Osborne Vixen - Zenith 7" display t-shooting issues

tony duell ard at p850ug1.demon.co.uk
Sat Nov 8 17:08:34 CST 2014

> A few weeks ago, I had to put a new AC cord on a Luxo magnifier lamp, the
> type with the 20W circular fluorescent tube. I was really surprised to find
> nothing but an iron-core choke in the base.. no starter device, nada.. just
> a choke in series with the line cord (and the lamp) I assume. It +is+ the
> type where you must hold down the power button for a moment to light it, so
> perhaps that's a (manual?) starting mechanism. But in any case, I was a
> little puzzled that's all there was to it, so to speak.

The normal fluorescent lamp starter is an automatic switch, effectively. Often it's a little
discharge lamp (argon filled, most of the time) with bimetallic strips for electrodes. At switch
on, the starter lamp strikes, the electrodes get hot, bend and touch. This energises the filaments
in the fluorescent tube. Since the starter is shorted out (by the electrodes touching), it cools down, the
electrodes spring apart. The sudden open circuit causes a large back emf from the ballast choke, which 
strikes fluorescent tube (the hot filaments in said tube emit electrons, making it a lot easier to strike).

Anyway, if your magnifying lamp is anything like the one I repaired years ago, there is a momentary
contact set on the on button. It's wired in the same way as the automatic starter switch in a normal
fluorescent lamp. When you press and hold the on button, the filaments in the fluorescent tube
warm up. When you release it, you open this circuit, causing the back emf from the ballast to
strike the tube.


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