CurtPalme.com Home Theater Forum

[Joust]

i didn't notice it either. thats because I paid my measly $10 to become a member and not have to see the adds. ya bunch of cheap bAA$tards

BTW the magnets in the yokes are made of ferrite i think. They are artificially magnetised very much like a video tape is when you record onto it.

[Nashou66]

I found a company that will custom make exact sizes . but i requested a quote for 9 magnets the exact size of OD 2.7 x ID 2.2 x W .5 inches . We'll see what they say.

Athanasios

[tse]

There is alot of technology in the focus mags. As far as I can tell this attachment is the patent for the KD type mags. Special stuff involved.

Scott

[Nashou66]

[tse]

Cool! At least it is close. If there is room on the tube neck to move the magnet back it might bring it closer to perfect focus but that might make it go the other way, too. It will probably take some means of fine focus adjustment to get the projected image right. Fine focus adjustment can be done with a DC power supply with variable current output. Just connect to the static winding and adjust current. Reverse connection if it goes the wrong way.

Scott

[Nashou66]

I didnt even have the winding in the magnet core, that is with a straight magnetic field. I'll try to night moving the magnet core back a bit but i think if i have the astig magnets in there correct positions(more forward about an inch) it might be the thing were looking for.

Connecting the dc source would be the same as the focus control of the red tube in the service menu correct?

i dont have a variable dc source to test it with.

Athanasios

[Nashou66]

The thing i like is the dots being round and the centers bright and perfectly in the center. so maybe the astigs are in the right place farther back?
hmmm?

Athanasios

[tse]

Believe it or not most tubes are very near perfect round dots and beam centering is usually near right even without the little neck magnets. Most need only a very small adjustment.

The statis focus places a DC current throught the winding. 50% is about 0 current, 0% is about -0.2A, and 100% is about +0.2A. That's for the Thompson magnet where the inductance is about 25mH. Lower inductance will take more current to do the same thing.

The formula is 1/2 x L x I^2 = energy

L = inductance in Henrys
I = current in amps
I^2 = current in amps squared

So with 0.2A current and static winding inductance at 25mH the energy is 500uJ.

If the static winding is 250uH it would take 2A of current to change the focus the same amount.

Yikes! Math.

There will be a test tomorrow.

Scott

[Nashou66]

well i think my next step will be putting the coils back into the magnet cores and wire it up to the harness but with out cutting the original thompson coil harness. I'll need to go get some clored wire and some clips to fit. So with all that math mumbo jumbo what your basicaly saying i better hope i have a higher inductance, closer to 25mH than 250microH.

Athansios

[cmjohnson]

By LARGE, when describing the toroidal coil/transformers used to magnetize and then demagnetize the yokes, I mean large
enough that the yoke assembly will completely fit inside the central core space.

This makes the outer diameter of the toroid pretty big, about a foot in diameter, and it's quite heavy.
At least, that describes the one Scott uses to adjust Thomson yokes.


I HAVE seen toroidal transformers in this size range that have been taken out of scrapped industrial equipment. So
you COULD find such items if you look around hard enough, without paying insane amounts of money to get them.

But you will have to find companies that specialize in electronics recycling and surplus, and get them to help you out,
I'd be willing to bet.


The magnets in the Kanto-Denshi yokes aren't all THAT strong. They're some grade of Alnico, and FAR from the strength
of even the weakest neodymium or other rare earth "supermagnets".



Here's a concept: You can do some experiments in making a DC electromagnet as part of your Advanced Frankenyoke
project. Use it as your outer field magnet and dump the original K-D or Thomson magnets altogether.

With a good variable power supply (or three), you could determine the correct static field strength for optimum
performance, and so long as you are sticking with one resolution (focus parameters are affected slightly at different
refresh and scan rates), you'd have a workable solution to the magnet problem. Not exactly an elegant solution,
but a solution nonetheless.

And if your meter is calibrated, you could note the exact current and voltage drive to that coil that gives optimal
results, then send it off to someone who can make magnetic field strength measurements like Scott, and have him
duplicate your voltage and current settings and then have a point of reference for calibrating the customized yokes.


Winding electromagnets isn't tough. It just takes patience. Anyone could do it.


CJ

[tse]

Attached is a pic of the focus magnet that ESP was using when I started working for them in 1982. IIRR it was a flat donut shaped piece of ferrite sandwiched in between two big steel washers. The three "ears" were smaller washers welded to one of the larger ones. Something like this would not be too hard to make in a home workshop. I tried it on a 'LUG tube and it actually worked! I didn't do any measurements or comparisons to more modern magnets, though.

Scott

[antorsae]

Scott,

What would happen if the 15K resistor is not connected? The earlier diagrams in this thread do not include it and yet people seem to be using these mods with success.

BR - Andres

[Joust]

[tse]

When the thermistor is connected it changes the static focus a small amount (<1%) depending on it's temperature. It might change the top (or bottom) extreme of static focus if not connected. Must not make much difference.

Scott

[Joust]

OH Yeah. I've been menaing to try that. But it certainly isn't required.

[cmjohnson]

I've found the CD with most if not all of the original photos in my first posts in this topic, which now are broken links. I'll attempt to restore
those pics and edit the posts to match over the next couple of days.

CJ

[antorsae]

Hi - I have one question for the magnetic experts: as per the instructions in this thread and AVS I opened the shell and reversed the direction of the coils so that the magnetic polarity is the same as with the Thomson yokes. I also had to do this on the G90 yoke as I had the same polarity as the 22-22 one.

My question is: what would happen if I hadn't reversed the coils? I believe cmjohnson mentioned that this was important as otherwise the tube may be damaged but the more I think about it, I think it would not have an effect... as the G90 and 1292 are driven with reversed (as the Marquee would see it) and the rest of the electronics are almost compatible (unless the deflection coils is doing something I don't understand).

I am saying this as I'd like to try the focus coils are they were designed (I suppose the static/dynamic focus closer to the tube face, stig closer to neck boards).

Let me know what you think!

Andres

[cmjohnson]

I now believe that it's merely important that all three coils have the SAME magnetic orientation in the same projector.

Whether it's "north to the front" or "south to the front", it shouldn't make much difference. Just as long as all three are
oriented in the same way in the same projector.

CJ

[antorsae]

Well... I couldn't wait so I tried the focus coils in their original magnetic position (the opposite as in the Marquee) in my test bench projector and THEY WORK! The did not fry and tube. I will install them in one of my Ultras to see how it compares to the "reversed" version.

[antorsae]