Yokes, what's the difference?

[jeyurkon]

Can anyone explain what is different about yokes that are designed for 53, 60, 70, and 90 degree deflection? Will a 70 deg yoke work in place of a 60 degree yoke?

John

[bandersen]

The difference is the amount the electron beam is deflected. Older CRTs (like the 10BP4) are generally longer so use a smaller 50º deflection angle. At the other extreme are the really shallow 120º tubes used in sets like the Predictas.

If you sub a 70º yoke for a 60º, I believe the edges of the picture will be off the screen.

[jeyurkon]

I guess I was looking for a bit more detail. You can drive a 60 yoke more and achieve 70 degrees or drive a 70 degree yoke less and get 60 degrees of sweep. But there must be more to it than that or they wouldn't differentiate between them.

John

[Don Lindsly]

The yoke is wound to achieve the desired deflection angles, both vertical and horizontal. You will notice the difference between a 90 and 70 degree yoke.

The second consideration is impedance. The yoke must match, or at least be close, to the design impedance of the flyback and vertical output transformer. That assures maximum energy transfer from the output circuits to the yoke. A mismatch will show up as non linearity, drive lines and/or distortion.

The third is the available output stage power. A 90 degree yoke will not always deliver enough width and height without the additional power necessary to sweep the sharper angle.

There is some consistency within manufacturers, but not always. It is best to match the correct yoke and only use a substitute for testing.

[jeyurkon]

Originally Posted by Don Lindsly

The yoke is wound to achieve the desired deflection angles, both vertical and horizontal. You will notice the difference between a 90 and 70 degree yoke.

The second consideration is impedance. The yoke must match, or at least be close, to the design impedance of the flyback and vertical output transformer. That assures maximum energy transfer from the output circuits to the yoke. A mismatch will show up as non linearity, drive lines and/or distortion.

The third is the available output stage power. A 90 degree yoke will not always deliver enough width and height without the additional power necessary to sweep the sharper angle.

There is some consistency within manufacturers, but not always. It is best to match the correct yoke and only use a substitute for testing.

This is starting to help but I'm afraid I'm going to show the depth of my ignorance.

The deflection should probably be linear with current. But as the beam travels a longer distance through the field at larger deflection angles it would start to be bent more than would be proportional to the current. So, they must wind it so that the field drops off more rapidly at larger angles? What do they mean by cosine wound? Is that related to the above?

The impedance must be complex. Nearly purely inductive so wouldn't the power factor be zero?

The field would be determined by the number of amp turns. The inductance would probably go as the number of turns squared. I would think that you would want the inductance as low as possible while keeping the amperage reasonable. So that the yoke would look like a low reactive impedance and the power actually being dissipated in a load resistor. That is the reactive impedance would be small compared to the load resistor.

Is my thinking going wrong somewhere here?

John

[jeyurkon]

I've had second thoughts about my comments about needing low inductance. Driving it the way I envisioned would be inefficient.

Instead, the yoke probably does look nearly like a pure inductor and the parallel resistors that are added to dampen oscillations from the parasitic capacitance. The transformer will just transform this inductance so that the network of transformer and yoke will just look an inductance. This is then part of the resonant circuit that's being driven.

So the actual value of the inductance of the yoke would be important.

Does this sound right?

John

[old_tv_nut]

The principles of horizontal deflection and vertical are different.

For horizontal deflection, the ideal is a pure inductance in which current increases linearly over time due to a constant drive voltage. In reality, there are resistive losses that have to be overcome, but the drive is still applied by hard-switching a voltage supply onto the yoke and then diode conduction (by the damper) of the current in the other half of the sweep. Tube and transistor circuits differ in that tube circuits generally are higher impedance (higher voltage and lower current) than transistors.

For vertical deflection, the operation is much more of a linear amplifier arrangement, although there is enough inductance to cause a pulse voltage on retrace. By the way, this makes it possible to use feedback techniques to make the current perfectly linear and eliminate a manual vertical linearity adjustment (which I recall was done in some Zenith designs).

[jeyurkon]

Originally Posted by old_tv_nut

The principles of horizontal deflection and vertical are different.

For horizontal deflection, the ideal is a pure inductance in which current increases linearly over time due to a constant drive voltage. In reality, there are resistive losses that have to be overcome, but the drive is still applied by hard-switching a voltage supply onto the yoke and then diode conduction (by the damper) of the current in the other half of the sweep. Tube and transistor circuits differ in that tube circuits generally are higher impedance (higher voltage and lower current) than transistors.

For vertical deflection, the operation is much more of a linear amplifier arrangement, although there is enough inductance to cause a pulse voltage on retrace. By the way, this makes it possible to use feedback techniques to make the current perfectly linear and eliminate a manual vertical linearity adjustment (which I recall was done in some Zenith designs).

In other words for the horizontal they take advantage of di/dt=v/L. But the slow di/dt required for the vertical would require a huge value of inductance, so it's easier to treat it as more of a linear circuit?

The damper must live a hard life handling the dump.

John

[old_tv_nut]

Originally Posted by jeyurkon

In other words for the horizontal they take advantage of di/dt=v/L. But the slow di/dt required for the vertical would require a huge value of inductance, so it's easier to treat it as more of a linear circuit?
>>>>Correct

The damper must live a hard life handling the dump.
>>>>The horizontal output tube or transistor must do a controlled turnoff of the current. The damper does it "naturally".


John

I never worked on tube deflection design, so I don't know all the details, but for solid-state horiz circuits, the turn-off speed of the output transistor and the damper diode are critical to keep dissipation down and to avoid killing the transistor due to exceeding the "safe area" (simultaneous current/voltage limits). Generally, the horizontal drive waveform includes a negative pulse at turn-off (not simply going to zero), to pull stored charge out of the HOT base and shut it down as quickly as possible.