cs121ad@ux1.cso.uiuc.edu (09/22/90)
I do not know if this is the correct place to put this but.... I have a zenith dumb terminal (z29) where in the tube keeps losing the signal for some reason. Does anyone know where I can get schematics for it (zenith serial # 9587597) or better yet could anyone tell me which chip is the signal amp? Maybe replacing that would solve my problems. Please response via e-mail because I dont read this group often. //mike neuliep wires@uiuc.edu
BOYDJ@QUCDN.QueensU.CA (Jeff Boyd) (09/24/90)
I have exactly the same problem with a z49. It's been sitting in a closet waiting for me to find time to attack it with the voltmeter, but I could remove the guesswork with schematics.
mbeck@AI.MIT.EDU (Mark Becker) (09/24/90)
Hello * I had responded via private e-mail to the gentleman with the Heathkit terminal and fading display. Since there's been some followup to the group, I thought a summary would be useful. This information applies to the Heath H-19a terminal. I had the 'fade out/fade in' problem some time ago while doing classwork. The display would fade out over about a 10 second period, fiddling with the brightness control wouldn't help, and it would fade back in sometime later. Was driving me nuts. Especially when trying to debug the class project for Intro to Databases. Filiment power for the CRT is developed from a six-turn winding around the flyback transformer. This winding is routed through a Molex connector to the main analog board in the bottom of the set, via some traces, through another Molex connector to the small board under the CRT, and finally to the CRT pins via a small 7 (9?) pin push-on connector. In my case, one of the molex connector pins sticking up out of the main analog board had developed a crack all the way around the pin. Depending on the temperature of the pin, it would sometimes make connection, sometimes not. It worked okay when cold.. but would start getting flaky maybe a half-hour later. Then the display would start to oscillate between full brightness and nothing-at-all. It was fairly easy to remove the analog board... remove the logic board, undo the screws, and carefully manipulate the analog board out from under the CRT. Resolder the offending pin, reassemble, and apply power. The Molex connector used by Heath in that terminal has a pin/socket scheme with a point-contact connection. The point-contact, under the right conditions, will develop a great deal of heat due to resistance at the point. This heat can get high enough to melt the solder off the pin. The connector bodies might discolor a little (didn't in the Heath but did in another piece of equipment here) Next time it happens, look inside the unit and see if the filiment is glowing. If it isn't then you might have the same problem I did. Hope this has been of some help. Regards, Mark mbeck@ai.mit.edu