Osborne Vixen - Zenith 7" display t-shooting issues

[drlegendre .]

Good News, everyone! ;-)

 The Osborne Vixen 7" display has been repaired.. And our friend Chuck has
earned not one, but two gold stars in the process.

To be fair, it's one of those situations wherein if I'd had an appropriate
replacement part on-hand, it would all have been over in the first few
hours.. because it turns out that the dang horiz. opt. transistor WAS the
culprit all along! It had some kind of thermal fault, that made it give up
the ghost as the current began to flow in earnest.

Per Chuck's suggestion, I picked up (10) MJE13007 transistors from an eBay
vendor - cost was less than $5 USD all-done. This was less than the cost of
a single NTE 379 - the part that allegedly crossed to the mystery Zenith
1070 item.

The packet arrived today, and I wasted no time installing one of the 13007s
in place of the original Horizontal Output transistor - a Zenith part
marked "1007". Anyway, the display came right up, looking great, and the
new HOT runs very, very cool - maybe a 10 degree rise over ambient?

So far, so good. A big round of thanks to Chuck, and everyone else who
added some useful info to the thread. Thing is, I still have qty. 9 of
these 13007 parts sitting around - would anyone like to have a spare or two
on the shelf?

On Sat, Nov 8, 2014 at 9:31 PM, drlegendre . <drlegendre at gmail.com> wrote:

> 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
>>
>
>