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B&W TV Strange brightness behavior in Marconi 702

dtvmcdonald · 2023-02-06T18:14:49Z

My Marconi 702 has a strange behavior at turnon. When it warms up, the screen is very slow to fill up, spreading out from the center. This is not strange, its reported by lots of owners of these sets. What's strange is the behavior of the brightness control. Just after the screen fills up, its still a bit dimunless the brightness is all the way up, when its fairly much normal. But as I turn the brightness down, its very very strange. As it gets turned down, the picture washes out and essentially disappears into a gray screen. As I turnit down more I would have expected the picture to either go negative or just get darker and darker andthen go black. But what it does do is reappear as a regular positive image, with the brightest parts getting darker very very slowly and the darkest parts soon going quite dark. As the set warms up for another 10 minutes this behavior goes away, and at brightness settings lowerthan the previous "contrast goes to zero" settings the picture does just get darker quickly. What could cause this? I was thinking a gassy CRT, but I see no sign of a glow in it. I was thinking abad contact on a CRT pin or perhaps the detector diode. When I first got the set there was a bad contact, but a good cleaning seems to have made it stable. The set has plenty of drive to the CRT. The actual picture has a perfectly normal appearance with good brightness and contrast. Another symptom is that one corner of the picture is somewhat out of focus. I've tried magnets to fix that, thinking maybe a magnetized housing that a strong degausser can't fix, but they don't help. I'd like opinions first before I go into it with my scope. I would have only a few minutes a day to try as it needsto cool all the way down before another restart. I hope this is the right section for this query.

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Black & White Television plus Standards converters

Last Post by Till Eulenspiegel 2 years ago

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Till Eulenspiegel

(@till)

Posts: 4927

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I think it might well be a miss-aligned CRT gun electrode. Set will be absolutely fine aligned to single sideband operation. Will give more RF gain which might have might actually been done in the old days to achieve distant reception from the AP transmitter. Narrower bandwidth helps to reduce the noise figure.

Till Eulenspiegel.

Posted : 26/02/2023 9:38 pm

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