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Re: Monitor problem

Giorgio Morocutti wrote:

On Wed, 06 Feb 2008 01:02:39 -0800, "Michael J. Mahon"
<mjmahon@aol.com> wrote:



First, a few questions:

1) When the monitor is warmed up, about what percent is the
  brightness control turned to in order to begin to see the
  "background" (black) raster?  10%?  60%?



Circa 50%




2) If the brightness control is set to just below the point
  where the background raster is visible, what percent of
  the contrast control is required to get a good display?



circa 25%



3) After setting the monitor up as in question 2), what setting
  of the brightness control is sufficient to make the entire
  screen (including the text) black?



circa 33%


Al of that looks typical.


4) Did you notice any change in the normal settings of the
  brightness and contrast controls when the problem began?



unfortunately, no !


Hmm.


Without knowing the answers, there is one more capacitor that
is a candidate for your problem:  C501, a 10uF @ 150v electrolytic
near the horizontal output transformer.  It's the filter capacitor
for the negative grid bias.  Note that the positive side of the
capacitor is connected to ground, and the negative side (usually
the "can") is at about -100v relative to ground.



I have made it, but no change.
(Only the new cap is 350VL rather than150VL)

>

Thanks Michael!


I'd feel better if we found the problem--but at least we've ruled
out a couple of things.  ;-)

I'm running out of ideas, in the sense that from here on, the
failures that cause the problem become weird, and that opens up
a lot of possibilities.

I don't know what kind of test equipment you have, but if you
have a voltmeter with at least 1 MegOhm input impedance, it would
be a good time to start measuring the voltage at the wiper of
the brightness control, both during normal operation and just
after the monitor is turned off.  (This voltage will be changing
pretty fast, so a digital voltmeter is at a disadvantage.)

(BTW, I'd recommend using test clips so you don't have to hold
probes while making the measurement.)

The other useful measurement under the same conditions would
be the voltage at the cathode of the CRT, noted on the schematic
as the RED wire at the top of the schematic near the CRT symbol.

Either the cathode voltage is dropping too fast at power down
or the grid voltage is rising too fast.  (It's actually the
difference between them that is significant.)

I've even run across a few reported cases where the CRT itself
was causing this problem--which would be a good reason to
either ignore it or engineer a special solution.  ;-)

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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