CurtPalme.com Home Theater Forum

[cmjohnson]

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For those of you who still do such things, here's something for you to look at.


You no doubt remember Bill Blue's guide to setting up Marquee magnetics, as well as Guy Kuo's Holy Focus guide.

I've been doing magnetic setups on literally dozens of Marquee CRT assemblies over the past few weeks, and every assembly has included 2/4/6 pole CPC ring sets.

I've learned a neat trick to ensure perfection in the setup of the CPC rings.

It's as simple as this: Do your final adjustments at MAX CONTRAST.

By doing this, this brings out details in the big round dot at high focus levels (Go straight to 100 focus for this) and
those details in the dots are the final arbiter of the quality of the CPC ring setup.

This only works if your CPC rings have the 6 pole triangularity adjusters.

What you're looking for is a triangular feature INSIDE the out of focus dot. And that triangular feature should be symmetrical and well centered in the dot. It only appears at high contrast levels.

Here's an example of this done perfectly.

Sorry about the scratches, they're on the lens used for testing the assemblies and there's no point in replacing the lens.

[cmjohnson]

Oh, I would also mention that sometimes you can NOT get both perfect raster centering AND perfect magnetic alignment of the focus yoke at the same time. This is because there are eight individual magnets in the focus yoke and if they are not all well matched in their magnetic properties, it will skew the alignment of the magnetic field. This is a deficiency in the design of the focus yoke, and the way to correct it is to send the yokes back to VDC to be recalibrated in their focus yoke calibration fixture.

If you can't get the raster centered up well with good flare with the CPC rings in the neutral positions, your focus yoke has this problem.

[gjaky]

[barclay66]

[cmjohnson]

I haven't previously seen the part about looking for the triangular feature inside the defocused blob at high contrast levels.

If it was there I don't remember seeing it.


I've found focus yokes that were so out of adjustment that they were rejected as unusuable. That's unique to the Thomson yokes
used on Marquees. All the other types seem to never have this problem.

Yeah, you can do raster centering. But if you have to electronically compensate for a major issue with the focus yoke, then
you're going to run the associated circuits harder and hotter and reduce reliability.

I'm glad I have a large stock of focus yokes. I pre-test them and the ones that are too out of whack get put in the bin to go to
VDC for recalibration at a later time.

[gjaky]

I’ve checked on my test Marquee, it indeed have raster centering, as Barclay said, but there is no scale of the control, so how do you know where it is? I mean, if you perform the CPC-Focus centering method how do you know if you not counteracting with an electronic adjustment extreme that was left there before?
Anyway, the static raster shift is one of the least stressing electrical adjustment, the vertical deflection circuit does not generate much heat at all, especially in 16:9 aspect ratio. The horizontal deflection currents are in the range of 10-30A, adding ~100mA static current to those coils for centering will not really change the things much there either
In the NEC PGs there were raster centering magnets mounted on the deflection coils (two magnet rings) so you could move around the raster without any electronic adjustment and without affecting focus, astig etc. On the NEC XG they abbandoned these magnets and used all electronic corrections instead.

As for astig and focus how I do:
I align the focus yoke to the sharpest when the center focus setting is around midpoint. Then set contrast to 100%, display a dot pattern and set focus to 100%, set the 2 pole CPC magnet so the big blob have a bright dot in the exact center, then the 2 pole is centered (you can verify this by setting the focus down to 0% and back to 100%, then the raster/dots should not move away, change shape yes, but not move away). Then set the focus to 0% and continue with 4 pole CPC, and set the blob to most round like, sometimes however this is not possible so you end up with a big triangle, then the 6 pole CPC can bend this out so it would be a hexagon, (but most ofthen you can set the thing pretty much perfect even with the 4 pole) then go back to focus midpoint and I can set raster centering (electronically) now without compromise on astig or focus, without endless iteration on the settings.

[barclay66]

[gjaky]

As it seems on the VDM U4 adds a DC offset to the output amplifiers resulting a vertical centering feature.

[cmjohnson]

The very best and most precise way to set focusing and centering is to start by removing the CPC rings and unplugging the focus yoke.

This allows you to get the best possible mechanical centering without uneven flaring of grid lines due to focus yoke misalignment.

Then plug the focus yoke in and focus for sharpest central image.

Then reinstall the CPC rings. Note, if you can NOT get properly circular dots, your CPC magnets are likely defective. It is not difficult to damage their magnetic field patterns.

I bought a magnetic field visualization sheet off ebay for a few bucks and use it to check the magnetic patterns of CPC rings that
are not acting properly. In every case where the ring doesn't work well, I've found individual rings with obvious field problems.

I've even been able to remagnetize some CPC rings using neodymium supermagnets. With the visualizer sheet, you can see your progress as you practice. I then take apart useless flare only ring sets and use their knobs, and find sets without triangularity, pull the blank center pair of rings out, magnetize them to become 6 pole rings, test, reassemble, and test again on the projector.