Coil inductance
0µH
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What you see on the Mac
Yoke current
0%
Picture width
0in
Too wide
Our Mac Plus draws its picture too wide for the glass. We traced the cause to one part: the ferrite slug in the width coil. This page shows what we found and the physics behind it.
The coil, the yoke and the circuit all work. The ferrite slug inside the width coil has lost its magnetic effect, so turning it can no longer pull the picture in.
The Apple and File menus on the left and the disk and Trash icons on the right are cut off. Every app overflows by the same amount, so it's the display hardware, not software.
We turned the slug through its whole travel inside the winding. The width did not move at all. A healthy slug changes the width a lot over that range.
With the steel driver in the coil, the picture shrank to the right width, then snapped back when we pulled it out. So the coil carries current and responds to magnetic material. Only the slug doesn't.
A crack works like an air gap that cancels most of the slug's effect, and a brass or plastic replacement does nothing. Either way it still turns freely and looks normal.
Next: confirm by holding a fridge magnet to the slug (real ferrite is attracted), then swap in the slug or the whole L2 coil from our donor Mac (M0001), which uses the same part.
These two diagrams follow the same controls. On the left is a slice through the width coil showing its magnetic field. On the right is the current the yoke receives during a few scan lines.
Every turn of wire wraps the current's magnetic field around the inside of the coil, so a coil concentrates field along its axis. That field holds energy. To change the current you have to build up or drain that energy, and that takes time.
This resistance to change is inductance. A good mental picture is a flywheel: the coil's field is the flywheel's spin, and inductance is how heavy the flywheel is.
Air is a poor path for magnetic field. Ferrite is a ceramic full of tiny magnetic regions that swing into line with the field and add their own, so field passes through it hundreds of times more easily than through air.
When the slug sits inside the winding, the same current produces far more field. More field for the same current means more inductance: a heavier flywheel. When the slug is outside the winding, the field barely reaches it, so it does almost nothing.
inductance = field held ÷ currentAbout 22,000 times a second the horizontal circuit pushes current through the width coil and the yoke, which are connected one after the other. The push is the same each time: roughly the same voltage for the same length of time.
Through an inductor, current climbs at a rate set by voltage ÷ inductance. Same push, heavier flywheel, less speed at the end. So the more inductance the width coil adds, the less current the yoke reaches by the end of each line.
current ramp rate = voltage ÷ (Lyoke + Lwidth coil)The yoke turns its current into a magnetic field across the tube's neck, and that field bends the electron beam sideways in proportion to the current. The peak current at each end of the ramp sets how far left and right the beam reaches, which is the picture's width.
Put together: slug deeper → more field per amp → more inductance → shallower current ramp → smaller beam swing → narrower picture.
Field has to cross every gap in its path, and air resists it hundreds of times more than ferrite. A hairline crack across the slug forces the field through air, which undoes most of the slug's effect even though the slug still looks whole and still moves. Pick Cracked ferrite and sweep the slider: the width barely changes.
Steel is magnetic, so a steel tool in the coil adds field and narrows the picture. Steel also conducts electricity, so the coil's changing field drives swirling currents in it that waste energy as heat. That's why the picture moves while the tool is in, and why a metal tool is the wrong one to adjust with.
Plastic is invisible to the field. Brass isn't magnetic but does conduct, so the swirling currents in it push the field out and slightly lower the inductance. A substituted brass or plastic slug moves freely but can't bring the width in.
Illustrative model, not a circuit simulation. Fixed points: the 7.0 in target width and 11.90–12.75 V range come from Apple's Mac Plus service manual; the coil's 12–44 µH range is a bench measurement of a Plus board's L2 with the slug out and in (Retro Repairs & Refurbs); the Plus scans about 22,000 lines per second. The yoke inductance, the material factors and the rest of the scaling are chosen to show the behaviour. The diagnosis comes from our own bench observations; the magnet test and slug swap will confirm it.