irwin@m.cs.uiuc.edu (11/16/89)
/* Written 3:17 pm Nov 14, 1989 by usenet@cps3xx.UUCP in m.cs.uiuc.edu:sci.electronics */ >A "friend of mine" (no really!) has a Space Wars video game which is >slightly defective, and has enlisted my help to bring it back up to >mint (1977!) condition. The problems lie in the video section. >Specifically, the VectorBeam (tm) vector display. The display is very >dim and can only be viewed in a totally dark environ. The yoke amp is >fine, as everything is in the right place...just very dim. ....Some text deleted... >mail at >beyer@frith.msu.edu >or lemense@msupa.msu.edu >I'm lost in a sea of High Voltage! >Tom LeMense /* End of text from m.cs.uiuc.edu:sci.electronics */ Tom, I serviced TV for one heck of a lot of years, and my intuition tells me that your problem is not high voltage, but rather a low emission CRT. The filament in a CRT heats the cathode, which is coated with a white powdered substance to increase emission. The surface of the substance becomes oxidized and emission drops. Heating the filament to an above normal temperature increases emission and helps to burn off the oxidation. This was the pricipal of the picture tube rejuvenator which was a picture tube tester carried around by the service man. He could "shoot the tube" and nurse it back to health. This used to be a problem with old TV sets and there used to be "Pix tube brightners" on the market that would help the situation. What they were was a step-up transformer with a plug and socket attached, where all signals to the CRT were straight through, except the filament. The filement voltage went to a transformer, which had a ratio of a little more than 1:1, so a 6.3 volt filament voltage was stepped up to maybe 7 volts. Some of the more elaborate ones even had adjustable taps with a lead and several holes that it could be plugged into, to change the ratio. To use the brightner, one simply unplugged the socket from the CRT neck, and connected that to the plug on the brightner. The socket from the brightner was then connected to the CRT neck. To prove if this is your problem, you could disconnect the filament leads from their source and power the filament with an external adjustable power supply. DC will work, even if the tube is powered with AC in the device. If you slowly increase the voltage to the filament, watching it glow in the neck of the CRT, and monitor the amount you apply, you can see if a slight increase helps. Measure what the normal voltage is on the filament, under normal operation, so you have a feel of what you should apply. Increase this amount by 10 to 15 percent and see if you get improvement. This will tell you if it is the CRT or not, and you might be able to burn off the oxidation and improve results. 30 seconds at 8 volts, and then drop back to 6.3 and see if it holds is the way to do it. Note, that is assuming that 6.3 IS the voltage. Some CRTs have other voltage levels on the filament, like 2.5 volts, so proceed with caution. Hope this helps. Al Irwin Univ of Illinois Dept of Comp Sci irwin@m.cs.uiuc.edu