

     If you have an ATARI SM124 Mono monitor that has recently 
gone belly up, read on. In particular, this pertains to those of 
you who have tweaked the coils and trim pots in your monitor to 
get a full raster display, although it's quite possible that 
unmodified monitors have been affected as well. Also, if do have 
a modified monitor that's behaving fine, you still may want to 
consider the suggestions described in this article, as it could 
save you a surprise monitor failure that (you _KNOW_ ) will 
definitely happen at the worst possible time.

     Finally, this text describes a problem particular to the 
monitor manufactured by "Gold Star Co", which has the Brightness, 
Contrast, and Off/On Volume controls recessed on the right side- 
panel. There may have been other manufacturers of SM-124's with 
completely different electronics, so if ANY of the physical 
component layout descriptions written here do not seem to jive, 
then all bets are off and the information given here may or may 
not have any practical value. In addition, the monitor 'fix' I'm 
going to describe should only be done by a trained electronic 
technician, or at the very least a knowledgable electronics hacker 
with good soldering AND de-soldering skills!

     Disclaimer: You can easily get knocked squarely on your butt 
     by the voltages present in your monitor. Weather or not it 
     kills you probably depends on just what you fall on, your 
     present health, and where the resulting flying monitor lands. 
     In any case, your decision to implement the changes described 
     herein is completely your own. I disclaim all responsibility 
     for your health, safety, or any damage to equipment or 
     property caused by fires, implosions or explosions, 
     earthquakes, global flooding, or galactic imbalance resulting 
     from your use of any information in this writing. I think 
     that about covers it ;)

     So... once upon a time, we all got our nifty HI-RES mono 
monitors, powered them up, and the very first thing we all said 
was, "Ah.. MAN!!... This monitor is screwed up". Perhaps stronger 
language was used, but the fact was that we all thought something 
had to be wrong. Alas though, the tiny picture on that screen was 
exactly what the ATARI mystics had intended! Of course, it wasn't 
long before many daring hackers boldly removed the monitor's 
covers, cast their warrantees aside, and discovered a way to 
expand the tiny picture to full size. The information spread 
quickly through the worldwide internets, spider-nets, and fish 
nets, and soon all mere mortals had full screen displays and could 
actually read that 6 X 6 font! And so, the all lived happily ever 
after. But then one day years later, a curse fell upon the land, 
and darkness fell upon the faces of many of these monitors.
 
     Ok, enough with the story narrative. By now, you're beginning 
to get the picture (no pun intended). The modification to fill the 
screen does indeed place an additional stress on a portion of the 
monitors circuitry, which coupled with a design error quite common 
to many types of computer monitors, ultimately caused the failure. 
The good news is that I've now seen quite a few monitors with this 
identical failure which is 100% repairable, and should leave you 
with 'good as new' results. The bad news is that you do need some 
technical skills, but even if you don't, finding a techie friend 
to help will be abundantly easier having a possible procedure for 
repair. 

                              page 1




     Brief technical summary. Right near the coil you adjusted to 
spread the horizontal width of your display, there's a 2.2 uF 
capacitor. More specifically, it is a non-polarized electrolytic 
capacitor, which is actually 2 polarized capacitors wired 'back to 
back' in a single enclosure. As the screen is widened, the peak 
voltage level across this part increases, thereby causing more 
internal stress. This situation is further aggravated by the type 
of part as well. All capacitor types have certain advantages and 
disadvantages and when it comes to electrolytic types, the 
advantage is smaller size. It's disadvantage however is that at 
higher frequencies, it has high 'leakage' current and acts more 
like a resistor. The Horizontal frequency passing through this 
capacitor is in fact quite high, and the resistive qualities at 
these frequencies cause the capacitors to generate significant 
heat. Over time, that heat will cause the part to fail even in un-
modified monitors, but the modification certainly speeds up the 
process.

     If your monitor has fallen victim to this failure, in a way 
your lucky. There's a fair chance that the problem (as said 
earlier) would happen one way or another anyway, and at least now 
you have a possible fix. First, you need to disconnect the monitor 
and open it up. If you have the chassis I'm describing there will 
be two screws on the upper sides, visable from the back; two 
more screws on the bottom; and a single screw on the back above 
the power plug, which is a removable 'spade' type AC connector. 
After removing the screws, you can remove the back cover, but be 
sure to be careful of the speaker connections. The speaker is 
mounted to the inside of the cover, and connects to the monitor's 
main PC board via a small removable plug. It's a bit tough to get 
to, but trust me, it IS removable.

     Once the cover is free, there are 2 fuses to check first. 
From now on, all references to right and left will assume you are 
behind the monitor, with the screen pointing away from you. There 
is a small PC board mounted vertically on the right side of the 
chassis which contains a 0.75 amp fuse. There's also a 2 amp fuse 
immediately to the right of the speaker plug you removed on the 
main board. Check both fuses with a DVM or other suitable 
instrument. Most likely, you'll find that it's the 2 amp on that 
is gone, but either one blown is possible. You can buy these tiny 
fuses at Radio Shack, and while your there, you can pick up a much 
more modern replacement for the capacitor we mentioned earlier. 
There's also a resistor that often cooks when this failure occurs, 
so you might as well get all this stuff at once. If the problem 
turns out to be something else, at least you'll be prepared for 
the eventuality. In any case, read on to do a 'walk through' 
before buying, to make absolutely sure your chassis physically 
matches the one we're discussing.

     At your local Radio Shack, pick up the following items:

          fuse      (pkg of 4)     3/4 amp             270-1048
          fuse      (pkg of 4)     2.0 amp             270-1052
          fuse      (pkg of 4)     5.0 amp*            270-1056
          capacitor  buy 2!        1 uF, 200v metal    272-1055
          resistor  (pkg of 2)     680 ohm, 1/2 watt   271-021

                    * optional purchase, see text.


                              page 2




     Since we have plenty of extra fuses now, start by just 
changing them, and setting up the monitor (still uncovered) with 
your ST. If the display comes up normally, leave everything 
powered up and wait a few hours. If all goes well, you can either 
make the fix were going to describe anyway to save future grief, 
or put the other parts away, along with this text, in case the 
problem returns.



     Assuming the problem did not go away (no display and/or fuse 
blown again), there are two paths you can take from here. You 
_can_ just go ahead and change the suspected components without 
really knowing if they are at fault, since they'll probably need 
replacement eventually anyhow. The other possibility is that you 
don't want to to this until you're a bit more sure it's necessary. 
If that's the case, read the next few paragraphs. Otherwise, just 
skip over to "Making the Repair".



     If you do want to do some checking, here is a possible (but 
not foolproof) procedure to see if the suspected capacitor is 
indeed the culprit. If the 0.75 amp fuse on the vertical board was 
blown, temporarily replace it with a 2 amp. If the 2 amp fuse on 
the main board has blown, temporarily replace it with one of the 5 
amp fuses. Set up the monitor, still open, near your ATARI, and 
connect the monitor cable to your machine. Boot the ATARI with the 
monitor still off. We want to keep our 'powered up' time with the 
larger (wrong) fuses to a minimum. Now, hook up the power cord and 
turn on the monitor, being ready to instantly kill power at the 
first sign of any unsafe condition such as smoke or electrical 
arcing.



     With the higher current fuses installed, the display will 
probably still be out, but the green pilot light on the front 
panel _should_ light. If it doesn't, then either the new fuses 
have blown, or a problem exists somewhere else in the circuit. Of 
course, the pilot light could just be shot, but if the higher 
current fuses do actually blow, then forget about the remainder of 
this discussion and just try implementing the suggested repair.   
Also, if the screen display _does_ appear with the larger fuses, 
don't be to happy yet! Twice now I've seen the 'crippled' 
capacitor seem to work fine for awhile before the final breakdown. 
So weather the screen display comes up or not, proceed to the next 
step as long as the fuses are not blowing.













                              page 3




     Locate Capacitor C714. It will be the round cylinder on the 
left side of the main board (from the back), in-between the coil 
you adjusted when you increased the width and the flyback 
transformer. The coil is the only adjustable coil on the board, so 
you can't miss it, and hopefully you know that the flyback 
transformer is the thing-y with the wire that goes into the top of 
the picture tube! Locate the capacitor... but don't touch it yet! 
The flyback puts out thousands of volts, and if there is a flaw in 
it's insulation, it could arc right into you hand. Just leave the 
unit powered up for a few minutes, and then turn it off. Now, with 
your finger, move close to, and finally touch the capacitor from 
the top. What you're looking for is a _HOT_ part, so be careful! 
If in fact the part is really hot, it probably has shorted and 
needs to be replaced. 


     Also, immediately to the right of the coil, also on the main 
board is a diode (D-709) and a resistor (R-221). Take a good look 
at the resistor to see if it looks stressed or burnt. If you want, 
you can do an in-circuit check with a DVM to see if it has opened. 
You can make this measurement from the bottom of the board if you 
want since it's pretty cramped on top. Even if it has not died, 
you should consider replacement if it appears stressed, or the 
underside of the board appears burned and discolored at the 
location of the resistor.



                        Making the Repair


     Making the repair is simply a matter of changing the parts, 
BUT... there's some special considerations here. You may or may 
not be aware that radio shack does have 2.2uF, 50 volt non-
polarized electrolytic capacitor in stock! But using that part 
would be dumb since the conditions that caused it to die in the 
first place have not changed AND, the part was a poor design 
choice to begin with! We're going to use the two 1.0 uF metalized 
film capacitors which are NOT electrolytic, AND have a working 
voltage 4 times that of the original part! We will have to 
parallel both capacitors to get 2.0uF, which is reasonably close 
to 2.2 and in fact works just fine. First though, you'll have to 
de-solder the existing C-714, which will probably be a bit of a 
chore. Do that, and also get rid of the existing R-221 if it 
shows the signs of heat stress (or is open) as we discussed 
earlier. You can just cut it out if you want, since we'll be 
soldering the new parts to the underside of the board.

     Now, take the two 1.0uF capacitors, and solder them together 
in a neat parallel arrangement, that has a pair of leads bent to 
accommodate the spacing of the leads on the original capacitor. 
Look at the underside of the board and observe the original C-714 
solder pads, as well as the clearance to the bottom when the cover 
is replaced. Give it a little thought and come up with a 
mechanically sound and visually sensible combination. The 
capacitors again, are not polarized so direction does not matter. 
And remember... parallel, NOT series!!! Solder the parallel 
combination to the C-714 pads under the board. Try to push the 
leads right into the holes left by the removal of the old 
capacitor.

                              page 4




     R-221 should also be replaced from the bottom side of the 
board unless you're really patient, and have some small tools. 
It's much easier to just trim the leads and tack solder it in 
place under the board. If the board looks a bit burned in this 
area, you may want to clean it up a bit prior to installing the 
new resistor.

     Finally, don't forget to put the proper fuse values back into 
BOTH locations. The original fuse values are important protection 
and do not normally blow unless there is a real problem. I 
definitely do not recommend leaving the larger fuses values in 
place. Do a bench check on the ST again, still with the cover off. 
You may need to re-adjust the horizontal width, or the position 
tabs on the neck of the picture tube to center and set up the 
display. Once this is done, you can put the cover back on, and 
you're back in business. If the fix does NOT help, just double 
check your soldering and connections to make sure you didn't wire 
something wrong.

     Hope this fix was helpful. If so, I'd like you to drop me an 
EMAIL, since I'm curious as to just how many fried monitors out 
there had this problem. Also, if you're not able to make the 
repairs yourself and live anywhere in Islip, Long Island, NY area, 
send me E.Mail and maybe I can help you out. My GE mail address is 
R.CONSTAN, or Internet: rconstan@maestro.com.

                              -Randy Constan, Elfin Magic Co.


































                              page 5


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