CurtPalme.com Home Theater Forum

[Meemil]

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Hi,

I've just noticed that my RGB zone focus range is pretty poor on some of the sides on different colors and blue's left/right side focus is so wacky I can't even see the scanlines there with internal test pattern. That's not optical issue though as they appear when I ramp the global focus to specific point.
Clearly the problem seems to generate from FCM and I came up with a possibility that the blue caps are weakening. Before I go and purchase replacements, I'd like to ask if that's expected? I've seen similar blue ones go bad on neckboards too.

Thanks, Eemil

[garyfritz]

This may not be your problem, but remember blue is SUPPOSED to be fuzzy. Blue is electrically defocused to spread the beam across a larger area of phosphor so you can get more light out before the phosphor saturates and maxes out its light output. Otherwise blue starts to top out long before you get to 100 IRE, and you end up with less blue (relative to midrange IREs) at high levels. That's what causes the infamous "blue hump" -- you calibrate to a high IRE like 80 or 90, but the blue is relatively stronger in the midrange, so mid-intensity colors look cold and lifeless. With defocused blue, the blue response curve is more like red & green, so you can get more consistent color balance across the IRE range.

[Meemil]

[garyfritz]

OK, I kinda figured that. You can experiment with the blue focus and see exactly where it's sharply focused -- that's when it's the dimmest! Defocus it a bit and it gets a lot brighter.

With a properly-defocused blue, you will get some blue fuzz around objects. It can be annoying around white-on-black text, like movie credits, but it's not usually noticeable in most actual video content. Your eye doesn't see blue all that sharply anyway. Take a look at Christmas lights some night -- red lights are sharp and clear, but blue lights look like fuzz. And nobody defocused your Christmas lights. Your eye just doesn't focus well on blue.

[Meemil]

[dvh99]

[garyfritz]

[Curt Palme]

Seeing scan lines clearly will depend on your resolution, the tube type and what lens you have on them. No question that the LUG tubes are sharper than the LCPs/. The sharpest E'home I've seen is with the LUG tube and HD10E lenses. that's a 9500 Ultra of course. If you're running 1080p and 8" stock tubes (180DMB), I'm not even sure if you'll see scanning lines at 1080p, but it's been years since I played with an 8500 at this point.

[Curt Palme]

Seeing scan lines clearly will depend on your resolution, the tube type and what lens you have on them. No question that the LUG tubes are sharper than the LCPs/. The sharpest E'home I've seen is with the LUG tube and HD10E lenses. that's a 9500 Ultra of course. If you're running 1080p and 8" stock tubes (180DMB), I'm not even sure if you'll see scanning lines at 1080p, but it's been years since I played with an 8500 at this point.

[cmjohnson]

As I recall from my own experiments back around 13 years ago, I found that you can resolve resolutions that are pretty high (2048x1536 or even 2048x2048) on the tube face of an 8500 but the HD-8B lenses will soften that up considerably.

By "Resolve", I mean that in the central area you can visually differentiate the one on/one off scan line patterns.

And that was at a pretty low contrast level. Not a level you'd run when viewing a movie at a normal screen size and brightness.

The tube will do it, it's just a matter of focus retention and optics resolution. Which are very much issues with the 8500.

I'd say that 1600x1200 (full frame) is more of the PRACTICAL limit for an 8500 under normal viewing conditions.

[Meemil]

Thanks for your inputs.

Actually the problem persists even with 31khz internal pattern. I also have hd-145 lenses so optics are not the problem. As I said, when I ramp up the "global focus" (in picture menu) I can get the sides sharp but not with the RGB zone focus adjustment.

[cmjohnson]

Check optical focus and scheimpflug. Look in the lenses and see if the image on the tube face is appreciably sharper at the edges than you see on the screen.

[Meemil]

CJ, the issue is not optical, I've checked the tube face and I see the same thing.

I replaced the caps today and noticed that I can get a tighter focus on the tubes! However, the blue problem persists. The left and right zone of the focus doesn't make any change to picture at all. I tried swapping the blue and red cables going to the focus module and the distortion moved to red. Obviously there's a problem in the board, but I'd really like to figure it out as I've gone and recapped the thing now.

Thanks, Eemil

[dvh99]

swap the focus connector from the blue and green and see if the problem goes from the blue to the green tube.

if so some fets needs to be replaced iirc on the fgm.

[dvh99]

oops just read you have done that. still swapping out those fets could solve the problem.

[cmjohnson]

If you can't adjust any outer zone focus, you have no focus signal getting to the dynamic winding.

Central focus is only a DC signal going to the static winding. All outer zone focus is a waveform amplified on the focus board and sent to the dynamic winding.

It's not impossible that you may have a bad focus yoke with an open dynamic winding but I've never actually seen that happen. The focus signal power level is low and unlikely to burn out a dynamic winding.

Move the focus plug to the green channel and when you then try to adjust green focus, see if blue focus changes.

[Meemil]

Thanks for the helpful feedback!

Great idea Dvh! I don't see any darkening on the fets but I'll go and test them!

CJ, Tube & coils seem fine. As I said when I swapped the connectors I could tighten the dynamic focus areas on blue.

Eemil

[gjaky]

BTW couldn't that be the blue tube is from an old M8000? They say the focus yoke was improved with the newer models.

[Meemil]

Small update: tested the mosfets, all returned good. Wanted to confirm this so quickly swapped green and blue channel's fets
Anyhow, that fixed nothing.

Gjaky: I'm sure that's not the case as swapping the cables between tubes gives nice focus for blue and any color thats connected into the blue channel on FCM gets the unresponsive dynamic focus.

Just too bad that I don't have a swap FCM.

Thanks so much for the responses
Eemil

[gjaky]

OK.
Maybe is this applies to you?
http://www.curtpalme.com/docs/ElectrohomeMarqueeTechnicalBulletin_Tb98-02.pdf

[Meemil]