Strange Convergence Problem

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.......

Reply to
Chris F.
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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.

Reply to
ian field












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

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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.


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