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Video signal at brightness control of a CRT...

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Externet

Externet

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Hi all.
A test instrument has a monochrome CRT display, selects with cursors, its operation modes and displays data.

There is a control for brightness, and probably some internal trimmer for contrast. There is also a RGBIHV CGA jack for using an external color monitor.

What is likely the signal at the brightness or contrast controls wipers ? Could it be composite ? If so, I would like to tap there and feed video to a composite monitor instead of a RGB.

The intention is replacing its original CRT with a 7 inch LCD display, color or not. Suggestions are welcome.

( https://http2.mlstatic.com/monitor-...00-D_NQ_NP_914230-MLC26696623254_012018-F.jpg )
 

Surely not composite color video. Why would anyone use it inside the instrument, the poor color signal resolution simply ruins a sharp text and graphic display. Just convert the RGB output to monochrome video.
 

...beside that, if the signal was composite, dropping the level with a potentiometer would also drop the sync levels which should be constant regardless of image content.

Controls in CRT displays usually change the DC levels on the CRT itself and/or the gain of the video amplifier stages.

Brian.
 

Thanks, gentlemen.
I was able to find a little information. The internal CRT video signal is digital, generated from a proprietary ASIC output pins 'shade1' and 'shade2', fed to two ACT244 buffer ICs and mixed.
If there is any hope to make such signal composite, please advise.

Attached 'videoout portion'
 

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  • Video out page455.jpg
    Video out page455.jpg
    433.9 KB · Views: 111

I assume the 'VIDEOOUT' is an analog signal with four levels, those being from each combination of digital output from the ACT244s. In that case the controls will set the maximum and difference levels between the video steps. Not quite the same as contrast and brightness and the levels will be highly dependent on the load resistance on the VIDEOOUT point. Your LCD display would need an analog video input and independent sync inputs and you would probably have to add an analog buffer stage to the video connection.

Brian.
 

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