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Strange Convergence Problem

Discussion in 'Electronic Repair' started by Chris F., Nov 23, 2008.

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  1. Chris F.

    Chris F. Guest

    I'm working on a 1989 Philips 27" set with a very unusual problem. The set
    was initially dead, and after getting it going I noticed the convergence was
    way out of alignment. But strangely, neither the yoke nor convergence
    magnets have moved from their factory positions, and there are no linearity
    or pincuishoning problems with the raster itself. With a cross-hatch pattern
    displayed, I observed the following; the green vertical lines are perfectly
    straight, but the red and blue lines are on opposing sides of the green
    lines at the top, and slowly reverse sides towards the bottom. The
    convergence cannot be aligned even remotely correctly. I've never seen
    something like this on an in-line CRT before, except once. I once had a 19"
    set with convergence that could not be corrected, and I accidently
    discovered that tipping the set sideways caused the convergence to correct
    itself. Since the yoke and magnets were solidly mounted, I assumed there was
    an internal defect in the CRT, and junked the set.
    I should point out that this very heavy console set was subjected to a
    bumpy 40-mile ride here, in the back of a pickup truck, and I'm wondering if
    the CRT may have sustained internal damage. Please tell me I'm wrong.......
     
  2. ian field

    ian field Guest

    Years ago I used to repair TVs for a back street bodger, your problem
    reminds me of a Hitachi we had in to refurbish for re-sale. It had a rainbow
    pattern in one corner of the screen that went away when the set was turned
    upside down, so we simply took the tube out and put it back upside down, we
    slackened the clamps on the scan yolk and turned it 180 degrees to get the
    picture back the right way up and wedged a linoleum tile between the anode
    cap and the PCB - the picture was perfect.

    The shadow mask is a thin sheet of metal with holes to match the phosphor
    dots/stripes, but it is spot welded to a pressed steel frame which in turn
    is fitted into the glass with sprung leaf clips for thermal expansion.

    You could try laying it face down on a folded blanket in the truck and
    driving it round some bumpy roads - or you could try the upside down tube
    trick.
     
  3. hr(bob)

    hr(bob) Guest

    A loose shadow mask would screw up the purity, but would not have the
    effect on convergenc that was noted...Convergence makes up for the non-
    centered electron beams going thru the deflection yoke. I would try
    moving the deflection yoke, tube, and convergence assemblies to see if
    that might help. IT may look like the tube and the coils around it
    have not moved, but a 30 mile trip could easily shift things a little
    bit and that's all it takes to make a big difference int he
    convergence.

    Bob Hofmann
     
  4. John-Del

    John-Del Guest

    Bad deflection yoke. There are convergence shunts mounted inside the
    yoke housing, and the mastic goo that locates them dries up, shrinks,
    and cracks allowing the shunts to shift. You might also notice the
    yoke is probably noisy during operation.

    John
     
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