monitor question (problem diagnosis)

[Dave Dunfield]

At 09:57 07/11/2005 -0500, you wrote:
>Re: " What does a single vertical line in the middle of a display indicate?"
>
>This is a symptom that is normally never seen.
>
>It would, in theory, indicate loss of horizontal deflection.  The problem is
>that loss of horizontal deflection in virtually all monitors also causes
>loss of high voltage, which keeps you from seeing ANYTHING.  [The high
>voltage is obtained from the horizontal output transformer ("flyback"
>transformer) in virtually every TV set and monitor made.)
>
>One possibility:  If the horizontal deflection yoke was open, there could be
>a loss of horizontal deflection while you still had output from the
>horizontal output transformer.  Usually, an open yoke would shut down the
>horizontal output and high voltage, but it's the only way I can think of
>that you might get loss of horizontal deflection and still have high
>voltage.

I have a MAC Plus with the same symptom - seemed odd to me as well, so I asked
a local guy who used to repair monitors. He didn't think it was that odd, here
is what he responded with (I haven't had time to look further at it yet):


>#2 - I've got a Mac Plus which displays a thin VERTICAL line - I've not 
>seen this before, because since it has HV, the horizontal oscillator must
>be running! - any guesses?

This obviously has separate circuits for EHT and horizontal 
drive/deflection, and the deflection circuit has a bad drive component or 
bad solder joint. Usually the HOT goes due to high ESR caps in the primary 
side of the transformer driving the HOT, or the solder joints go bad because 
of the high drive currents and heat. Also check the flyback transformer 
(actually, all) solder joints with a 10x loupe.  You really need an ESR 
meter and to go through all the electrolytics on the board. I regret selling 
mine a few years back; that was an invaluable piece of test gear.


Regards,
Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
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