Osborne Vixen - Zenith 7" display t-shooting issues

[drlegendre .]

Tony,

Yeah, I see now how the button acts as a manual starter. Fluorescent
lighting is one of those things I've just never bothered to think about -
so I read the Wiki article on it, and now I've got it down. It's such a
ubiquitous technology, that I've never had the occasion to need to know
much about it.. just replace the bulb when it won't light, and if that
doesn't do it, check the starter.. failing that, replace the ballast or the
whole dang fixture, if it's one of those workbench cheap-o units.

Oh - and there is one more component in the Luxo. There's a cap wired in
parallel with the tube, probably does double-duty as power factor
correction and saving the starter switch contacts from arc damage when they
break open and the coil kicks back.

On Sat, Nov 8, 2014 at 5:08 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:

> > 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.
>
> -tony
>