Christian wrote: cold the with I used a lot of 4010s and 4014s in my days at Tektronix, and saw this type of problem on numerous occasions. The primary terminals for the Control Data Cyber 73 (and later, an added Cyber 175), and Vax 11/780 (VMS during the day, BSD 4.x Unix at night (for the hacks to play with), intermittently) were 4010s and 4014s. There were user areas with a bunch of these terminals grouped together. These user area terminals were very heavily used. Most people didn't have terminals at their desk in those days. The flood system that erases the screen has a limited lifetime. I know there was some adjustment inside both of the terminals (but you'd need a service manual...perhaps bitsavers?) that would adjust the flood current. However, if adjusting this didn't work, they'd check a couple of test points on one of the circuit boards, and if all the numbers came back right, the factory service guys would not fuss with it anymore, and start right away on replacing the tube. A good factory service person could replace the tube in a 4010 (took longer for a 4014) in about 20 minutes. My guess would be that, given the many, many years this terminal has been around, that the tube is just plain worn out. It is a possibility that some of the components in the flood drive circuitry have changed characteristics over the many years...resistors and capacitors can do this, as well as active components due to heat and power cycling. I'm sure that there's service documentation or schematics online somewhere. If bitsavers doesn't have it, I know that there is a website dedicated to Tektronix equipment, and has scans of lots of manuals and such, but can't recall the URL (don't have it bookmarked). Such material would at least let you know if the tube is tired, or if perhaps something else in the flood drive circuitry has gone astray that might be repairable. Rick Bensene The Old Calculator Web Museum http://oldcalculatormuseum.com