Quote:
Originally Posted by ohohyodafarted
Great discussion guys. Thanks!
To more enlighten you as to if it's drive line or barkenhausen: The line looks like the "drive line" in the photo above. It gets stronger as you turn up the H drive control, and get dimmer when you turn it down, however it would not go away with the control at minimum.
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BTW, the line was right in the center of the screen left to right and ran all the way top to bottom
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A drive line effect will occur from top to bottom since it is high-level and the same effect on every horizontal trace. Barkhausen is an oscillation that is at RF and gets back into the tuner. It is sensitive to conditions such as bias voltages (and magnetic fields, as shown by the magnet trick), and could come and go between top and bottom.
I think the distance from the left to the drive line can depend on the particular circuit, since a more efficient H scan circuit means the damper will be supplying retrace current for a longer time before the stored energy is used up.
Aside:
I never worked on tube H outputs, only transistor monochrome. I don't recall having to make any special accommodations in new designs to prevent drive lines, but that may have been because Moto already had a driver design with a duty cycle that was OK for different output stages. We did have to optimize the turn-off current for the H drive to the H output transistor. There needed to be a negative pulse at the leading edge of the turnoff to pull charge out of the junction of the (saturated) H out transistor (HOT), but its magnitude and width had to be right so as to turn the HOT off as quickly as possible, but quit before causing reverse breakdown. This meant a different component choice depending on the particular model of transistor and sweep current. If the turn-off wasn't fast enough, the dissipation in the HOT would go up, due to the HOT still drawing some current while the collector pulse voltage was rising. A hotter HOT would switch less efficiently, and a thermal runaway would be possible. If the drive current waveform design was really wrong, the HOT turn off could be so slow that the combination of rising pulse voltage and slowly falling current could exceed the safe area of the HOT and kill it instantly. We did a lot of x-y scope tracing with the HOT collector voltage and current as the x and y axes while adjusting the drive design for fastest switching. You could always tell which scopes were used in sweep design because they were the ones with a hole burned in the phosphor at the lower left corner of the graticule; someone would always forget to turn down the scope beam current when turning off the set.