Re: Blown or blocked blue gun in Amplifone?

From: Andrew Welburn <andy_at_andysarcade.net>
Date: Thu Dec 20 2012 - 07:12:57 EST

  The three electron beams move in unison in a CRT, its just the
individual colours (each beam aligned to a different colour reactive
phosphor on the face) being switched on and off that gives you the
coloured vectors you see. So the blue beam hasn't caused your
burning/bubbling in the glass, its just the beam itself has at some
point in the past.

  The nebula you describe is indeed a fuzzy mess, it cannot burn the
phosphor in its cloudy haze, any more than a raster image can burn an
area of phosphor. You need a beam sitting statically over a very
localised area to get burn, and you need an intense beam at a pinpoint
to burn/bubble the dotmask/phosphor away.

  I have an amplifone tube from a dedicated major havoc that has a
burned and bubble phosphor/dotmask and a coin-sized dark patch in the
centre pretty much just like you describe... Mine has the added effect
of having a warped dotmask so that there is uneven purity over the
entire tube face, its just a rainbow of coloured vectors. Its ok for
testing/fixing monitor parts, but no good for playing anything.

  Have you tried *very* gently tapping the neck of the tube with a
rubber mallet with th tube facing down to see if you can dislodge
whatever it is blocking the gun? it does sound like a blockage if
red/green are unnaffected. I'd say its very rare, but not impossible. If
you dislodge the material, it can fall into the bottom of the tube and
out of the way and wont trouble you again unless you put the tube face
up again.

  19" Amplifone tubes are more like regular raster tubes in their
deflection angle, so i think finding another tube might not be that
hard. The Amplifone tube does have a finer dot pitch than regular std
res raster monitors, but it'd still work but look a little blockier if
you didn't find a medium resolution monitor tube with the finer dot pitch..

  ymmv

Andrew Welburn
http://www.andys-arcade.com

On 20/12/2012 07:31, David Shoemaker wrote:
> So I found a tube to borrow to test some boards in my major havoc.
>
> Red and green look pretty good but blue looks like nothing more than a
> nebula of color primarily centered in the middle of the tube. I suspect
> a hunk of crap is blocking the blue gun from getting a clean shot at the
> screen.
>
> I tried my rejuvenator and got good response on all 3 guns and no
> lightning when I tried to clear shorts.
>
> Am I on base here or totally whacked? Any suggestions on what I might
> be able to do to clear the problem?
>
> Oh one additional point, the center of the tube has phosphor burn (as
> expected) but it almost seems like there is a bubble in the glass. I
> suspect the constant blue gun hit is not doing good things for the glass J
>
> Anyone have any luck swapping tubes from other monitors over to these?
> I realize it is going to be an issue with the 100 vs 90 degree but if I
> can get the monitor in at least 90% it would be a viable test unit while
> I try to track down a “correct” tube.
>
> Thanks,
> David
>
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Received on Thu Dec 20 07:13:07 2012

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