CurtPalme.com Home Theater Forum

[RogerH]

I noticed something weird while getting ready to do a wear-balancing burn on my other green XG-852 (AKA Runco 991) CRT.

While doing the setup photos I noticed some black spots on the CRT (not the glass cover plate) along with a thin vertical black line. The spots almost look evenly spaced vertically, and the thin black line is slightly to the right of the line of spots. These show up as magenta blemishes on a white screen.

The photo shows most of the spots and the line with some magnified views that I Photoshopped in.



My impression is that the spots are on the surface of the CRT rather than phosphor blemishes, but I could be fooled if the glass of the CRT face is not as thick as I think.

Has anyone seen this before? It looks like what might be left behind by inadequately squeegee cleaning the CRT before bonding the cover glass, if that sort of thing is done.

If it is just dirt, would I be able to carefully cut out a chunk of the black silicone seal on one edge of the glass seal in order to get a Q-tip head through the hole in? I don't know anything about the silicone seal (or if that's what it is) or the spacing of the cover glass, or if there is anything but air in there. But I am adventurous and I can work carefully.

I need advice from someone who has seen this or has been under the CRT cover glass.

[Nashou66]

The way the ones are on the left all in a row i would say Spot burn or something similar.

Athanasios

[CIR Engineering]

They look like phosphor burns to me.

Has the projector been shut down improperly before or has the power ever been cut to it while you were watching?

craigr

[ecrabb]

Deflection burns usually end up right in the center (or a line in the center depending on which deflection circuit dies)... But, it would be weird if it was anything else in a pattern like that.

If you have a nice bright flashlight, you should be able to move the beam of the flashlight around and look from the side, such that if the burns aren't on the phosphor and are instead some sort of material in the LC chamber or on the front of the tube glass, should cast a shadow on the phosphor.

Try taking a picture like you took, but with the projector off and the flash on the camera.

SC

[RogerH]

These two projectors are new to me, so I don't know their running history except for their 1300+ hours and what I can see worn in the tubes. I do know that they were run stacked with identical source material, so if there was any burn it must have been unique to one projector only-its mate shows no similar problem.

Under closer examination with a magnifier, the largest spot (directly under the one I have magnified in the pic) is very black and sharply delineated-sharper than I would expect from spot burns (you can't judge this yourself from the photo which has imperfect focus). The sharp and irregular spot outlines compared to the beam spot size points to contamination, not spot burn.

I expected that this was on the CRT surface, but a distant point source of light (so that all I can see is the sparkle from the phosphor granularity) gives no indication that these are blemishes or contaminants on the outside of the CRT face. I also don't see any shift in the magnifier's focal plane between the phosphor coating and the spots or blemishes, but if the CRT glass is thin I might not be able to make any assumptions from this.

Again, the spots are way too detailed under magnification to have been created by a beam. My guess is that it is contamination inside the CRT face (under the phosphor-a manufacturing defect), or simple debris on the outer surface of the CRT itself that is giving no indication of shadow or parallax against the phosphor grain.

Does anyone know how thick the face glass of this type of CRT would be (not the cover glass, but the tube itself)? This could tell me if I can rule out surface dirt specks that might be fixable.

Maybe my CRT has some dead pixels...

[RogerH]

ecrabb,

This is an AC, not LC projector, but I am wondering if I am misunderstanding how AC and LC differ. I was assuming (perhaps wrongly) that the space between the CRT and the cover glass is air filled on mine (as well as the space between the first lens element and the CRT) .

Is this right?

[ecrabb]

No, AC and LC are both liquid cooled and have a fluid chamber in front of the CRT face. The difference is that in the LC projector, one side of the chamber makes the first optical element, while in the AC projector, both pieces of glass are flat. Both AC and LC are arranged: | phosphor/glass | glycol fluid | glass/plastic |

The glass on the tube face is pretty thick - probably 3/16"+. You'd see a shadow with a light if it were anywhere beyond the phosphor plane itself.

The spots certainly could be contamination or de-lamination, but that should be more organic, and not in a clear pattern like you're showing. Don't discount spot burn because it looks too sharp... Don't forget that the CRT spot size is much smaller than what you actually observe with a running projector because of blooming and nearby phosphor lighting - that's why CRT's ANSI contrast is so bad. A collapsed deflection circuit will result an amazingly sharp burn on the phosphor - much sharper than the image you're used to seeing.

SC

[Curt Palme]

Check to see if there is a VDC sticker on the green tube. It could be a rebuild.

If this is from the guy I'm thinking of, he bought one set off me, and had the other already. He was doing a stack, and ended up with issues on his original set at some point after he stacked them. (don't quote me on that, it was a couple of years ago).\

VDC rebuild tubes did have phosphor issues, typical of the dot you're seeing, but not the line.

[Curt Palme]

Check to see if there is a VDC sticker on the green tube. It could be a rebuild.

If this is from the guy I'm thinking of, he bought one set off me, and had the other already. He was doing a stack, and ended up with issues on his original set at some point after he stacked them. (don't quote me on that, it was a couple of years ago).\

VDC rebuild tubes did have phosphor issues, typical of the dot you're seeing, but not the line.

[RogerH]

ecrabb & Curt:

These are not VDC tubes. I have every reason to believe that these are low-time, original and unmolested projectors (even though used a bit hard in their short lives).

What ecrabb is saying about the liquid on the tube face makes sense. I didn't see the point of the first lens element in an LC coupling to the cover glass if there was an air space between it and the CRT. Thanks for pointing that out.

I am also thankful that I didn't find that out on my own via the method I proposed earlier...

Unless phosphor burn can look like jagged, black, and broken/irregular specks of charcoal under a microscope, I'm calling this contamination crap under the phosphor. There is no parallax or shadow movement as lighting changes position. If phosphor burns to pitch black in discrete granules, it is possible, but the detail on the speck is something like an order of magnitude finer than a scan line width, and there is no gradient around it

Whatever it is, it sounds permanent. It's hardly noticeable at the moment against the sheetrock screw heads on my shop wall.

Actually, now that I think about it, I'll bet the specks are overspray from that fantastic black Runco/Earl Sheib paint job on the NEC skins. Paint went everywhere, and they clearly didn't believe in spending more than a half a minute masking. It is (or should be) an embarrassment.

[CIR Engineering]

If you look at your photo you can see the scan lines in the photo. The scan lines are thinner than your spots. I don't know why you think the spots are larger than the beam. It sure looks like spot burn to me.

It isn't paint overspray. Nothing can get in there once assembled.

Also, the fact that you aren't seeing shadows means that the problem is on the phosphor, be it a contaminant or a burns.

Really it makes no difference at the moment if it's contaminant or burn. What I would do if I were you is keep my eye on it carefully. If you start seeing more of these it is most likely being struck repeatedly. At that point you can try and figure out what is causing the burns. If you see no new ones just don't worry about it.

craigr

[RogerH]

The paint overspray comment was an attempt to be funny, hence the emoticon. My fail. I do understand that paint, even from a bad Runco paint job, will not get inside a CRT.

The point I was trying to make earlier is that you cannot see the detail in the photo that I see under the microscrope. What I see is something like this quick Photoshop drawing:



If it is burn, then burning looks very grainy on a microscopic level, or it burned a chunk of phosphor right off the face of the glass. I am saying that the details that I see seem way too small for the sharpest beam to have possibly burned.

[ecrabb]

Interesting... So, there's a whole row of regularly-spaced dots like what you just posted?

SC

[RogerH]

The dots are not perfectly spaced vertically but they do seem to all lie on a line. There are only 3 or 4 dots. This imaginary line is offset horizontally a bit from the faint visible line that you see at the top of my original photo.

I know that the overall pattern reeks of an image burn, but when I look at the spots under high magnification they just look like contamination or internal mechanical damage to the phosphor.

I just examined another tube under the microscope that has some seriously brown logo burn, and it certainly looks nothing like what I have described here.

[km987654]

[CIR Engineering]

[RogerH]

I finally studied the spots under high magnification with external lighting (instead of having the CRT on), and I am going to have to agree now that they could be burns. There is a repetition to the spot positioning that had to have come from a scan flaw.

I can see the individual particle sizes of the phosphor coating adjacent to the dark spots, and it looks like individual jagged particles get cooked and leave the adjacent, mostly thermally isolated particles untouched. This could account for why I am seeing jagged edges that are much finer resolution than the spot size itself could burn. I always thought that the phosphor was milled to a much finer powder, and I thought the grain that we see on images was really more from internally etched glass. It looks like the phosphor powder is quite coarse, on a microscopic scale.

Curiously (maybe related, maybe not), I did find that C4411 and C4412 on the V-Drive board for this projector (XG852) have been replaced, which is surprising for a 1300hr chassis. These 220uF 63v caps filter the power to the vertical STK amp. I have seen these caps fail by shorting (probably from heat/age) and taking out one of the series resistors R4401 or R4403, stopping the vertical drive but without destroying the STK amp. It would have to be a very odd, partial vertical failure to contribute to burn spots like this, but it does makes me wonder if there is a connection. I always replace these two caps (and the corresponding two 25v caps for the horizontal centering STK module on the same board) just as a preventative/maintenance measure, but I would guess that there was some kind of failure at some point that was attributed directly to these caps.

Anyway, I am satisfied (but not happy) that these spots are not repairable, so I can get on with my life. They are no worse than a few isolated dead pixels on a digital, and at least they aren't stuck ON.

[CIR Engineering]

[jkruger]

Watch a few thousand movies and then see it it has gotten any worse. If nothing has changed, then replace the tube. If it has gotten worse then find the problem causing the spots.

[Curt Palme]

I completely agree, run the set for 100+ hours as it is, then if they haven't gotten worse, assume an internal tube flaw and replace tube.

Check the red G2 connections on the HV board and tube socket, as a loose one can cause that.

[Curt Palme]

I completely agree, run the set for 100+ hours as it is, then if they haven't gotten worse, assume an internal tube flaw and replace tube.

Check the red G2 connections on the HV board and tube socket, as a loose one can cause that.