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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 27th Nov 2023, 4:53 pm   #1
Niechcial,Steve
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Default Video below black level- Sony 5-303W

Trying to find this one has driven me crazy! The picture has bad cogging on line which varies in respect of picture content. The picture is good apart from this- lots of gain, good high frequency response, albeit with slight caption streaking.
Looking at the video on a scope there is clearly some picture content below black level on the sync pulse and below the back porch. This is also visible post sync separator.
Feeding the correct level of composite video in at the base of the video driver produces a normal picture. That basically exonerates everything after the video detector. Nevertheless, I have also changed all the electrolytics in the video/sync circuits. There is nothing visible on the supply lines or chassis rail. The video detector itself reads as you would expect both ways. I have disconnected the agc line and experimented by putting a varying voltage across it. The gain varies as expected, but the cogging remains at all signal levels. I presume this must be an IF fault of some kind, but I have no idea what!
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Old 27th Nov 2023, 6:43 pm   #2
Freya
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Default Re: Video below black level- Sony 5-303W

Have you read the restoration of the same model on oldcrap.org, this detailed report suggests that once the capacitors in the IF stage have been replaced that they need re aligning.
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Old 27th Nov 2023, 8:06 pm   #3
Niechcial,Steve
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Default Re: Video below black level- Sony 5-303W

Thanks Freya. I'll takea look
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Old 27th Nov 2023, 10:15 pm   #4
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Default Re: Video below black level- Sony 5-303W

This sounds more like overloading of the tuner/IF than an alignment issue. Have you tried connecting your signal source via an attenuator?

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Old 28th Nov 2023, 9:43 am   #5
Niechcial,Steve
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Default Re: Video below black level- Sony 5-303W

Hi Paula. Thanks for that idea. The agc on this model is not applied to the tuner which runs at full gain constantly. I agree it does sound more like some cross-modulation tuner problem, but attenuating the aerial input down progressively to the point when the picture virtually disappears makes no difference to the cogging. Also, as I say above, applying manual gain to the agc line reduces the gain as it should, but does not change the cogging. I have a tuner from Sony's earler model which is also at IF 27 Mhz. I'll connect that and see what happens.
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Old 29th Nov 2023, 8:34 pm   #6
Niechcial,Steve
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Default Re: Video below black level- Sony 5-303W

Here are some scope traces, Trace 1 (top) is the input to the modulator. Trace 2 (bottom) is the base of the of the video output (same as input to sync separator , but ac isolated). I'm wondering whether the problem is a spurious HF oscillation
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