CRT removing Red and blue signal? - Page 1
Forum section: electronics / beginners

author: MrZwing
author_rank: Regular Contributor
timestamp: March 01, 2017, 07:35:49 pm
content: Helloi wonder what would happen if one would remove the connections for red and blue signal from a CRT tube? would it only send the green color and thus make it "look" like a Green CRT? or would it just make the tv not work at all untill every signal is hooked back up?/MrZ

author: MrZwing
author_rank: Regular Contributor
timestamp: March 01, 2017, 07:37:06 pm
content: Or just hook up all signal to only the Green so all signal info is displayed as Green? would that improve "green monochrome" or mess it up?

author: Ian.M
author_rank: Super Contributor
timestamp: March 01, 2017, 07:47:48 pm
content: Some tube TVs check the cathode current for each colour (usually on an off-screen scanline) and shut down if they detect a tube fault, but others will run happily with one or more cathodes disconnected. Feeding a Luma signal to the RGB component input G with R and B grounded is a much safer way of getting a greenscreen effect, but the shadowmask + phosphor dots seriously compromises the horizontal resolution, aliasing with any HF content in the video signal so its nothing like an actual greenscreen mono monitor.

author: MrZwing
author_rank: Regular Contributor
timestamp: March 01, 2017, 07:51:14 pm
content: Ahh as i suspected with the image quality. sad green monochrome displays are so hard to find for a decent price these days.. i just love how they look and want almost that every visual thing i build need to be green monochrome.well i guess i will just have to become rich and buy all the screens i can get my grubby mitts on.

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2017, 08:04:51 pm
content: Generally it's possible to do, occasionally there's even a service option that disables red and blue. A color CRT with only one gun running still doesn't look the same as a monochrome CRT because it has a shadow mask and the phosphor is different. Mono tubes are still out there, I bought a NOS monochrome CRT from Video Display Corp last fall to replace a tired one in my Wyse terminal , the shipping can be a killer on those though.Save CRTs when you find them though, they're not being made anymore and nobody is rebuilding them either. When they're gone, they're gone.

author: MrZwing
author_rank: Regular Contributor
timestamp: March 01, 2017, 08:11:33 pm
content: sad indeed that noone can see that CRT have a place.. at least to keep old equipment running.

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2017, 08:16:30 pm
content: I love CRTs, same with Nixie tubes and other "obsolete" glow-y things. It has astonished me how quickly they have disappeared, they were everywhere, so common you couldn't give them away and then suddenly they were almost all gone. I regret throwing away a bunch of them years back when they were so common that nobody wanted them. Back in the 90s-early 2000's you couldn't even give away a monochrome monitor.

author: MrZwing
author_rank: Regular Contributor
timestamp: March 01, 2017, 09:40:21 pm
content: oh yeas nixie and tubes in general are beautiful practical works of art that makes everything look 100% more scientific.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 01, 2017, 09:56:33 pm
content: On the physical tube?Depends if it's driven by cathode or grid. An open-circuit grid will float to a modest voltage, allowing some cathode current.Most monitors are cathode driven, AFAIK, in which case the current would go to zero (of course!).The point about current-detect circuitry is a good one.If it's VGA, removing the R and B will simply have them default to black. You might get something that looks like...https://www.seventransistorlabs.com/Images/ScopeBig.jpg(which isn't actually disconnected R and B, the scope just doesn't drive them.)Tim

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2017, 10:53:46 pm
content: While I don't doubt they exist, I never personally encountered a TV or monitor that detected cathode current, at least not one I had to repair. On a monitor of course you can simply disconnect the unwanted signal lines between the monitor and the source. On a TV you can probably put a switch in the wires between the main and neck board. I recall repairing a number of TVs that had missing colors due to bad solder joints or failed transistors that drive the cathodes. More often that caused a color to be stuck on full with retrace lines rather than dead but both could happen.

author: ConKbot
author_rank: Super Contributor
timestamp: March 01, 2017, 11:34:46 pm
content: All the CRT monitors I've had keep running just fine missing a color. VGA cable jack screws come loose, and it rocks to the side and everything goes cyan when red gets disconnected. I suspect it would keep going just fine with blue missing too.

author: Ian.M
author_rank: Super Contributor
timestamp: March 01, 2017, 11:40:12 pm
content: IIRC it was a 'feature' of some older Sony TV models to compensate for tube ageing and do an auto white-balance. If it couldn't get a balance within a certain range, it would trip the protection circuit. It was a total P.I.T.A if you were trying to get an extra year or two out of a failing tube.