Re: Sams PhotoFacts for the IIgs Monitor

["Michael J. Mahon" <mjmahon@aol.com>]

a2aviator@gmail.com wrote:

> Lastly, back in the day, one trick with CRTs was to use a CRT
> Rejuvenator on the thing, this was pretty much a waste of time. The
> CRTs used in the IIgs monitor were pretty much immune to these
> machines and if it did work, your symptoms came back in several
> months. The IIgs RGB monitor had a really short life compared to other
> monitors, they just didn't last long at all, and interestingly, we all
> used them and didn't notice because everyones monitor it seemed was
> looking the same. ;-)
>
> Basically they would loose brightness and focus ability, take as long
> as a half hour to warm up to a usable picture, and that caused people
> just to leave them on.. which.. made them go even faster.

> http://www.starkelectronic.com/bk490.htm

CRT rejuvenators had several modes of operation, addressing
different problems that could develop in a CRT.

The most common problem was the degeneration of the electron-
emitting surface of the heated oxide cathode, usually as a
result of bombardment with positive ions created by the electron
beam interacting with residual gas in the CRT.  The positive ions
are accelerated backward along the electron beam path until some
of them strike the surgace of the cathode, changing its work function
and damaging its ability to emit electrons.

A rejuvenator momentarily applied a much larger than normal heater
voltage to the cathode heater, briefly overheating it and causing
the ion-damaged surface layer to partially evaporate or anneal,
partially restoring the emissivity of the cathode, giving the CRT
a new lease on life.

Of course, the reversal of damage was seldom complete, and the
damage process (ion formation) was still at work (and generally
increasing in speed as the parts of the CRT continue to outgas),
causing the improvements to be temporary.

Another device commonly used to restore emissivity was a "booster"--
essentially an autotransformer that permanently boosted the heater
voltage from a nominal 6.3VAC to 7.5-9VAC.  The higher cathode
temperature caused it to emit electrons more effectively, but, of
course, the higher heater temperature decreased the life of the heater.
Fortunately, most CRT heaters were significantly overdesigned, and
burnout was seldom a problem.

I recently used the "booster" approach on a "weak" Monitor ///.
I moved the heater wire from the +12v regulated supply to the
14-volt or so unregulated side, and the brightness improved
to normal levels.

-michael

NadaNet file server for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

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