spam@eneevax.UUCP (11/02/83)
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I have had experience with 3 Atari 800 computer exhibiting the
'horizontal bars' problem. All 3 had begun doing this after
memory was added. This looked suspiciously like a power supply
problem; the units had passed the factory test with the 16k
originally installed, but were not up to spec with the additional
load of more memory.
An oscilloscope on one of the offending units confirmed
that there was oscillation on the +5 and +12 lines, so I got out
my trusty soldering iron, and soldered some high-quality, silvered
mica capacitors across those lines. They are available at the front
of the computer, under the keyboard. Usually there are already
some small glass capacitors that look like diodes there. Leave them,
and parallel any needed capacitance. It doesn't take many picofarads,
just fast ones.
This fix totally cured all the computers, maybe it will help
yours.
--Sir Spam the Roadie
(John Rehwinkel)
...!umcp-cs!eneevax!spam
PS: My computer, which is dropped/frozen/baked often, never has any
problems of any sort, the ones that get sick are the ones that
get the velvet-cushion treatment. Apparently, Apples need this,
but the Ataris take umbrage at being treated like unliberated
women.wje@inuxe.UUCP (W Erwin) (11/10/83)
I have also experienced the horizontal bars on my atari 800. However, the common fix of putting capacitors across the power supply didn't help my situation. I also looked at the power system with a scope and could find nothing unusual. I even had another 800 open at the same time and compared the two systems. Then i resorted to basics - see if anything feels unusually warm. I found that one of my large filter capacitors (4700 UF i believe) was much warmer than the other. The capacitors on the other 800 i was using for comparison were cool. I would have thought that a defect of this kind would reflect in high ripple and/or low input voltage into the regulator but neither was the case. At any rate, replacing the capacitor fixed the problem. Good luck.