Hello Col,
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Originally Posted by Retired
Thank you Dave; I've played with every knob I can find but it remains the same;
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OK, that's a pity!
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I can adjust to obtain one picture but this deletes the test card and just gives a lot of rubbish.
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It's possible that the frame hold could be set so that the frame 'rolls' very quickly, that could give the effect you describe. Adjusting the frame hold
shouldn't delete the picture! It's also possible that you may have been adjusting the line hold, having that set off-frequency could also cause that effect. Once you have the set fully working, you should be able to observe the effect of adjusting the line and frame hold controls to see just what they do!
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I'm interested to learn of the frame timebase perhaps running at 25 instead of 50? Is this Hertz; 50 cycles per second?
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Yes, the frame timebase normally runs at 50Hz (= 50 fields per second). As the line and field frequencies are locked together with a specific relationship to give an interlaced display, you get two fields per frame, the frame rate being 25Hz (= 25 frames per second).
Depending on the range of adjustment of the frame hold (or because a/some components(s) have altered in value) you may be able to adjust it so that the time base runs at 25 Hz, you would then get two fields displayed one above the other, instead of interlaced!
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Would this also have to to with rectification such as half and full wave rectification if so then would a faulty diode throw the time base out;
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No, the field frequency is determined solely by the field timebase, which is synchronised to the the field sync pulses in the video signal. As the mains rectification is half wave anyway, you couldn't get a condition where one diode of full-wave rectification had failed!
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I'm thinking in terms such as the single diode used to bridge the faulty PY32 valve?
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No, the PY32 is a single diode as half-wave rectification of the mains supply is used.
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Sorry if this is a truly dumb question but there is all-sorts whizzing around in my head as I try to understand what is going on.
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There is no such thing as a 'dumb' question!
If you are not clear about how something works, you have to ask questions, no matter how 'dumb' the may appear to you!
Regards,
Dave.