rich@island.uu.net (Rich Fanning) (01/11/90)
I have an original 128K Mac which will soon be reaching its fifth birthday.
Alas, in the last few weeks something odd has happened to the brightness
of the screen. Before, the brightness control adjusted it from completely
dark to fairly bright. It now goes from fairly bright to REALLY REALLY BRIGHT.
Is there a simple fix?
--
Rich Fanning | "Thinking is more interesting than
{uunet,sun,well}!island!rich | knowing, but not so interesting as
| looking" - Goetheken@argus.UUCP (Kenneth Ng) (01/13/90)
In article <1264@island.uu.net>, rich@island.uu.net (Rich Fanning) writes:
: It now goes from fairly bright to REALLY REALLY BRIGHT.
: Is there a simple fix?
Good shades? :-)
--
Kenneth Ng: Post office: NJIT - CCCC, Newark New Jersey 07102
uucp !andromeda!argus!ken *** NOT ken@bellcore.uucp ***
bitnet(prefered) ken@orion.bitnetemuroga@m.cs.uiuc.edu (01/15/90)
/* Written 9:55 pm Jan 10, 1990 by rich@island.uu.net in m.cs.uiuc.edu:comp.sys.mac */
/* ---------- "128K Mac screen too BRIGHT!" ---------- */
I have an original 128K Mac which will soon be reaching its fifth birthday.
Alas, in the last few weeks something odd has happened to the brightness
of the screen. Before, the brightness control adjusted it from completely
dark to fairly bright. It now goes from fairly bright to REALLY REALLY BRIGHT.
Is there a simple fix?
--
Rich Fanning | "Thinking is more interesting than
{uunet,sun,well}!island!rich | knowing, but not so interesting as
| looking" - Goethe
/* End of text from m.cs.uiuc.edu:comp.sys.mac */
If you open up the mac, there is an adjustment screw on the left side
to control maximum brightness. Maybe yours just needs readjustment.
Eisuke Muroga
Department of Computer Science | 1304 W. Springfield Ave.
University of Illinois at Urbana-Champaign | Urbana, IL 61801
ARPA emuroga@m.cs.uiuc.edu
CSNET emuroga@uiuc.csnet
USENET uiucdcs!uiucdcsm!emuroga
BITNET emuroga%m.cs.uiuc.edu@uiucvmd.bitnetdan@mipon2.intel.com (Dan Casali) (01/17/90)
I've had this happen to two add-on monitors; one was under warrantee and was simply swapped; the other was an out of warrantee two-page grey scale monitor (Viking) that showed this symptom last week immediately after I heard the sharp snap of a high voltage arc from within. I thought this must be a common failure mode, and watched with interest for responses to your posting but saw none. This weekend I did a bit of research, then opened up the case and had at it with my scope. Up front, let me say that if your monitor suddenly took a turn for the worse, it is unlikely it will respond to a simple re-adjustment. You got video blight, guy. If you have a voltmeter, a scope, and a basic understanding of transistors, you might be able to debug the problem. I did. On most monitors, the brightness control affects overall electron emission (bias) and the contrast control affects video amplifier gain. If you have a schematic (fat chance), track the video signal thru each amplifier stage and look for decent gain at each stage. The final stage must put a signal of over 50V peak to peak on the video tube control grid. [NOTE: The video tube acts like a large capacitor, and retains a charge of ~15-20K volts even after the monitor is turned off. It is not lethal but it is extremely startling. If you have to get near the HV lead learn how to discharge the tube. ] If you dont have a schematic (I didn't) you can try working backwards from the video tube pins. Identify the video signal (if you can see anything on your screen, it'll be there), then trace back until you find a stage that isn't amplifying. In my case, it was the drive transistor to the final amplifier. Signal in = signal out; not normal behavior for an amplifier. Snip out transistor and check resistances with a voltmeter. In general, c-b and e-b resistances should show about 30:1 ratio between forward and reverse measurements. Lots of luck. You have almost nothing to lose by giving it a shot. And it felt great when it came back up. (By the way, if there was some precipitating event that lead to the failure, be sure to work on eliminating that as well. I'm pretty sure the arc I heard went to the heat shield on the transistor that died. I put on a different shaped shield, blew out dust, and sprayed the whole mess with clear Krylon plastic. Go for it.... -dan casali