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Amiga Collections: Topik
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Topik - Disk 04 - Disk and Technical Stuff (19xx)(Topik Public Domain)(PD)[WB].zip
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Topik - Disk 04 - Disk and Technical Stuff (19xx)(Topik Public Domain)(PD)[WB].adf
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Electronix
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1084Repair
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1989-06-20
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5KB
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90 lines
So, your vintage 1084 Amiga monitor has these symptoms: every few minutes
you hear a snapping or popping sound, the screen display brightens in one
small spot briefly (especially noticeable if a screen-blanker has darkened
the screen!) This is caused by the monitor high voltage (around 25,000
volts!) used for the picture tube jumping ("arcing") to ground, not
necessarily a good thing!
My three-year old 1084 was giving these symptoms badly about a year ago,
until I finally fixed it. Other people using my computer had made
(negatively) remarks about this for some time, but I'd gotten used to it.
The frequency of the arcing increased until it was occurring about eavery
90 seconds (I finally timed it). It still didn't bother me until I was
working on some newsletter articles by leaving the machine on for 2-3 days
at a stretch (to allow me to add thoughts when they occurred without
re-booting) and would return to find my machine crashed. After several
trips with my Amiga (without monitor!) to the repair shop where the
crashes wouldn't recur, I wondered if the monitor arcing could be causing
this! Of course the technician at the repair shop and I both agreed this
was possible and certainly worth taking a look at...
As an electronic technician myself I felt comfortable giving it a shot
myself... after a good while I found the problem, fixed it, and my
erratic crashes disappeared!
Caution: these monitors have some very high voltage inside, and have a
bad bite! My favorite Amiga technician hates to work on them ("once
bitten.."). You may wish to simply pass this article on to your Amiga
technician, but if you work on it yourself, remember to BE CAREFUL! The
high voltage lingers long after the monitor has been turned off and
unplugged.
First, I measured the time between arcs, and found it to be approximately
90 seconds. Then removed the back from the monitor and sat in the dark
with it turned on until it arc'ed (nap till the 80 second mark, then pay
close attention, it should arc soon, right?). It took a while, and I had
to finally put the monitor face down, with the bottom of the circuit board
in view. At last, I saw the area where it was arcing, and visual
inspection showed a very small "burn" mark on a p.c. board trace, just
happened to be the ground, where it passed by a bent tab that secured a
metal shield (that surrounds the high voltage stuff).
I considered adding an IFF picture to illustrate this, but your monitor
may be arcing in a different location; let me know if your monitor arcs
there too, and I'll add a picture to this.
Looking further, I found the the "second anode" high voltage lead (the
large red wire going to the picture tube) was touching (or very close to)
the piece of sheet metal that shields the high voltage fly-back
transformer (where the large red wire comes from). The insulation on the
wire was breaking down, there was a small discolored area on the metal
shield there, and this was charging the plate up until it arced to the
p.c. board ground trace!
----------------------------------------------------------------------
The fix entailed:
routing the high voltage lead away from the metaal shield (rotate it
slightly where it enters the picture tube)
wrapping the "bad" spot on the high voltage lead with electrical tape.
remove the high voltage shield and spray the area where the high voltage
lead passes with something called "corona dope" (available at TV or
elecronic repair parts shops, the brand I used was like purple spray
paint... I just checked my Radio Shack catalog, they don't carry it).
spray the bottom of the printed circuit board with the same corona dope,
and when replacing the high voltage shield, don't bend that support tab as
close as it was!
Tips on using the corona dope: the instructions say thin coats are better
because they dry quicker; apply multiple thin coats (allow each coat to
dry before applying the next) rather than one or two thick coats!
There is a sort of "putty" for the same application (fixing/preventing
arcs in TV set high voltage sections) that may be useable here, but I've
never had an opportunity to use it.
------------------------------------------------------------------------
The hardest part of all this was finding where it was arcing; remembering
to put my eyes where they needed to be when it was about time for another
arc, and then finding the light switch without moving my eyes so I could
see where the arc had occurred (a flashlight would have been nice).
"Copyright 12-4-88 by John Palmer"
Feel free to redistribute for non-commercial purposes, etc...
JohnP on PeopleLink's AmigaZone, johnpalmer on BIX