JustGreg
Joined: 07 Mar 2006
Posts: 3098
Location: Kenosha, WI
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| Posted: Sun Jun 27, 2010 8:36 pm Post subject: |
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| cmjohnson wrote: |
It's not impossible that it could be an unbalanced yoke drive signal, too, which would point at an issue on that channel of the HDM.
I've already ruled that out Chris. I swapped plugs with green.
In the case where it could be due to a focus coil, which is mechanically set up correctly, then it's possible that the static magnetic field
strength from the internal permanent magnets could be non-uniform. In the case of the Thomson focus coils, this is very possible as
the magnet structure consists of six cylindrical magnets between two steel plates, in a squirrel cage arrangement. If some of those
magnets were to be well off their proper strength, it would cause the total magnetic field shape to be off, and geometrically similar to
the field generated by a tilted yoke.
Before I pull the tube to check the deflection alignment I'll try another focus coil. I did check the resistance between the other 2 tubes and there was a very small deviation. I suppose it wouldn't take much either to cause the hump in the image.
Check the yokes. Focus first because they're easier to get to. Check alignment with the coils disconnected. If the sag is still there,
swap the deflection yoke drive cables between this tube and one other. (It's generally best to leave green untouched since it's your
point of reference.) If the sag switches to the other tube, it says bad HDM. If the sag doesn't change, it could still be either the
focus coil or the deflection yoke. swap the focus coils between this tube and another tube. Then try swapping focus coils. If the sag
still doesn't move, then the last thing left is the deflection yoke. In this case it might be better to just pick up a spare deflection
yoke from Curt (I'm sure he's got a ton of them) rather than have to go to the effort of stripping down TWO tube stacks and realigning
both of them later.
I have known good spares of everything. See above reply.
Warning: The HDM generates up to 1500 volts at a high frequency. (The horizontal deflection frequency.) Contact with a live connection
carrying this signal will result in an instant burn almost exactly like poking yourself with a sharp and hot soldering iron. Been there, done
that, have the scar to show for it.
Duly noted. I wear high voltage gloves but even then I treat it all like I'm not wearing any protection.
And, while it is safe to swap focus leads around on a running projector, do NOT attempt this with the HDM leads. NEVER. Even if you
don't get your hands burned off, you'll probably destroy the HDM and maybe much more.
A no-no I was already aware of but it's good that you say it to educate others.
You can mess with convergence, focus, and astig coils and connections while the PJ is running, but under no circumstances is anyone
to ever mess with the deflection coil connections on a running projector. And if you should have to do alignment and rotation of a
deflection coil, which will be necessary if you remove the coil from the neck but will usually be glued in its correct position from the
factory, then you may want to apply a nice thick layer of RTV silicone to the exposed pins on the yoke and let it cure up completely
before reaching in there and tweaking the deflection yoke. RTV is cheap insurance and will make a very good insulator if you use
enough of it. I highly recommend putting RTV over deflection yoke pins as a matter of routine safety, when there is no integral cover
on the yoke assembly.
I never thought of that. Not a bad idea seeing as nobody should ever have to do any work to them.
CJ
CJ |
I noticed on new tubes there's a gray epoxy being used to lock the deflection yoke in place as well as hot glue. If the problem ends up being a tilted deflection yoke I wonder how the tube ever made it through QC/final testing? That yoke certainly couldn't move with all that gray crap on it....even if it were shipped by DHL. 
Thanks guys. I have my work cut out for me...right after I mow the lawn that is.  
_________________
Greg
"Is it ignorance or apathy? Hey, I don't know and I don't care!" --Jimmy Buffett
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