CurtPalme.com Home Theater Forum

[Nashou66]

Hmm, thats not what I found.

This is the values i measured, remember the static coil in mH not uH, I think this is where you made a mistake.

2-4 52.6 uH
3-5 296 uH
6-8 52.6 uH
7-9 296 uH

I have about 9 barco coils and all are very close.For the correct static you need close to 30mH thats milli Henries and not Micro henries. the dynamic is close. so only the Static needs rewinding.

Now the astig i dont know what to make of it.

the Thompson is about 156uh, the Sony is about 56uh but works, the Barco 3203 is 296. I have not tried that as my test bench PJ did not have an astig, now I do have one. But i need to find my rewound coil, i have a few different versions I need to test them again and mark them.

Athanasios

[cmjohnson]

I'm sorry, I must issue a retraction here. My readings were substantially in error. I was reading .3 mh and .4 mh, and I thought I was
on a different scale.

It does explain the limited range of focus adjustments. However, the static magnetic value is still right.

I'll rewind these coils and report back.

CJ

[cmjohnson]

A funny thing just happened. I rewound the coils on a yoke that was delivering SHARP focus with the coils disconnected.

I wound them to 45 microhenries and 30 millihenries, and the values are right on within 1 percent according to my meter, which
has been checked out at a calibration lab and certified to be within 3 percent on all scales.

After reassembly and installation, that SAME yoke now doesn't focus properly in the center of the raster with the coils unplugged.

Something changed during the assembly/disassembly process. It may not have been much, but it was enough to change its performance.


What's even stranger, is even with the values being correct, I still have very limited actual focus adjustment range. A great deal less
than I'm used to seeing.


I'll figure it out but not tonight. Leave it for tomorrow after work.




Probably I'll just start over with another of the un-modded yokes, and mark it carefully for assured perfect reassembly.


CJ

[dvh99]

https://www.curtpalme.com/forum_archived/viewtopic.php@t=22271.html

its the way the focus module is designed you dont have good focus with the coils not connected to the fgm.
look at scotts post and graph.

[Nashou66]

Chris try this.

First leave the pastic case off and spin the assembly around the tube, focus will change. And if you can wrap the assembly with rubber tape do so,
touching a bare magnet structure on the tube will shock and scare the hell out of ya!

And

[cmjohnson]

I'll try that and also will look into reversing the order of the coils. It's just not right that these measure right to spec and the field strength is really close (but it does need to be adjusted, and I think I can do that) but it doesn't focus right.

I'll also compare DC resistances. It should be no higher than the DC resistance of the stock yokes. If it is higher, that will restrict current
carrying ability and thus efficiency. This would indicate that the wind needs to be redone in a larger gauge magnet wire.


CJ

[cmjohnson]

Problem solved. It helps when you don't mix up the wiring between the static and dynamic coils.

I also metered out the DC resistance of the coils and the dynamic coils are about the same as the stock yoke's dynamic winding,
but the static coils I wound are MUCH lower in DC resistance. which is what we want. That's GOOD. It's about 40 ohms vs. 60 ohms.


Now, the results are every bit as good as I might have hoped for. No blooming at contrast of 100. None. Oh, the lines do get a little bit thicker, but there's no loss of corner definition to speak of. You've seen blooming, and with this, there isn't any.


My results here are good enough that I declare victory. And here's the formula:


Obtain KF3203 yokes. Disassemble them. Remove windings from the RED coil (dynamic) until there are 29 windings remaining,
and then check it with your inductance meter. You want 45 microhenries. One wrap will make a difference. Wrap it back up.

Remove the yellow winding and totally remove all the wire from it. Wrap 790 turns of no. 32 enamelled magnet wire around it, trying
to keep the layers smooth and even if you can but it doesn't have to be a work of art.

Wrap that coil back up, but be sure to splice some heavier lead-in wires to the ends by using some of the scraps you took off one of
the spools.

Carefully reassemble the magnetic assembly and remove the 9-pin header connector from the little PC board. You will connect a
Marquee yoke harness to the coil by soldering its wires in place of the original connector.


If you are careful and look at the little wire connection PC board, it will guide you in what's what. It shows pairs of connections labelled
X, Y, S, and D. S is the Static pair. D is the Dynamic pair. X and Y are the astig windings. The green and brown leads on the
original focus yoke are for a thermistor in the focus yoke which is used to compensate for temperature changes. You can transfer that
thermistor to the new yoke if you want to but I never bothered. So I leave those wires cut and unterminated.


That's my formula for an Optimized Frankenyoke, which seems to deliver everything I was hoping for.


CJ

[tse]

Some light reading material. A patent for one of this type of focus magnet.

Scott

[cmjohnson]

Noting the date of that patent, it has expired as patents are good for only 17 years. Being filed in 1992, it expired last year.

So, it appears that there's no legal obstacle to reproducing THAT design, anyway. If you wanted to invest in the tooling required,
and serve the very small market that remains.

CJ

[cmjohnson]

One thing to note is that if you intend to use the little PC board that's part of the K-D yoke assembly to connect up your Marquee wiring harness, be aware that the PC board is not an expensive one. It has thin traces and the board is just phenolic, and single sided. This makes it pretty easy to rip a trace off the board simply by handling the wires after they're soldered on. That board can't stand much abuse.

This caused me to have to do some troubleshooting for a while as I had a yoke that wasn't doing anything even though it had been
carefully checked during assembly.

I elected to scrape away the solder mask and bridge solder across the breaks I found, and handle the assembly with care after that
point. Once it's installed and adjusted it'll be perfectly reliable.


Maybe a better option would be to connect the Marquee yoke cables to a connector that mates up with the PC board on the yoke, if
that's practical for you.

Incidentally, the astig windings in the KF3203 yokes work perfectly well in a Marquee. No problems at all.


CJ

[cmjohnson]

Based on my experiments over the past few days, one thing I've learned is that the KF3203 yokes are probably a little bit stronger than
optimum, magnetically speaking. Their magnetic field should be softened up a little in order to be closer to optimum for Marquee usage.

I have a collection of KF3203 yoke assemblies that have been magnetized to specific, known, documented strength values. When I put
one in that is the same measured strength as a correctly calibrated stock Marquee yoke, at 294 Gauss, I'm unable to get the corners
and edges to focus. With a slightly weaker one, I can. I'm still working my way through the bunch to see what magnetic strength
gives the best focus values.


CJ

[cmjohnson]

I have to say, it's turning out to be very frustrating to get consistently good results out of these. If you have a FY with the same field strength as a good stock yoke, and with the same coil values as well, it is not any sort of guarantee that it'll deliver superior performance,
in fact, it's not even guaranteed that it'll deliver equal performance as the stock yoke.

I'm now reasonably sure that the KF3203 yokes need to be a little weaker than a Thomson yoke, in terms of basic magnetic field strength.

I have one that is exactly the same strength as a recently calibrated Thomson yoke, at 294 Gauss, and it won't fully focus properly.

Slightly weaker ones will focus but at 254 Gauss, which is the weakest magnet set I've tried yet, it won't even come CLOSE to focusing.
With that one, the grid lines are so fat that they overlap.


It's looking like the happy medium is around 270-275 Gauss, or thereabouts.


I'm going to regroup a little and focus first and only on the static winding, starting with a magnet structure that seems to be optimal.

Once I know what works best for the static winding, which actually may not be 30 milliHenries, then I'll start testing dynamic coils.

In both instances I'll over-wind the coils and then see how they work, then take some winds off to drop the value by 10 percent or so,
and try again.

Once the static and dynamic coils are both doing a good job, independently of each other, then it would be time to try to run them both
together and see how they interact.

But it's important to note that the dynamic coil only provides a small adjustment in focus, relative to the static setting. You simply
won't get massive focus changes out of the dynamic coil.


One simple way to try to figure out what the right strength value is, is to probe the dynamic signal strength with an oscilloscope and current probe, if you have such items. By comparing what a Thomson yoke requires for good edge focus, and what you're seeing
with the FY yoke, you can figure out if your dynamic winding is too large an inductance, or not large enough.

If I can focus the center OR the edges, but not both at once, then this implies a mismatch between the static/dynamic coil inductances that are too great for the drive circuitry to compensate for. If you can focus the center but reach 100 before sharpening up the
outlying grid lines, then the dynamic coil either has too high DC resistance or it simply doesn't ehxibit enough inductance for the job.



CJ

[Nashou66]

Chris from the 3 I have played with i got good focus with the correct static winding values. What I found is that you need to make sure the dynamic winding is as uniform as possible . Otherwise you will have uneven focus on the edges, also make sure the wound assembly order is the same as a stock yoke. that is static first then dynamic, this does help out a little. one other trick is to ream out the end of the plastic bobbin that is on the convergence yoke side so it fits over it when the convergence clamp is removed.

The electronics work where they either add or remove a magnetic field to the focus yoke. So if a yoke with no current going through the coils is focused then you should be able to get good overall focus if the winding is wound evenly across the bobbin.

Athanasios

[cmjohnson]

According to Tse, the ideal situation is a SLIGHTLY defocused image with the magnetics unplugged. The magnetic core alone should be just a little bit weaker than required for a sharply focused center. This is optimal.

Frankly, it's been so long since I've done much work with stock yokes, I can't even remember what typical edge focus values are.


I do have a second projector for use as a test stand, and time is running short for me to get this machine back on the ceiling where it
belongs, so what I may do is simply use my stock calibrated yokes in place of any FYs that aren't cutting it, and swap them out at
a later date when I've got them adjusted correctly.


Athan, you said you are looking for a set of LC tanks for an AmPro 4600? I now have those for you.

[dvh99]

[cmjohnson]

The big advantage of a properly adjusted set of FYs is sharper focus at ANY contrast level, and the ability to retain sharp focus at high contrast levels without any blooming.

Tse has done some measurements on a sample in operation and noted that the beam profile is in fact thinner and more closely resembles a square wave than the typical Gaussian curve as is standard with the Thomson coils. This translates to higher MTF at any given scan line
pitch, and higher resolution capacity.

It's worth pursuing. It's not as easy to get the optimal results as I might have hoped, but there are no insurmountable technical obstacles. It's just a matter of adjusting values until it all locks together. The ideal coil inductance values for Thomson yokes may
not be the ideal values for the KD yokes. It seems that this is the case.

The problem is I'm trying to solve an equation with four variables: Base magnetic strength, static coil inductance and performance,
dynamic coil inductance and performance, and the interaction of the static and dynamic coils when run together.


However, having DEFINED the problem, a solution may be calculated.

The next step is for me to make actual measurements of the static field strength of a Thomson yoke coil while in operation at a value
that results in optimal focus with a typical CRT. With that information it will then be possible to measure the static field strength of
the FY candidate under the same setting conditions, and figure the difference and adjust coil values using the results. Then try again.

I've been pursuing this project using mostly the empirical method, but it's time to switch to the experimental method. I know the
project will work, but it'll take luck or some good research to come up with the proper yoke formula. I'd rather go with research.


I also think that the focus range in the Marquee focus system is a bit larger than it needs to be. If the focus amps can
be scaled back so it's not so excessive, then an added benefit is a greater degree of precision of adjustment within
the new adjustment range. The adjustment steps would naturally be finer.


CJ

[Nashou66]

It really has to do with how well you can layer the dynamic coils Chris. The better you can can get each and every individual strand to line up the better uniformity you'll have in focus from side to side. This is where a professional coil winding machine would do best, just like the ones used to originally wind them. I'm not 100% sure of my theory but from reading some papers on magnetics and magnetic coil construction placement of the wire does make a difference.

Id wind a few more coils with the same value and test those against each other and see what you come up with.

Did you use that online calculator i Posted earlier in the thread?



Athansios

[dvh99]

are you using a proper inductance meter?
cheap measurement devices can deviate 10%.
the following link is for a lc meter that you can make yourself, it is precise and cheap.

http://ironbark.bendigo.latrobe.edu.au/~rice/lc/

[cmjohnson]

My inductance meter is within 3 percent on all scales according to the local NIST-traceable calibration lab. Good enough.


I did use the online calculator but always rely on the meter. I found the calculator be close but in the real world, the coil form
may not be exactly the same as the theoretical one which does make a difference.

It's important to note that if there is a magnetic object inside the coil, it will increase the measured inductance,
so take it off the winder shaft when checking the current value.

My dynamic windings are perfectly laid. What I do on the dynamics is to trim back the existing coil to 19 turns and then measure it,
and then take off one winding to get to 45.something uH as needed.

The static winding comes out at 30.1 mH with 790 turns of #32 wire. I get a good wind but it's hard to maintain a perfect lay for several
layers. Oh, I could do it, but I'd spend an hour per coil just winding them. Maybe if my coil winder wasn't a Chinese POS which works
but the shaft is eccentric and nothing is a model of high quality manufacturing. But it gets the job done.


I'm going to try some dynamic windings that are a little bit underwound, and a little bit overwound, to see what happens.


CJ

[Nashou66]

I also found getting the Dynamic focus the hardest to adjust correctly. One thing I did not add is the thermosistor. That might have something to do with getting the best focus, but that wave form gets added into the static focus not the dynamic. However the two do interact with each other slightly so that could be a factor.

Scott mentioned in another thread that you can adjust the resonant caps for a specific frequency and i assume it go for the
value of the coil used as well. So it might be easier to try to find the perfect cap value for that frequency you plan to use with the inductance vale used on the dynamic coil.

I think C119 is the one used for the range of HD we use, its 68nf now, maybe try 10nf intervals + and - of 68nf. Using a variable cap here would be a good idea to dial it in. I would adjust focus as best as possible with the 68nf cap then remove it and replace it with a variable cap that is maybe 40nf to 100nf and dial it in while watching the grid.

Athanasios