CRT removing Red and blue signal?

[MrZwing]

Hello

i 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

[MrZwing]

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?

[Ian.M]

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.

[MrZwing]

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.

[james_s]

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.

[MrZwing]

sad indeed that noone can see that CRT have a place.. at least to keep old equipment running.

[james_s]

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.

[MrZwing]

oh yeas nixie and tubes in general are beautiful practical works of art that makes everything look 100% more scientific.

[T3sl4co1l]

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

[james_s]

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.

[ConKbot]

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.

[Ian.M]

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.