18.1 SM124 MonitorReparaturanleitung für den SM124
Monitore SM124, SM144, SM146
Der SM124 ist ein sehr kontraststarker 12" Monitor, bei dem
etwa 10" für das Bild (etwas klein) genutzt werden. Bei
diesem Monitor kann man von aussen nur die Lautststärke, den
Kontrast und Helligkeit regeln. Der SM124 hatte von Haus aus keinen
Monitorfuß. Atari hatte aber für kurze Zeit einen passenden
Fuß im Angebot, der direkt am Monitor befestigt wurde.
Es gab auch mal einen SM125 . Das war im Prinzip ein SM124, nur
mit Monitorfuß. Wurde aber nur in sehr geringen Stückzahlen
verkauft und dann zugunsten der seperaten Monitorfuß-Lösung
(s.o.) aufgegeben.
Mit dem SM144 kam dann als Ablösung ein 14" mit etwa
12" nutzbarer Bildschirmfläche.(!nl) Ein wesentlicher
Nachteil gegennüber dem SM124 ist die wesentlich geringere
Kontraststärke aber vor allem die Unschschärfen im
Randbereich. Auch das Bild war meistens recht schief. Er hat aber auch
einen Vorteil:
Man kann die Bildgrösse ohne öffnen des Gehäusses
vornehmen. Dieser Monitor kam im Prinzip für den Mega STE raus.
Er hatte einen Monitorschwenkfuß, der genau auf die
"Brotdose" gepasst hat.
Kurz nach dem SM144 kam der SM146, beide wurden sogar für
längere Zeit parallel verkauft. Er ist auch ein 14" Monitor
mit 12" genutzter Fläche. Er ist genau wie der SM144 nicht
so kontraststark wie der SM124, hat aber nicht das Problem mit den
unschärfen und dem schiefen Bild wie der SM144. Auch hier gibt es
neben den Reglern für Helligkeit, Kontrast und Lautstärke
(alle 3 Regler vorne) noch hinten 3 Regler für H-Phase,
V-Größe und V-Bildfang. Auch dieser Monitor hat einen
speziell an den Mega STE angepassten Monitorschwenkfuß.
Alle Monitore werden inzwischen nicht mehr gebaut. Will man einen
Gebrauchten kaufen, sollte man vom SM144 ausser acht lassen. Ob SM124
oder SM146 ist reine Geschmacksache.
Von dem Poti für die Bildbreite und -höhe würde ich
generell die Finger lassen. Wenn die Bildbreite und -höhe
vergrößert wird, ändert sich die Stromaufnahme des
Zeilentransformators und beim SM124 hat das die Zerstörung einer
Diode im Leistungsteil zur Folge. Sprich: Danach gibt es kein Bild
mehr.
Abbildung 1 - SM124 Monochrome Monitor
Bild: von Bernd Maedicke
Das Orignale Anschlußkabel des Monitors ist ein spezielles Kabel, es besitzt eine Koaxialleitung und einige normale Kabeladern. Solche Kabel bekommt man ausschliesslich im Fachhandel. Zur Not reicht ein dünnes Koaxialkabel und eine 4-6 adrige abgeschirmte Steuerleitung. Beide Kabel sind dann halt zusammen zu binden.
Wichtig ist die Mono-Detekt Leitung.
Dazu ist im 13poligen Monitorstecker eine Drahtbrücke von Pin
4 nach Pin 13 herzustellen. Wird das nicht gemacht läuft der
SM124 nicht und die Videohardware kann Beschädigt werden.
Nicht alle SM Monitore stammen vom gleichen Hersteller, Stecker
und Belegungen können von dieser Beschreibung abweichen.
MONOCHROME MONITOR FIX
Randy Constan, Elfin Magic Co.
If you have an ATARI SM124 Mono monitor that has recently gone
belly up,
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,
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..
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.
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
If your monitor has fallen victim to this failure, in a way your
lucky.
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,
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.
Since we have plenty of extra fuses now, start by just changing
them,
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,
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.
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.
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...
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.
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:
Look for an electrolytic capacitor that is bulged, burned or
leaking brown stuff. Most likely located near the power supply
transformer. If it was an SM124 monitor, it would probably be the
2.2mfd 50 volt bipolar capacitor labeled C714. I've never had any
problem with an SM125 so am not sure what specific capacitor it might
be. The electrolytic capacitors do go bad with age though. Fortunately
they usually show physical signs of failure, and they are inexpensive.
Copyright © Robert Schaffner (doit@doitarchive.de) Letzte Aktualisierung am 23. Mai 2004 |