Videokarma.org TV - Video - Vintage Television & Radio Forums - fdt-5bx5 Violently Exploding Capacitor

Quote:

Originally Posted by old_tv_nut (Post 3261755)

Another paper talks about using a permanent magnet to cancel out DC effects in a power supply that produces both AC and DC in an inductor. Haven't found anything that talks about deliberately using a magnet in an AC inductor.

I was basing this off of the advice of an EE whom I interact with a bit over on the Vintage Computer Federation forums.

In responding to another person's question about this same kind of issue, Hugo wrote:

Quote:

The linearity coil is different from a regular inductor, in that it contains a permanent magnet. So its inductive properties are in themselves not linear.

The idea is to correct for the non-linearity that is intrinsic to the H scanning system in all magnetically deflected CRTs.

There are two basic problems with the linearity in magnetic H scan, the first is that the yoke has an increased sensitivity for higher deflection angles, this results in general compression of the raster centrally or relative expansion at the R and L sides, this anomaly is corrected by the S correction capacitor in series with the H scanning coils of the yoke.

The remaining linearity problem is more interesting, as in the case of H scanning: essentially a rectangular voltage is switched across the yoke by the HOT (horizontal output transistor). The current climbs initially at least linearly at a rate of V/L where V is the supply voltage and L the inductance, but starts to taper a little near the right side of the scan. This partially compensates some of the yoke increased sensitivity. Unfortunately though, when the HOT is switched off for flyback, the field in the yoke gets reversed after flyback, beam on left side of screen, and the rate of change of current with time is initially higher than it was on the right side of the raster, in the initial part of the scan, as the current then passes (decays) via the damper diode back to the power supply to scan the left side of the raster.

The net effect is, the scanning raster, without a linearity coil in series with the yoke, in a transistor VDU, will always be a little stretched on the left side, and a little compressed on the right, even if the the S correction capacitor is ideal.

Some linearity coils, the magnet is a metal cylinder with a hex hole that rotates adjacent to the coil. After a while many manufacturers made fixed linearity coils, non adjustable with the magnet built in.

If you cannot find the exact part one option could be to search for TV linearity coils on ebay and find an adjustable one that can also be rewound with a similar sized wire and number of turns, to the one you are trying to replace. Generally these coils use fairly thick wire as adding DC resistance degrades the linearity.

A width control inductor on the other hand, has no magnet and simply reduces the overall H scanning current and reduces the width of the raster proportionally without changing the linearity.

The post I took this from is here.