Giorgio Morocutti wrote:
On Fri, 01 Feb 2008 15:27:04 +0100, Giorgio Morocutti <giorgio.morocutti@alice.it> wrote:On Thu, 31 Jan 2008 17:32:59 -0800, "Michael J. Mahon" <mjmahon@aol.com> wrote:Michael J. Mahon wrote:Looking at the schematic, I'd suspect C206, a 220uF, 90v electrolytic that filters the supply controlling the cathode voltage. It looks like this value was chosen high enough to keep the cathode biased above the grid for long enough to prevent the un-scanned spot from appearing. There are other possibilities, but that's what I'd try first. The simplest thing to do is to substitute another known-good capacitor of at least 200uF and at least 90v rating. If that cures the problem, then replace the capacitor. (Bridging the old cap with the new one might work, too, but there's a chance that it's electrically leaky, which would interfere with bridging.)Thanks Michael!I have replaced the old cap with new, but nothing is changed. :-( Have you another suggestion? Thanks
You're welcome, but let's see if we can get it fixed! 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%? 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? 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? 4) Did you notice any change in the normal settings of the brightness and contrast controls when the problem began? 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. -michael NadaPong: Network game demo for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."