From: knop@dutecag.et.tudelft.nl (Peter Knoppers)
Subject: Re: SM124 problems
Summary: Fix included.
Organization: Delft University of Technology, Dept. of Electrical Engineering
Date: Wed, 11 Nov 1992 10:06:00 GMT

cs2320ak@utagraph.uta.edu (cs2320ak) writes:

>I looked in the FAQ but couldn't find anything about this.  I know I've
>heard just about everyone who owns an SM124 monochrome monitor complain
>of this problem, which is when the background is all black using white
>characters or graphics, the brightness of the characters fades to near
>nothingness.  I would like to know if there's something I can do about
>this problem, as in getting it fixed for cheaper than a new monitor
>would cost.  Is this a design flaw?  Thanks in advance.

This fix was posted a long time ago (aug 1988) by Werner W. Braun. I don't
think he would mind my reposting it as he wrote "I hope this will help
someone else"...

I have personally applied this fix to two SM124 monitors and it cured the
problem completely. Both monitors still work fine, many years later.

Peter Knoppers - knop@duteca.et.tudelft.nl

------message <8808101333.AA23692@ucbvax.berkeley.edu> follows------
Some days ago i sent a plea for help to all Atarians in netland:

> Is someone out there in netland who knows a difference between older and the
> newest SM124 B&W-monitors which could explain the following behavier:
> Mega ST4 and old (Oct. 87) Monitor:
>     In reverse mode (light text on black background) is brightness dependent
>     on information on screen !
> Mega ST4 and new (Mar. 88) Monitor:
>     The symptom does not occur !

Our dealer didn't know of this problem, he told me all SM124 monitors are
returned to Atari for repair because they get no parts from Atari. So they
have no experience with these monitors. (Thanks to V.A.Brandt who gave me
an important hint)

I decided to take an oscilloscope and did some measurements.
As far as i know at two places the brightness of a CRT is controlled:

           1) the first gate (hope this is the right technical term)
           2) the cathode voltage/current.

The voltage between cathode and 1st gate should be constant, by adjusting this
you can control brightness of the monitor.

Therefore i measured the voltage of the first gate against ground: it was
constant, so this was apparently not the cause of the trouble.
Then i measured the cathode voltage, which should be +70V as the schematics say.
I found these voltage to be screen-image dependant:
           with a light screen i found +70V,
           with completely dark screen i found +85V.
The cathode current varied between 0mA (no bright, no contrast) and 17mA (full
bright and full contrast). So i think the problem is a bad regulation of
cathode voltage. If the load is too small (dark screen or only some light
characters on a dark screen, the cathode current is small) cathode voltage
goes up and voltage difference between cathode and 2nd gate goes down --->
screen fades.

And now my fix:
Beware --- I'm a simple minded chemist who ownes a soldering iron but is not an
EE, so i will take no warranty for any side effects of my fix. It works with
our monitor, but i fixed it today and so have no experience on the long run.
It's a simple fix and should do no harm to your SM124. What do i do?
I put a little extra load (1.5mA) at the high voltage transformer, so that the
current never sinks under 1.5mA and cathode voltage never goes up again. With
maximum brightness and contrast i measured 17mA, in normal operating condition
about 8-13mA, so this extra current should never overload the transformer. But
i give no warranty for this! Be carefull if you turn on full brightness and full
contrast.

The procedure is given for a monitor manufactured in oct '87, the newer ones
have a different PCB layout.

1. Open the monitor by removing the 5 screws.

2. Look at the small board which is plugged on the end of the CRT. There you
   find the names of the parts printed. Look for C308 (47uF/100V) at the lower
   right edge of the board. If the board is covered with a shielding, you may
   have a newer monitor and this fix possibly does not apply.

3. Get a resistor 47k ohms 0.25 W and solder it across the pins of C308.
   Put the resistor at the right boarder of the board so that it does not
   touch the monitor box later.
   Pay attention to the small space between the monitor box and the board.
   Possible Damage of your monitor!

4. Close the monitor, cross fingers and switch on. Problem should have gone.

PP: I also found some differences between the circuit drawings and the actual
    board, but this should be no problem. But i supplied R302 with 1k/0.25W,
    but this is not necessary for the fix

     part        was        should be
   ---------------------------------------
     R302       open         1k
     R311        68R         47R
     R312       270R         240R
     R313       820R         680R


I hope this will help someone else, forgive the rather long posting.

Werner W. Braun     xbr3d815 @ ddathd21.bitnet
Newsgroups: comp.sys.atari.st