Video Streaking

[Niechcial,Steve]

I have a horizontal streaking across the picture of a transistor set (see attached- ignore the daigonal white line which is camera artifact) . When I have had this in the past, leaky electrolytics in the video coupling have been the cause but I have changed those. Does anyone have any other ideas please? Thanks
Attached Thumbnails

[Studio263]

When that happened in my Beovision 1400 it was leakage in the tube, according to forum member Bobby Ball who has the set now. Does it change as the set warms up?

[Maarten]

It can be leakage or stray capacity anywhere in the video path. Maybe a germanium transistor or diode.

[Niechcial,Steve]

Thanks Maarten and Studio 263. The tube is a directly heated one, so no possibility of heater/cathode leakage. I was thinking possibly a transistor or the video detector diode. I've alreay tried changing the video o/p transistor. Does anyne think this could be pre-detctor?

[Maarten]

Come to think of it... Did you check all electrolytics in the video path (if any) or in the power supply lines for the video path?

[Niechcial,Steve]

I had previously changed all the electrolytics in the video path including the two coupling line and frame blanking- though they all tested ok on removal. Without much hope I have just changed the 0.22 cap coupling the video output to the tube, and lo and behold-fault cured! The cap measured 33 pf on my meter. It is a non-electrolytic plastic encapsulated type (Paper? Polystrene?). Needless to say the contrast has also shot up. Thanks all for the help.

[Niechcial,Steve]

End result

[Maarten]

Nice!

[Niechcial,Steve]

Thanks!

[pbengtech]

I was going to suggest using a slave TV switched to AV ( with isolation of course) and resistors to get the levels right, as a video probe. That way you could check the signal was correct at the start of the video path and follow it along.
I used an old 12v supplied battery/mains tv for the purpose.
Well done for finding the faulty component.
Phil B

[raditechman]

I cannot remember the model, but there was a Rediffusion (possibly Plessey chassis) television where the video coupling capacitor to the CRT was mounted next to the PCL84 video output valve, and did not fare well with the heat, I think it was a .1mfd and gave streaking effect. I nice quick field service repair as it became a "stock" fault.

John

[Niechcial,Steve]

Thanks Phil and John. A video probe would have been a good next step and would have led me straight to the video o/p. I was very lucky to hit on the o/p cap considering it's considerably over-rated and runs cool.