Osborne Vixen - Zenith 7" display t-shooting issues

[Simon Claessen]

Cool! that is good news. and thanks for sharing this. another thing to 
lookout for.

simon

On 23-11-14 09:56, drlegendre . wrote:
> 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
>>>
>>
>>
>

-- 
Met vriendelijke Groet,

Simon Claessen
drukknop.nl