Floppyshop 2/Floppyshop - 2.zip/Floppyshop - 2.iso/diskmags/5791-.end/dmg-6001/dead_mon.txt

Text File | 1994-05-14 | 15KB | 331 lines

     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: [e-mail].

                              -Randy Constan, Elfin Magic Co.


































                              page 5


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