Re: Apple IIgs color monitor

[Rubywand <rubywand@swbell.net>]

Alberto Cavalcoli writes ...
> 
> I'm sorry if anyone have posted this question before.
> I have a problem with my Apple IIgs RGB Color monitor: after 30 mins of
> works the monitor become too brightness and lost focus. I have tried to
> replace all the electrolytic capacitors and two big resistors (R413 -
> 470 ohm, R901-6,8 Ohm), but the monitor do not show any signs of getting
> better.
> Have anyone fixed this defect ?
 ....

     Replacing the power supply filter capacitors is a fix suggested by Eric
Jacobs. It seems like a good try and may work in many cases, especially for
monitors that have been sitting around in a closet for a few years.

     A few years ago, our original IIgs monitor exhibited somewhat similar
behavior. I did not try replacing the power supply filter capacitors-- did not
know about that approach at the time. A decent fix was to clean off dust and
adjust the Intensity and Focus controls on the back of the black High Voltage
Module. (Also, I had already resoldered all of the HV Module connections on the
main circuit board.)

     The monitor works well enough-- i.e. is bright and sharp-- but it does
require a bit of brightness adjustment after several minutes of operation and
the display has to be slightly compressed vertically in order to maintain
linearity. (So, I was glad to get a replacement GS monitor from a sister who
decided to go all-PC!)


     All of the stuff I've read about the brightness and lack of focus problem
seems to agree that, one way or the other, the cause is likely to be a High
Voltage which is not high enough. This could go back to a weak low-voltage
supply. Or, it might relate to some weak component in the HV circuitry-- like
an electrolytic capacitor in the Horizonal oscillator which drives the HV
supply.

     One component which hasn't been memtioned is the temperature sensitive
surge resistor often found in the AC line input going to a TV or monitor
low-voltage power supply. (By the way, the "low voltage" supply voltage is not
all that low in some places.)  This special resistor, sometimes called a
"globar", usually looks like a fat disc capacitor. It is supposed to start out
around 20 to 50 Ohms or so when power is first turned ON and, then, quickly
drop to a much lower value as it heats up. If there is one in the IIgs monitor
and it is defective then all of the power supply circuits would be affected.


     Again, if you (or Ron or Alberto) decide to work on your IIgs monitor, be
sure to have an open, non-conductive work surface out of the reach of children
and check the other case opening and safety measures outlined in the FAQs--
like wear goggles, keep heavy tools away from the CRT neck, do not touch two
conductive circuit points at once, etc.. 
(See  http://home.swbell.net/rubywand/Csa2MONITOR.html .)




Rubywand