Michael J. Mahon wrote:
Giorgio Morocutti wrote:On Thu, 31 Jan 2008 01:16:32 -0800, "Michael J. Mahon" <mjmahon@aol.com> wrote:This is the result of the CRT not remaining biased off after power is turned off. I'd look for an open capacitor or resistor in the vicinity of the grid and/or cathode bias circuits. (I haven't found the schematic yet.)Can I send you in .jpg format via email ? (circa 900kB) ThanksSure--that would help! (My email address is un-munged.)
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.) -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."