Magnavox Magnalux

[electra225]

Magnalux is Magnavox's automatic contrast and brightness control system. I believe it was available in the 1961 models, but don't quote me on that. Magnavox totally changed their TV chassis for the 1962 model year. We will, then, restrict this discussion to the 1962 models, assuming there are at least 1961 and 1963 whose models may well be so equipped. Magnavox called their 1962 console TV chassis the 36 series. Mine is a C36-20-11. The 36 series, 20th version, 11th edition of the 20 version. Confused? There are 23 versions of the C36 chassis used in Magnavox console TV sets and Stereo Theaters in 1962. Three series of Stereo Theaters were available, the 200, 300 and 400 series. They all used a version of the C36 TV chassis. Only one version of the C36 that didn't have Magnalux. This chassis was found in entry-level, 21" consoles and maybe some 19" or 21" table model sets. C36-01.

There is exactly ONE paragraph in the Magnavox manual on Magnalux. They think the system is so simple any idiot should be able to understand and troubleshoot it. It took THIS idiot three days to finally get the concept thru his thick noggin. The way Sams drew the system was confusing and I found a mistake in the drawing, which helps nothing. They gave screen voltage on the video amp as 50 volts, it should have been 150 volts. Then test point 36 wasn't even on the drawing, although it was listed. Test point 36 is where the Magnalux system gets its B+, 280 volts. I redrew the schematic on a clean sheet of paper more the way I think the electrons would flow. This made more sense and allowed the light to come on, finally.......

Magnalux works in the video amp. The heart of the system is an LDR, light dependent resistor, that varies in resistance depending on ambient lighting. It increases or decreases the gain of the video amp, varying video strength and contrast, while also controlling brightness on the pix tube screen. The output of the video amp is impressed on the cathode of the pix tube, the brightness control output is impressed on the grid of the pix tube. The cathode should always be positive to the grid on the pix tube. When you ground the grid of the pix tube, it is driven to full brightness. If you cut off voltage to the cathode, the tube goes dark.

In operation, when light strikes the LDR, its resistance determines video, contrast and brightness. Add light, the brightness increases, the video increases, the contrast increases. Cover the LDR, the opposite effect takes place. The Magnalux circuit is in parallel with the brightness control, Yet, how it works is that the resistances of the LDR and brightness control are added. Although the operator can set the brightness "range" with the control, the LDR can override the brightess control. This system may sound bizarre on its face. In practice, it works nicely. Many of the old black and white sets were unwatchable in high ambient lighting. The picture on this set is perfectly viewable even in direct sunlight. This set may arguably produce the nicest black and white picture I have ever seen.