus a little at the top and bottom, but the right is still bad.

It could still be a geometry problem, but that it no longer requires an ion trap worries me. Unless the vacuum is extremely good I'd eventually get an ion burn.

John

[jeyurkon]

No pictures, yet I don't have any time left to work on it today.

I tried the 12LP4A from the Sonora. It was made by NU-Videon or something like that. It was better, but basically behaved the same except that it really does need an ion trap.

Using the double field trap I wasn't able to adjust it properly. At the brightest position there was a shadow. It was not as bright at the position that got rid of the shadow, but the focus was good.

I then used the single field trap that was on the tube. This tube really does require a double, but the single field gave the brightest image without the shadow. The CRT is dim and blue opposite the anode connector. It looks a lot like the damage described in the article I posted about adjusting ion traps where the beam hits one of the electrodes and sputters material on to the screen. I fear that's a result of it's being used in the Sonora with a single field trap.

I think the offset raster and bad focus on the right is a geometry problem. I earlier posted that I was surprised that the cross brace still fit after moving the bridge back. Well, it is slightly off shifting the bridge to the left. I'll need to make a new brace.

It may also account for the bad focus on the right. When I mis-adjusted the ion trap I could get a good focus everywhere, dim raster though. This makes it seem like a depth of field problem. When the ion trap is mis-adjusted, only a small portion of the beam exits the gun lowering it's acceptance. That is, there is a smaller range of angles of electrons leaving the gun. This gives a larger depth of field. With a small depth of field the focal plane has to be aligned properly to be in focus everywhere. With a large depth depth of field the focal plane can be tilted with respect to the screen and still have a good focus.

This is conjecture and I'll have to see what happens when I get the bridge back into its proper position.

I'm still worried about the rebuilds not using an ion trap any more though. I wonder how long they would last this way.

John

[jeyurkon]

I tried the used 12LP4 from the Sonora again, mostly to take a photo and try having the bridge in the correct horizontal position. That's the photo on the left. Ignore the vertical non-linearity, my bad.

The bridge position didn't help the focus. It did turn out to be related to the depth of field of the focus of the beam, but not because of a rotation of the screen around the focal plane.

You'll notice a bluish stain in the bottom right corner. This is opposite the HV connector. The HV connector is on top in the Sonora so this stained region is outside of the normal scan in that set. In the Sylvania I have to have the connector at a 45deg angle so that's why it shows up here. Its a really odd stain/ion burn because when you look at it closely you can see that the phosphor grains are coated on one side. I have no idea how this occurred.

The RCA data sheet for the 12LP4 gives more information than the others. It gives focus and ion trap strengths for 9KV and 11KV. The proper field strength for the ion trap at 9KV is about 14% less than it is at 11KV. That means it should be 30 Gauss instead of 35 Gauss. That's probably why I can't adjust the double field ion trap properly because I'm at 8.8KV. If I try this CRT again I'll make a shunt for the trap to reduce its field. I should be able to do at least 9.5KV with the Sylvania. That's supposed to be less than the peak value obtainable. Perhaps I missed something when I worked on the HV section. Maybe the RF transformer is getting lossy. The Sylvania manual also says to try different 6Y6 tubes since they're not all created equal. Or, maybe it thinks it's a Philco.

Since the focus field should be smaller at 9KV and it should be smaller also for a tetrode CRT than a triode CRT I decided to try to reduce the focus field more than I was able to easily do before. I was afraid that I would have to add more shunts, but there was still some adjustment room left.

Now the focus isn't perfect, but it's fairly uniform across the screen. There is some astigmatism. The focus magnet is close to the yoke at the moment. I will move it farther back which should help and eliminate the shadowing on the left.

The Sylvania is capable of better resolution than you see here, though it's not bad for a set of this vintage. It's mainly a focus problem. If I can get the HV up to 11KV I think things will look much better.

They did a really good job of rebuilding the two CRTs I have. They have excellent emission and are nearly identical in performance. My only concern is that they no longer use an ion trap.

John

[jr_tech]

Great progress!
Wow what a saga! So where did the Horizontal position pot end up ... still pushing the raster hard to the right? Another 1 to 2 KV should sharpen things up a bit if you can get it and still get enough horiz and vertical scan. Does the "shunted" focus assembly produce a fairly uniform field?

jr

[jeyurkon]

The pot is pretty much in a neutral position now. Basically where it was for the 10MP4. I have more than enough scan width left over in both horizontal and vertical.

I don't know how uniform the focus field is. I'll have to borrow a Gauss-meter. I didn't add any shunts. There are two versions that this same chassis uses. One has two adjustable shunts and another has only one. This one has two so it probably helps to keep the field uniform.

John

[miniman82]

Originally Posted by jeyurkon

Or, maybe it thinks it's a Philco.

Hey! My Philco resembles that statement.

[jeyurkon]

Scotty is a great guy! He says the SW30 is the correct gun for a 12LP4, but it is a straight gun and should only be used for an aluminized 12LP4. Since my CRTs aren't aluminized a bent gun should have been used.

He is willing to rebuild the two CRTs again for only the cost of rebuilding the two bent guns. This is more than reasonable since I would have had to pay the rebuilding costs for the guns anyway since bent guns aren't available.

I can highly recommend Scotty for any rebuilding. Also, the emission for the two CRTs were perfectly matched and in my opinion atests to the high degree of control that he has in the rebuilding process.

John

[jeyurkon]

Originally Posted by miniman82

Hey! My Philco resembles that statement.

I was wondering if I'd get a rise out of anyone.