Magnavox CD4756 (T989 chassis) dark bands down left edge

[old_tv_nut]

If you adjust centering, both sweep ringing and video will move together, so this cannot be used to separate the cause. Instead, can you turn the H hold back and forth a bit to change the horizontal phasing? If the picture moves but the rings stay still, they are related to horizontal sweep somehow - however, they still could be multiple things, ringing in the sweep or a horizontal transient getting into the video.

Needing a bigger cap across yoke/flyback sounds to me like something wrong with the yoke or flyback.

Edit: I don't know what the H drive waveform should look like. Can you verify that it's the correct duty cycle? Is there a drive adjustment coil you can try?

[James_S]

Originally Posted by old_tv_nut

If you adjust centering, both sweep ringing and video will move together, so this cannot be used to separate the cause. Instead, can you turn the H hold back and forth a bit to change the horizontal phasing? If the picture moves but the rings stay still, they are related to horizontal sweep somehow - however, they still could be multiple things, ringing in the sweep or a horizontal transient getting into the video.

Needing a bigger cap across yoke/flyback sounds to me like something wrong with the yoke or flyback.

Edit: I don't know what the H drive waveform should look like. Can you verify that it's the correct duty cycle? Is there a drive adjustment coil you can try?

I'll check this when I have time and get back to you. I'm hopeful that the yoke and flyback are ok as it could be difficult to find a replacement for either one. The picture mostly looks pretty good and everything seems stable, it has run for hours and nothing gets super hot or seems to drift over time.

[James_S]

Originally Posted by old_tv_nut

If you adjust centering, both sweep ringing and video will move together, so this cannot be used to separate the cause. Instead, can you turn the H hold back and forth a bit to change the horizontal phasing? If the picture moves but the rings stay still, they are related to horizontal sweep somehow - however, they still could be multiple things, ringing in the sweep or a horizontal transient getting into the video.

Needing a bigger cap across yoke/flyback sounds to me like something wrong with the yoke or flyback.

Edit: I don't know what the H drive waveform should look like. Can you verify that it's the correct duty cycle? Is there a drive adjustment coil you can try?

Ok this is a very clever idea. I tried fiddling with the H-hold adjustment and the picture shifts back and forth while the dark bands remain stationary so that suggests that it is some kind of ringing in the horizontal scan. That also suggests that this and the excessive HV are related. I don't see any adjustment for the horizontal drive, I will look around with the scope a bit more in that area of the circuit and see if I can find anything.

I've worked on a lot of monitors and TVs over the years and I can't recall ever encountering this symptom, it's really got me puzzled.