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From: andyc@hplsla.HP.COM (Andy Cassino)
Newsgroups: comp.sys.atari.st
Subject: Re: SM124 Monochrome monitor problem (an another one).
Message-ID: <5440163@hplsla.HP.COM>
Date: 15 Mar 91 22:40:28 GMT
References: <1483@ahds.UUCP>
Organization: HP Lake Stevens, WA
Lines: 22

| I recently purchased a 'previously owned' SM124 monochrome monitor.  It seems
| to work quite nicely except for one minor problem.  When the display is a
| black background and there is very little text on the screen, the image is
| quite dim.  As more characters are displayed the image brightens as if the
| brightness control were being turned up.

I received e-mail from one Ian D. Hawkins who apparently can read this group
but not post to it. With his permission I am posting his explanation and 
solution for this problem...

"It's easy to fix the fading problem.  It's caused by the drifting of the
 cathode supply voltage.  This is fed to the card on the end of the picture
 tube to a connection marked b2.  It should stay at 70 volts,  but drifts
 high when the screen is nearly black.  the grid thus appears more negative
 and so the current drops off.  Fix it by regulating the 70 volt line with,
 for example ,  a couple of 36v zeners in series.  They dont draw much
 current, only a few mA.  V2 and V3 are ground connection on the tube-end pcb."

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    % Andy Cassino                                       %
    % Hewlett-Packard - Lake Stevens Instrument Division %
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