Electronics > Repair
Repairing a video monitor in an IBM 3279 terminal
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jmaynard:
Several hundred volts is ridiculously high voltage to get across with one's hand... but no, I do not plan to touch anything on the tube base with anything but a Fluke Scopemeter probe.
inse:
No progress on the monitor so far?
jmaynard:
The heater is glowing, so that's good.
Nothing on the screen at any point, including both full brightness and the monitor test (turning the brightness all the way down to activate a momentary switch). One of the lugs on that switch is broken off, however, so I'm not confident of that result.

I do get a short green flicker of horizontal lines on the screen when I turn the unit off. Not entirely sure what that tells me, though.

Going to go back and probe the power supply voltages to make sure everything is good. I'm beginning to think the monitor might not have a problem at all.

Edit: Power supply voltages are all good. I'm seeing sync at the monitor input. Trying to get video signal probed, not entirely trivial.

Edit 2: Adjusting the G1 Bias control got the separator line to display! And when turning the unit off, the raster was visible for a moment as it shrank on the screen. I'm going to say the monitor is good and my problem (if any) lies elsewhere.

Edit 3: I got it into offline test mode. The green is extremely dim even with the Gi Bias control all the way at one end of its travel. Red and blue are brighter. All three need G1 Bias and the brightness control to be all the way at the end of travel to be visible. There aren't any crntrols labeled as drive, though. Wonder if the CRT is just tired and worn out, or...?

Edit 4: Well, that's a setbck. I had it in test mode showing a red screen...and then activated the display override switch. It went "fsssh" and quit, and now the power supply is acting fried: +5 comes up but intermittently drops out, and the ready light never comes on. Time to pull the power supply and see what fried.
inse:
What a bummer!
What is the override switch for?
jmaynard:
It's a test: when activated, it's supposed to display a complete, unblanked raster.

Pulled the power supply out, found two power electrolytics that appear to have let the magic gunk out, ordered replacements for all of the power electrolytics. Should arrive Tuesday. A mere $22.
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