Toshiba SV-771 has white dot/streak in video output...damaged head?

[technoman9]

Did that last night, no dice. There is a small divot worn into the carbon brush and the drum part is not rotating on center, but no amount of pushing, pulling, or repositioning this brush makes any difference in the white streaks.

I was thinking about this last night, trying to map what I see on screen to the signal coming out of the VCR. I know VCRs record video as diagonal stripes on the tape, alternating between two heads. The question is, what is recorded within one stripe? Is it a complete set of 262.5 horizontal lines worth of information (aka, a field)? Since the head swipes from bottom to top of the tape, is the bottom of the swipe the beginning of a field? I wondered if it was dirt on the tape path but I'm not seeing any, nor any tape path issues during playback.

[old_tv_nut]

One field per swipe.
If it's on the tape, it would be part way between the top edge and the bottom, edge, proportional to the vertical position in the picture.

Tape damage would also show up in both fields, but the examples you posted seem to be in alternate fields.

I think this is too minor to be tape damage as such. Damaged tapes typically produce a much wider noise stripe and really wreck the picture as the wrinkle passes by, and produce a buzzing sound as the heads run over the wrinkle.

I still can't come up with an idea of what's going on.

Grasping at straws: Is there a stationary part of the drum that you can get to and ground?

[technoman9]

I too am really coming up short, but appreciate all of the suggestions so far. I did attempt grounding the stationary part of the dump using a test lead but it had no affect. I also tried finding the RF output to see if something shows up there but haven't found that pin yet.

I finally got mgross0's spare SV-771 so now I have a spare machine to swap the Video and RF boards out if and see if that makes a difference. That, or I need to send it to 12voltvids on YouTube and let him take a crack at it.

[colectorad]

This issue is common on 21st century VHS machines. I've been able to clear it by skewing the A/C head adjust boss. However, the issue returns after the cassette has been ejected and reinserted.