Re: Apple //c "Doing the wave"

[aw133@lafn.org (James Poore)]

In a previous article, jlevenso@Rosie.UH.EDU (jonathan levenson) says:

>I have a "Unidisk" //c (ie, no ram expander connector, but can
>accept unidisk drives, fast modems)
>
>I have connected two different NTSC type monochrome monitors
>to the composite output port on the //c and get strange "waving"
>behavior.  Specifically, about 15 scanlines at a time gradually
>shift approx 15-20 pixels to the left, then they realign to center.
>The problem is usually minor upon powerup, but gradually worsens
>as the computer heats up.  I've taken a brief look "under the
>hood" and have found no obvious problems, aside from some dust.
>I have removed as much of the dust as possible, and also reseated
>all of the socketed chips.  Still, the problem persists.
>
>Is this a common //c problem that can be solved relatively easily?
>Could I fix it by swapping out a logic board of some kind?
>Or is it just easier to go buy a new //c?
>(I would like to get this one working however..)
>
>Thanks for help in advance,
>JML
>
>
>Note: I have connected the same monitors to my //GS composite out
>and do not get the "waving" behavior, so I'm pretty convinced
>it's the //c.

And you are definitely right.  What you have is 120Hz ripple in the power 
supply(s).  There are usually 1 or 2 reasons for the ripple.

1) The Filter capacitors for the the different voltages are leaky and 
should be replaced.

2) A voltage regulator is shorted

Problem 1 is found by measuring the AC and DC voltage on the supplies.
The DC voltages should be +/-5V and +/-12V.  The AC voltage should read 0 
or very close to it.  If the AC voltage is at .1 or greater, the caps are 
bad.  Also if the DC voltages are too low the caps are probably bad as well.

Problem 2 is a bit easier.  The DC voltage will read high i.e 12V will 
read 14-15 volts; the 5 v probably around 7-8V.

Since the computer is working, I would NOT suspect the +5V supply, but 
the -5V and the +/-12 lines.

If the wave in the monitor gets worse when you use a disk drive, the it 
points directly to the +12v supply.

Jim

-- 
James M (Jim) Poore--Apple II's Forever--Especially IIgs's
Los Angeles Free Net---aw133@lafn.org
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